feat(compiler)!: Prohibit mutating options [LNG-277] (#960)

* Parse any nested type

* Refactor type system

* Fix restriction

* Refactor type resolution

* Return stream definition

* Update examples

* Refactor

* Refactor

* Refactor integration tests

* Export service

* Add integration test

* Fix args provider

* Add parser unit tests

* Add type resolution unit tests

* Add more unit tests

* DataTypeToken -> CompositeTypeToken

* GeneralStreamType -> MutableStreamType

* Refactor

* Refactor TypeResolution

* colType -> collectionType

* Refactor

* Fix PushToStreamSem

* BasicTypeToken -> ScalarTypeToken

* CompositeTypeToken -> BasicTypeToken

* Fix for nil

* Make stream collectible

* Refactor collectible type

* Use internalError

* Add unit tests

aqua-run/src/main/scala/aqua/run/CliFunc.scala

@@ -3,16 +3,16 @@ package aqua.run
 import aqua.parser.lexer.{CallArrowToken, CollectionToken, LiteralToken, VarToken}
 import aqua.parser.lift.Span
 import aqua.raw.value.{CollectionRaw, LiteralRaw, ValueRaw, VarRaw}
-import aqua.types.{ArrayType, BottomType}
+import aqua.types.*
 
-import cats.data.{NonEmptyChain, NonEmptyList, Validated, ValidatedNec}
 import cats.data.Validated.{invalid, invalidNec, validNec}
-import cats.{~>, Id}
+import cats.data.{NonEmptyChain, NonEmptyList, Validated, ValidatedNec}
+import cats.syntax.comonad.*
+import cats.syntax.either.*
+import cats.syntax.option.*
 import cats.syntax.traverse.*
 import cats.syntax.validated.*
-import cats.syntax.either.*
+import cats.{Id, ~>}
-import cats.syntax.comonad.*
-import cats.syntax.option.*
 
 case class CliFunc(name: String, args: List[ValueRaw] = Nil)
 
@@ -52,7 +52,15 @@ object CliFunc {
               .map(
                 NonEmptyList
                   .fromList(_)
-                  .map(l => CollectionRaw(l, ArrayType(l.head.baseType)))
+                  .map(l =>
+                    CollectionRaw(
+                      l,
+                      ArrayType(
+                        // FIXME: Type of Literal should always be a DataType
+                        l.head.baseType.asInstanceOf[DataType]
+                      )
+                    )
+                  )
                   .getOrElse(ValueRaw.Nil)
               )
               .toValidatedNec

aqua-run/src/main/scala/aqua/run/TypeValidator.scala

@@ -2,6 +2,7 @@ package aqua.run
 
 import aqua.raw.value.{LiteralRaw, ValueRaw, VarRaw}
 import aqua.types.*
+
 import cats.data.Validated.{invalidNec, validNec}
 import cats.data.{Validated, ValidatedNec}
 import cats.effect.kernel.Async
@@ -10,14 +11,11 @@ import cats.syntax.flatMap.*
 import cats.syntax.partialOrder.*
 import cats.syntax.show.*
 import cats.syntax.traverse.*
-
 import scala.collection.immutable.SortedMap
 import scala.concurrent.ExecutionContext
 
 object TypeValidator {
 
-  import aqua.types.Type.typesPartialOrder
-
   /**
    * Compare and validate type from Aqua file and type generated from JSON.
    *    Also, the validation will succeed if the JSON type is missing an array or an optional field.
@@ -69,7 +67,7 @@ object TypeValidator {
           case (l: OptionType, r) =>
             // if we have ?[][]string and [][][]string it must throw an error
             validateTypes(name, l.element, Some(r), Some((l, r)))
-          case (l: BoxType, r: BoxType) =>
+          case (l: CollectionType, r: CollectionType) =>
             validateTypes(name, l.element, Some(r.element), fullOptionType.orElse(Some(l, r)))
 
           case (l, r) =>

backend/definitions/src/main/scala/aqua/definitions/Definitions.scala

@@ -1,12 +1,12 @@
 package aqua.definitions
 
+import aqua.definitions.*
 import aqua.res.FuncRes
 import aqua.types.*
-import aqua.definitions.*
+
 import io.circe.*
 import io.circe.parser.*
 import io.circe.syntax.*
-
 import scala.annotation.tailrec
 
 // Represents the Aqua types
@@ -16,7 +16,7 @@ sealed trait TypeDefinition {
 
 object TypeDefinition {
 
-  implicit val encodeProdDefType: Encoder[ProductTypeDef] = {
+  given Encoder[ProductTypeDef] = {
     case d @ LabeledProductTypeDef(fields) =>
       Json.obj(
         ("tag", Json.fromString(d.tag)),
@@ -33,7 +33,7 @@ object TypeDefinition {
       )
   }
 
-  implicit val encodeDefType: Encoder[TypeDefinition] = {
+  given Encoder[TypeDefinition] = {
     case d @ ScalarTypeDef(name) =>
       Json.obj(
         ("tag", Json.fromString(d.tag)),
@@ -68,14 +68,14 @@ object TypeDefinition {
       )
   }
 
-  implicit val encodeServiceDefType: Encoder[ServiceDef] = { case ServiceDef(sId, functions, name) =>
+  given Encoder[ServiceDef] = { case ServiceDef(sId, functions, name) =>
     Json.obj(
       ("defaultServiceId", sId.asJson),
-      ("functions", encodeProdDefType(functions))
+      ("functions", (functions: ProductTypeDef).asJson)
     )
   }
 
-  implicit val encodeNamesConfig: Encoder[NamesConfig] = { case n: NamesConfig =>
+  given Encoder[NamesConfig] = { case n: NamesConfig =>
     import n.*
     Json.obj(
       ("relay", Json.fromString(relay)),
@@ -88,13 +88,12 @@ object TypeDefinition {
     )
   }
 
-  implicit val encodeFunctionDefType: Encoder[FunctionDef] = {
+  given Encoder[FunctionDef] = { case FunctionDef(fName, arrow, names) =>
-    case FunctionDef(fName, arrow, names) =>
+    Json.obj(
-      Json.obj(
+      ("functionName", Json.fromString(fName)),
-        ("functionName", Json.fromString(fName)),
+      ("arrow", (arrow: TypeDefinition).asJson),
-        ("arrow", encodeDefType(arrow)),
+      ("names", names.asJson)
-        ("names", names.asJson)
+    )
-      )
   }
 
   def apply(t: Option[Type]): TypeDefinition = t.map(apply).getOrElse(NilTypeDef)
@@ -103,7 +102,7 @@ object TypeDefinition {
     t match {
       case OptionType(t) =>
         OptionTypeDef(TypeDefinition(t))
-      case t: BoxType => ArrayTypeDef(TypeDefinition(t.element))
+      case t: CollectionType => ArrayTypeDef(TypeDefinition(t.element))
       case StructType(name, fields) =>
         StructTypeDef(name, fields.toSortedMap.view.mapValues(TypeDefinition.apply).toMap)
       case AbilityType(name, fieldAndArrows) =>
@@ -198,7 +197,11 @@ case class NamesConfig(
 )
 
 // Describes service
-case class ServiceDef(defaultServiceId: Option[String], functions: LabeledProductTypeDef, name: String)
+case class ServiceDef(
+  defaultServiceId: Option[String],
+  functions: LabeledProductTypeDef,
+  name: String
+)
 
 // Describes top-level function
 case class FunctionDef(

backend/ts/src/main/scala/aqua/backend/OutputFunc.scala

@@ -3,8 +3,11 @@ package aqua.backend
 import aqua.backend.air.FuncAirGen
 import aqua.backend.ts.TypeScriptCommon.fixupArgName
 import aqua.backend.ts.{TSFuncTypes, TypeScriptCommon}
+import aqua.definitions.*
+import aqua.definitions.TypeDefinition.given
 import aqua.res.FuncRes
 import aqua.types.*
+
 import cats.syntax.show.*
 import io.circe.*
 import io.circe.parser.*
@@ -20,8 +23,6 @@ case class OutputFunc(func: FuncRes, types: Types) {
   val funcTypes = types.funcType(func)
 
   import funcTypes.*
-  import aqua.definitions.TypeDefinition.*
-  import aqua.definitions.*
 
   def generate: (AirFunction, String) = {
     val tsAir = FuncAirGen(func).generate

backend/ts/src/main/scala/aqua/backend/OutputService.scala

@@ -1,8 +1,11 @@
 package aqua.backend
 
 import aqua.backend.ts.TypeScriptCommon
+import aqua.definitions.*
+import aqua.definitions.TypeDefinition.given
 import aqua.res.ServiceRes
 import aqua.types.ArrowType
+
 import io.circe.*
 import io.circe.parser.*
 import io.circe.syntax.*
@@ -14,8 +17,6 @@ case class OutputService(srv: ServiceRes, types: Types) {
   private val serviceTypes = types.serviceType(srv)
 
   import serviceTypes.*
-  import aqua.definitions.TypeDefinition.*
-  import aqua.definitions.*
 
   def generate: String =
     val functions = LabeledProductTypeDef(

build.sbt

@@ -122,6 +122,7 @@ lazy val types = crossProject(JVMPlatform, JSPlatform)
       "org.typelevel" %%% "cats-core" % catsV
     )
   )
+  .dependsOn(errors)
 
 lazy val parser = crossProject(JVMPlatform, JSPlatform)
   .withoutSuffixFor(JVMPlatform)

compiler/src/main/scala/aqua/compiler/AquaCompilerConf.scala

@@ -7,4 +7,4 @@ import aqua.raw.ConstantRaw
  *
  * @param constantsList List of known constants
  */
-case class AquaCompilerConf(constantsList: List[ConstantRaw])
+case class AquaCompilerConf(constantsList: List[ConstantRaw])

integration-tests/aqua/examples/errorClear.aqua

@@ -7,7 +7,7 @@ service FailService("fail-srv"):
 
 func errorClearTest(node: string, relay: string) -> string, i64:
     stream: *string
-    code: ?i64
+    code: *i64
 
     on node via relay:
         try:

integration-tests/aqua/examples/funcs.aqua

@@ -39,7 +39,7 @@ func ifCalc() -> u64:
   <- res!0
 
 func cmp(a: i32, b: i32, pred: i8 -> bool) -> bool:
-  result: ?bool
+  result: *bool
 
   if a < b:
     result <- pred(-1)

integration-tests/aqua/examples/handleResultError.aqua

@@ -4,7 +4,7 @@ export handleResultError
 
 -- t = true, f = false
 func handleResultError(t: bool, f: bool) -> string:
-    opt: ?[]string
+    opt: *[]string
 
     if t == f: -- false
         opt <<- ["unreachable"]

integration-tests/aqua/examples/options/option_gen.aqua

@@ -2,7 +2,7 @@ service OptionString("opt_str"):
   checkOption(str: ?string) -> string
 
 func emptyString() -> ?string:
-  valueEmpty: ?string
+  valueEmpty: *string
   <- valueEmpty
 
 func checkEmpty() -> string:
@@ -11,7 +11,7 @@ func checkEmpty() -> string:
   <- res
 
 func stringAsOption(str: string) -> ?string:
-  valueEmpty: ?string
+  valueEmpty: *string
   valueEmpty <<- str
   <- valueEmpty
 

integration-tests/aqua/examples/stream.aqua

@@ -36,7 +36,7 @@ func returnNilLength() -> u32:
     <- arr.length
 
 func stringNone() -> ?string:
-    valueNone: ?string
+    valueNone: *string
     <- valueNone
 
 func returnNone() -> ?string:

integration-tests/aqua/examples/streamArgs.aqua

@@ -1,3 +1,7 @@
+aqua StreamArgs
+
+export retrieve_records, modify_stream, TestService
+
 service TestService("test-service"):
     get_records(key: string) -> []string
 
@@ -7,4 +11,8 @@ func append_records(peer: string, srum: *[]string):
 func retrieve_records(peer: string) -> [][]string:
     records: *[]string
     append_records(peer, records)
-    <- records
+    <- records
+
+func modify_stream(stream: *string) -> []string:
+  stream <<- "appended value"
+  <- stream

integration-tests/aqua/examples/via.aqua

@@ -1,3 +1,7 @@
+aqua Via
+
+export viaArr, viaStream, viaOpt
+
 import "@fluencelabs/aqua-lib/builtin.aqua"
 
 func viaArr(node_id: string, viaAr: []string) -> Info:
@@ -5,13 +9,12 @@ func viaArr(node_id: string, viaAr: []string) -> Info:
         p <- Peer.identify()
     <- p
 
-
 func viaStream(node_id: string, viaStr: *string) -> Info:
     on node_id via viaStr:
         p <- Peer.identify()
     <- p
 
-func viaOpt(relay: string, node_id: string, viaOpt: ?string) -> Info:
+func viaOpt(node_id: string, viaOpt: ?string) -> Info:
     on node_id via viaOpt:
         p <- Peer.identify()
     <- p

integration-tests/src/__test__/examples.spec.ts

@@ -33,7 +33,11 @@ import {
 import {
   abilityCall,
   complexAbilityCall,
-  checkAbCallsCall, bugLNG258Call1, bugLNG258Call2, bugLNG258Call3, multipleAbilityWithClosureCall,
+  checkAbCallsCall,
+  bugLNG258Call1,
+  bugLNG258Call2,
+  bugLNG258Call3,
+  multipleAbilityWithClosureCall,
 } from "../examples/abilityCall.js";
 import {
   nilLengthCall,
@@ -78,7 +82,10 @@ import { tryCatchCall } from "../examples/tryCatchCall.js";
 import { tryOtherwiseCall } from "../examples/tryOtherwiseCall.js";
 import { coCall } from "../examples/coCall.js";
 import { bugLNG60Call, passArgsCall } from "../examples/passArgsCall.js";
-import { streamArgsCall } from "../examples/streamArgsCall.js";
+import {
+  streamArgsCall,
+  modifyStreamCall,
+} from "../examples/streamArgsCall.js";
 import { streamResultsCall } from "../examples/streamResultsCall.js";
 import { structuralTypingCall } from "../examples/structuralTypingCall";
 import {
@@ -104,7 +111,10 @@ import { multiReturnCall } from "../examples/multiReturnCall.js";
 import { declareCall } from "../examples/declareCall.js";
 import { genOptions, genOptionsEmptyString } from "../examples/optionsCall.js";
 import { lng193BugCall } from "../examples/closureReturnRename.js";
-import {closuresCall, multipleClosuresLNG262BugCall} from "../examples/closures.js";
+import {
+  closuresCall,
+  multipleClosuresLNG262BugCall,
+} from "../examples/closures.js";
 import { closureArrowCaptureCall } from "../examples/closureArrowCapture.js";
 import {
   bugLNG63_2Call,
@@ -589,6 +599,18 @@ describe("Testing examples", () => {
     expect(streamArgsResult).toEqual([["peer_id", "peer_id"]]);
   });
 
+  it("streamArgs.aqua modify stream", async () => {
+    let streamArgsResult = await modifyStreamCall([
+      "passed value 1",
+      "passed value 2",
+    ]);
+    expect(streamArgsResult).toEqual([
+      "passed value 1",
+      "passed value 2",
+      "appended value",
+    ]);
+  });
+
   it("streamResults.aqua", async () => {
     let streamResultsResult = await streamResultsCall();
     expect(streamResultsResult).toEqual(["new_name", "new_name", "new_name"]);
@@ -934,9 +956,10 @@ describe("Testing examples", () => {
 
   it("via.aqua", async () => {
     let res1 = await viaArrCall();
-    let res2 = await viaOptCall(relayPeerId1);
+    let res2 = await viaOptCall();
-    let res3 = await viaOptNullCall(relayPeerId1);
+    let res3 = await viaOptNullCall();
-    let res4 = await viaStreamCall(relayPeerId1);
+    let res4 = await viaStreamCall();
+    expect(res1).not.toHaveLength(0);
     expect(res1).toEqual(res2);
     expect(res2).toEqual(res3);
     expect(res3).toEqual(res4);

integration-tests/src/examples/streamArgsCall.ts

@@ -1,5 +1,6 @@
 import {
   retrieve_records,
+  modify_stream,
   registerTestService,
 } from "../compiled/examples/streamArgs.js";
 
@@ -12,3 +13,7 @@ export async function streamArgsCall() {
 
   return await retrieve_records("peer_id");
 }
+
+export async function modifyStreamCall(arg: string[]) {
+  return await modify_stream(arg);
+}

integration-tests/src/examples/viaCall.ts

@@ -13,26 +13,21 @@ export async function viaArrCall(): Promise<string[]> {
   return res.external_addresses;
 }
 
-export async function viaOptCall(relayPeerId: string): Promise<string[]> {
+export async function viaOptCall(): Promise<string[]> {
-  let res2 = await viaOpt(relayPeerId, relays[4].peerId, relays[2].peerId, {
+  let res2 = await viaOpt(relays[4].peerId, relays[2].peerId, { ttl: 30000 });
-    ttl: 30000,
-  });
 
   return res2.external_addresses;
 }
 
-export async function viaOptNullCall(relayPeerId: string): Promise<string[]> {
+export async function viaOptNullCall(): Promise<string[]> {
-  let res3 = await viaOpt(
+  let res3 = await viaOpt(relays[4].peerId, null, {
-    relayPeerId,
+    ttl: 30000,
-    relays[4].peerId,
+  });
-    relays[2].peerId || null,
-    { ttl: 30000 },
-  );
 
   return res3.external_addresses;
 }
 
-export async function viaStreamCall(relayPeerId: string): Promise<string[]> {
+export async function viaStreamCall(): Promise<string[]> {
   let res4 = await viaStream(
     relays[4].peerId,
     [relays[2].peerId, relays[1].peerId],

js/js-exports/src/main/scala/aqua/js/Definitions.scala

@@ -2,11 +2,11 @@ package aqua.js
 
 import aqua.*
 import aqua.backend.*
-import aqua.definitions.{ArrayTypeDef, ArrowTypeDef, BottomTypeDef, FunctionDef, LabeledProductTypeDef, NamesConfig, NilTypeDef, OptionTypeDef, ScalarTypeDef, ServiceDef, StructTypeDef, TopTypeDef, TypeDefinition, UnlabeledProductTypeDef}
+import aqua.definitions.*
 import aqua.res.FuncRes
-import aqua.types.{ArrowType, BottomType, BoxType, LabeledConsType, LiteralType, NilType, OptionType, ProductType, ScalarType, StructType, TopType, Type, UnlabeledConsType}
+import aqua.types.*
-import io.circe.{Encoder, Json}
 
+import io.circe.{Encoder, Json}
 import scala.scalajs.js
 import scala.scalajs.js.JSConverters.*
 import scala.scalajs.js.annotation.{JSExportAll, JSImport}

js/js-exports/src/main/scala/aqua/js/JsonEncoder.scala

@@ -1,6 +1,7 @@
 package aqua.js
 
 import aqua.types.*
+
 import cats.data.Validated.{invalid, invalidNec, invalidNel, valid, validNec, validNel}
 import cats.data.{NonEmptyMap, Validated, ValidatedNec}
 import cats.syntax.applicative.*
@@ -9,7 +10,6 @@ import cats.syntax.flatMap.*
 import cats.syntax.functor.*
 import cats.syntax.semigroup.*
 import cats.syntax.traverse.*
-
 import scala.collection.immutable.SortedMap
 import scala.scalajs.js
 
@@ -17,18 +17,18 @@ object JsonEncoder {
 
   /* Get widest possible type from JSON arrays. For example:
   JSON: {
-          field1: [
+    field1: [
-                  {
+      {
-                    a: "a",
+        a: "a",
-                    b: [1,2,3],
+        b: [1,2,3],
-                    c: 4
+        c: 4
-                  },
+      },
-                  {
+      {
-                    c: 3
+        c: 3
-                  }
+      }
-                  ]
+    ]
-        }
+  }
-  There type in array must be { a: ?string, b: []number, c: number
+  Type in array must be { a: ?string, b: []number, c: number }
    */
   private def compareAndGetWidestType(
     name: String,
@@ -43,8 +43,8 @@ object JsonEncoder {
           case (la @ ArrayType(_), BottomType) => validNec(la)
           case (lo @ OptionType(lel), rtt) if lel == rtt => validNec(lo)
           case (ltt, ro @ OptionType(rel)) if ltt == rel => validNec(ro)
-          case (BottomType, rb) => validNec(OptionType(rb))
+          case (BottomType, rb: DataType) => validNec(OptionType(rb))
-          case (lb, BottomType) => validNec(OptionType(lb))
+          case (lb: DataType, BottomType) => validNec(OptionType(lb))
           case (lst: StructType, rst: StructType) =>
             val lFieldsSM: SortedMap[String, Type] = lst.fields.toSortedMap
             val rFieldsSM: SortedMap[String, Type] = rst.fields.toSortedMap
@@ -100,7 +100,10 @@ object JsonEncoder {
               .reduce[ValidatedNec[String, Type]] { case (l, t) =>
                 compareAndGetWidestType(name, l, t)
               }
-              .map(t => ArrayType(t))
+              .andThen {
+                case dt: DataType => validNec(ArrayType(dt))
+                case t => invalidNec(s"Unexpected type $t")
+              }
           }
         }
       case a if t == "object" && !js.isUndefined(arg) && arg != null =>

model/inline/src/main/scala/aqua/model/inline/ArrowInliner.scala

@@ -6,17 +6,17 @@ import aqua.model.*
 import aqua.model.inline.state.{Arrows, Exports, Mangler}
 import aqua.raw.ops.RawTag
 import aqua.raw.value.{ValueRaw, VarRaw}
-import aqua.types.{AbilityType, ArrowType, BoxType, NamedType, StreamType, Type}
+import aqua.types.{AbilityType, ArrowType, CollectionType, NamedType, StreamType, Type}
 
 import cats.data.StateT
 import cats.data.{Chain, IndexedStateT, State}
-import cats.syntax.functor.*
 import cats.syntax.applicative.*
 import cats.syntax.bifunctor.*
 import cats.syntax.foldable.*
-import cats.syntax.traverse.*
+import cats.syntax.functor.*
 import cats.syntax.option.*
 import cats.syntax.show.*
+import cats.syntax.traverse.*
 import cats.{Eval, Monoid}
 import scribe.Logging
 

model/inline/src/main/scala/aqua/model/inline/TagInliner.scala

@@ -8,7 +8,7 @@ import aqua.model.inline.state.{Arrows, Exports, Mangler}
 import aqua.model.inline.tag.IfTagInliner
 import aqua.raw.ops.*
 import aqua.raw.value.*
-import aqua.types.{BoxType, CanonStreamType, StreamType}
+import aqua.types.{CanonStreamType, CollectionType, StreamType}
 
 import cats.data.{Chain, State, StateT}
 import cats.instances.list.*
@@ -31,9 +31,10 @@ import scribe.Logging
  */
 object TagInliner extends Logging {
 
-  import RawValueInliner.{valueListToModel, valueToModel}
   import aqua.model.inline.Inline.parDesugarPrefix
 
+  import RawValueInliner.{valueListToModel, valueToModel}
+
   /**
    * Result of [[RawTag]] inlining
    *
@@ -242,7 +243,7 @@ object TagInliner extends Logging {
           (v, p) = flattened
           n <- Mangler[S].findAndForbidName(item)
           elementType = iterable.`type` match {
-            case b: BoxType => b.element
+            case b: CollectionType => b.element
             case _ =>
               internalError(
                 s"non-box type variable '$iterable' in 'for' expression."

model/inline/src/main/scala/aqua/model/inline/raw/ApplyFunctorRawInliner.scala

@@ -1,5 +1,9 @@
 package aqua.model.inline.raw
 
+import aqua.model.inline.Inline
+import aqua.model.inline.Inline.MergeMode.*
+import aqua.model.inline.RawValueInliner.unfold
+import aqua.model.inline.state.{Arrows, Exports, Mangler}
 import aqua.model.{
   CallModel,
   CanonicalizeModel,
@@ -10,14 +14,11 @@ import aqua.model.{
   ValueModel,
   VarModel
 }
-import aqua.model.inline.Inline.MergeMode.*
-import aqua.model.inline.Inline
-import aqua.model.inline.state.{Arrows, Exports, Mangler}
 import aqua.raw.value.{FunctorRaw, ValueRaw}
-import cats.data.State
+import aqua.types.{ArrayType, CanonStreamType, CollectionType, StreamType}
+
 import cats.data.Chain
-import aqua.model.inline.RawValueInliner.unfold
+import cats.data.State
-import aqua.types.{ArrayType, BoxType, CanonStreamType, StreamType}
 import cats.syntax.monoid.*
 import scribe.Logging
 

model/inline/src/main/scala/aqua/model/inline/raw/CollectionRawInliner.scala

@@ -1,11 +1,13 @@
 package aqua.model.inline.raw
 
-import aqua.model.{CallModel, CanonicalizeModel, NullModel, PushToStreamModel, RestrictionModel, SeqModel, ValueModel, VarModel, XorModel}
+import aqua.model.*
 import aqua.model.inline.Inline
 import aqua.model.inline.RawValueInliner.valueToModel
 import aqua.model.inline.state.{Arrows, Exports, Mangler}
 import aqua.raw.value.CollectionRaw
+import aqua.types.StreamMapType
 import aqua.types.{ArrayType, CanonStreamType, OptionType, StreamType}
+
 import cats.data.{Chain, State}
 
 object CollectionRawInliner extends RawInliner[CollectionRaw] {
@@ -20,11 +22,15 @@ object CollectionRawInliner extends RawInliner[CollectionRaw] {
     assignToName: Option[String] = None
   ): State[S, (ValueModel, Inline)] =
     for {
-      streamName <- raw.boxType match {
+      streamName <- raw.collectionType match {
         case _: StreamType =>
-          assignToName
+          assignToName.fold(
-            .map(s => State.pure(s))
+            Mangler[S].findAndForbidName("stream-inline")
-            .getOrElse(Mangler[S].findAndForbidName("stream-inline"))
+          )(State.pure)
+        case _: StreamMapType =>
+          assignToName.fold(
+            Mangler[S].findAndForbidName("stream_map-inline")
+          )(State.pure)
         case _: CanonStreamType => Mangler[S].findAndForbidName("canon_stream-inline")
         case _: ArrayType => Mangler[S].findAndForbidName("array-inline")
         case _: OptionType => Mangler[S].findAndForbidName("option-inline")
@@ -50,15 +56,15 @@ object CollectionRawInliner extends RawInliner[CollectionRaw] {
       }
 
       canonName <-
-        if (raw.boxType.isStream) State.pure(streamName)
+        if (raw.collectionType.isStream) State.pure(streamName)
         else Mangler[S].findAndForbidName(streamName)
-      canonType = raw.boxType match {
+      canonType = raw.collectionType match {
-        case StreamType(_) => raw.boxType
+        case StreamType(_) => raw.collectionType
-        case _ => CanonStreamType(raw.boxType.element)
+        case _ => CanonStreamType(raw.collectionType.element)
       }
       canon = CallModel.Export(canonName, canonType)
     } yield VarModel(canonName, canon.`type`) -> Inline.tree(
-      raw.boxType match {
+      raw.collectionType match {
         case ArrayType(_) =>
           RestrictionModel(streamName, streamType).wrap(
             SeqModel.wrap(inlines ++ vals :+ CanonicalizeModel(stream, canon).leaf)

model/raw/src/main/scala/aqua/raw/ConstantRaw.scala

@@ -55,7 +55,7 @@ object ConstantRaw {
       false
     )
 
-  def defaultConstants(relayVarName: Option[String]): List[ConstantRaw] =
+  def defaultConstants(relayVarName: Option[String] = None): List[ConstantRaw] =
     hostPeerId(
       relayVarName
     ) :: initPeerId :: particleTtl :: particleTimestamp :: nil :: lastError :: Nil

model/raw/src/main/scala/aqua/raw/ops/RawTag.scala

@@ -4,12 +4,12 @@ import aqua.raw.arrow.FuncRaw
 import aqua.raw.ops.RawTag.Tree
 import aqua.raw.value.{CallArrowRaw, CallServiceRaw, ValueRaw}
 import aqua.tree.{TreeNode, TreeNodeCompanion}
-import aqua.types.{ArrowType, DataType, ServiceType}
+import aqua.types.*
 
 import cats.Show
 import cats.data.{Chain, NonEmptyList}
-import cats.syntax.foldable.*
 import cats.free.Cofree
+import cats.syntax.foldable.*
 
 sealed trait RawTag extends TreeNode[RawTag] {
 
@@ -160,7 +160,7 @@ case class NextTag(item: String) extends RawTag {
   override def mapValues(f: ValueRaw => ValueRaw): RawTag = this
 }
 
-case class RestrictionTag(name: String, `type`: DataType) extends SeqGroupTag {
+case class RestrictionTag(name: String, `type`: Type) extends SeqGroupTag {
 
   override def restrictsVarNames: Set[String] = Set(name)
 

model/raw/src/main/scala/aqua/raw/value/ValueRaw.scala

@@ -1,9 +1,11 @@
 package aqua.raw.value
 
+import aqua.errors.Errors.internalError
 import aqua.types.*
+import aqua.types.Type.*
 
-import cats.data.{Chain, NonEmptyList, NonEmptyMap}
 import cats.Eq
+import cats.data.{Chain, NonEmptyList, NonEmptyMap}
 import cats.syntax.option.*
 
 sealed trait ValueRaw {
@@ -157,16 +159,27 @@ object LiteralRaw {
   }
 }
 
-case class CollectionRaw(values: NonEmptyList[ValueRaw], boxType: BoxType) extends ValueRaw {
+case class CollectionRaw(
+  values: NonEmptyList[ValueRaw],
+  collectionType: CollectionType
+) extends ValueRaw {
 
-  lazy val elementType: Type = boxType.element
+  lazy val elementType: DataType = collectionType.element
 
-  override lazy val baseType: Type = boxType
+  override lazy val baseType: Type = collectionType
 
   override def mapValues(f: ValueRaw => ValueRaw): ValueRaw = {
     val vals = values.map(f)
-    val el = vals.map(_.`type`).reduceLeft(_ `∩` _)
+    val types = vals.map(_.`type` match {
-    copy(vals, boxType.withElement(el))
+      case ct: CollectibleType => ct
+      case t => internalError(s"Non-collection type in collection: ${t}")
+    })
+    val element = CollectionType.elementTypeOf(types.toList)
+
+    copy(
+      values = vals,
+      collectionType = collectionType.withElement(element)
+    )
   }
 
   override def varNames: Set[String] = values.toList.flatMap(_.varNames).toSet

model/res/src/main/scala/aqua/res/ResolvedOp.scala

@@ -3,10 +3,11 @@ package aqua.res
 import aqua.model.{CallModel, ForModel, LiteralModel, ValueModel, VarModel}
 import aqua.raw.ops.Call
 import aqua.tree.{TreeNode, TreeNodeCompanion}
-import aqua.types.DataType
+import aqua.types.*
+
+import cats.Show
 import cats.data.Chain
 import cats.free.Cofree
-import cats.Show
 
 // TODO docs to all traits and objects
 sealed trait ResolvedOp extends TreeNode[ResolvedOp]
@@ -46,7 +47,7 @@ object FoldRes {
     FoldRes(item, iterable, Mode.Never)
 }
 
-case class RestrictionRes(item: String, `type`: DataType) extends ResolvedOp {
+case class RestrictionRes(item: String, `type`: Type) extends ResolvedOp {
   override def toString: String = s"(new ${`type`.airPrefix}$item "
 }
 

model/src/main/scala/aqua/model/OpModel.scala

@@ -4,13 +4,12 @@ import aqua.model.OpModel.Tree
 import aqua.tree.{TreeNode, TreeNodeCompanion}
 import aqua.types.*
 
-import cats.data.Chain
-import cats.free.Cofree
-import cats.Show
 import cats.Eval
+import cats.Show
+import cats.data.Chain
 import cats.data.NonEmptyList
+import cats.free.Cofree
 import cats.syntax.functor.*
-
 import scala.annotation.tailrec
 
 sealed trait OpModel extends TreeNode[OpModel] {
@@ -129,7 +128,7 @@ case class NextModel(item: String) extends OpModel {
 
 // TODO: Refactor out `name` and `type` to
 // something like VarModel without properties
-case class RestrictionModel(name: String, `type`: DataType) extends SeqGroupModel {
+case class RestrictionModel(name: String, `type`: Type) extends SeqGroupModel {
   override def usesVarNames: Set[String] = Set.empty
 
   override def restrictsVarNames: Set[String] = Set(name)

model/transform/src/main/scala/aqua/model/transform/pre/ArgsProvider.scala

@@ -18,7 +18,7 @@ object ArgsProvider {
     // Variable name to store the value of the argument
     varName: String,
     // Type of the argument
-    t: DataType
+    t: DataType | StreamType
   )
 }
 
@@ -46,18 +46,21 @@ case class ArgsFromService(dataServiceId: ValueRaw) extends ArgsProvider {
     )
   }
 
+  private def getDataOp(name: String, varName: String, t: DataType): RawTag.Tree =
+    CallArrowRawTag
+      .service(
+        dataServiceId,
+        name,
+        Call(Nil, Call.Export(varName, t) :: Nil)
+      )
+      .leaf
+
   def getDataOp(arg: ArgsProvider.Arg): RawTag.Tree =
     arg.t match {
       case st: StreamType =>
         getStreamDataOp(arg.name, arg.varName, st)
-      case _ =>
+      case dt: DataType =>
-        CallArrowRawTag
+        getDataOp(arg.name, arg.varName, dt)
-          .service(
-            dataServiceId,
-            arg.name,
-            Call(Nil, Call.Export(arg.varName, arg.t) :: Nil)
-          )
-          .leaf
     }
 
   override def provideArgs(args: List[ArgsProvider.Arg]): List[RawTag.Tree] =

model/transform/src/main/scala/aqua/model/transform/pre/FuncPreTransformer.scala

@@ -81,7 +81,7 @@ case class FuncPreTransformer(
       (name, s"-$name-arg-", typ)
     }
 
-    val dataArgs = args.collect { case (name, varName, t: DataType) =>
+    val nonArrowArgs = args.collect { case (name, varName, t: (DataType | StreamType)) =>
       ArgsProvider.Arg(name, varName, t)
     }
 
@@ -95,7 +95,7 @@ case class FuncPreTransformer(
     )
 
     val provideArgs = argsProvider.provideArgs(
-      relayArg.toList ::: dataArgs
+      relayArg.toList ::: nonArrowArgs
     )
 
     val handleResults = resultsHandler.handleResults(

model/transform/src/main/scala/aqua/model/transform/topology/Topology.scala

@@ -1,28 +1,28 @@
 package aqua.model.transform.topology
 
 import aqua.errors.Errors.internalError
-import aqua.model.transform.topology.TopologyPath
-import aqua.model.transform.cursor.ChainZipper
-import aqua.model.transform.topology.strategy.*
 import aqua.model.*
+import aqua.model.transform.cursor.ChainZipper
+import aqua.model.transform.topology.TopologyPath
+import aqua.model.transform.topology.strategy.*
 import aqua.raw.value.{LiteralRaw, ValueRaw}
 import aqua.res.{ApRes, CanonRes, FoldRes, MakeRes, NextRes, ResolvedOp, SeqRes}
-import aqua.types.{ArrayType, BoxType, CanonStreamType, ScalarType, StreamType}
+import aqua.types.{ArrayType, CanonStreamType, CollectionType, ScalarType, StreamType}
 
 import cats.Eval
 import cats.data.Chain.{==:, nil}
 import cats.data.OptionT
 import cats.data.{Chain, NonEmptyChain, NonEmptyList, OptionT}
 import cats.free.Cofree
-import cats.syntax.traverse.*
-import cats.syntax.show.*
-import cats.syntax.apply.*
-import cats.syntax.option.*
-import cats.syntax.flatMap.*
-import cats.syntax.foldable.*
-import cats.syntax.applicative.*
 import cats.instances.map.*
 import cats.kernel.Monoid
+import cats.syntax.applicative.*
+import cats.syntax.apply.*
+import cats.syntax.flatMap.*
+import cats.syntax.foldable.*
+import cats.syntax.option.*
+import cats.syntax.show.*
+import cats.syntax.traverse.*
 import scribe.Logging
 
 /**
@@ -370,7 +370,7 @@ object Topology extends Logging {
     reversed: Boolean = false
   ): Chain[Res] = peerIds.map { v =>
     v.`type` match {
-      case _: BoxType =>
+      case _: CollectionType =>
         val itemName = "-via-peer-"
         val steps = Chain(
           MakeRes.hop(VarModel(itemName, ScalarType.string, Chain.empty)),

parser/src/main/scala/aqua/parser/expr/ArrowTypeExpr.scala

@@ -2,13 +2,14 @@ package aqua.parser.expr
 
 import aqua.parser.Expr
 import aqua.parser.lexer.Token.*
-import aqua.parser.lexer.{ArrowTypeToken, DataTypeToken, Name}
+import aqua.parser.lexer.{ArrowTypeToken, BasicTypeToken, Name}
 import aqua.parser.lift.LiftParser
+import aqua.parser.lift.Span
+import aqua.parser.lift.Span.{P0ToSpan, PToSpan}
+
 import cats.Comonad
 import cats.parse.Parser
 import cats.~>
-import aqua.parser.lift.Span
-import aqua.parser.lift.Span.{P0ToSpan, PToSpan}
 
 case class ArrowTypeExpr[F[_]](name: Name[F], `type`: ArrowTypeToken[F])
     extends Expr[F](ArrowTypeExpr, name) {
@@ -19,8 +20,9 @@ object ArrowTypeExpr extends Expr.Leaf {
 
   override val p: Parser[ArrowTypeExpr[Span.S]] =
     (Name.p ~ ((` : ` *> ArrowTypeToken.`arrowdef`(
-      DataTypeToken.`datatypedef`
+      BasicTypeToken.`compositetypedef`
-    )) | ArrowTypeToken.`arrowWithNames`(DataTypeToken.`datatypedef`))).map { case (name, t) =>
+    )) | ArrowTypeToken.`arrowWithNames`(BasicTypeToken.`compositetypedef`))).map {
-      ArrowTypeExpr(name, t)
+      case (name, t) =>
+        ArrowTypeExpr(name, t)
     }
 }

parser/src/main/scala/aqua/parser/expr/FieldTypeExpr.scala

@@ -2,15 +2,16 @@ package aqua.parser.expr
 
 import aqua.parser.Expr
 import aqua.parser.lexer.Token.*
-import aqua.parser.lexer.{DataTypeToken, Name, StreamTypeToken}
+import aqua.parser.lexer.{BasicTypeToken, Name, StreamTypeToken}
 import aqua.parser.lift.LiftParser
-import cats.Comonad
-import cats.parse.Parser
-import cats.~>
 import aqua.parser.lift.Span
 import aqua.parser.lift.Span.{P0ToSpan, PToSpan}
 
-case class FieldTypeExpr[F[_]](name: Name[F], `type`: DataTypeToken[F])
+import cats.Comonad
+import cats.parse.Parser
+import cats.~>
+
+case class FieldTypeExpr[F[_]](name: Name[F], `type`: BasicTypeToken[F])
     extends Expr[F](FieldTypeExpr, name) {
 
   override def mapK[K[_]: Comonad](fk: F ~> K): FieldTypeExpr[K] =
@@ -20,7 +21,7 @@ case class FieldTypeExpr[F[_]](name: Name[F], `type`: DataTypeToken[F])
 object FieldTypeExpr extends Expr.Leaf {
 
   override val p: Parser[FieldTypeExpr[Span.S]] =
-    ((Name.p <* ` : `) ~ DataTypeToken.`datatypedef`).map { case (name, t) =>
+    ((Name.p <* ` : `) ~ BasicTypeToken.`compositetypedef`).map { case (name, t) =>
       FieldTypeExpr(name, t)
     }
 }

parser/src/main/scala/aqua/parser/expr/func/ArrowExpr.scala

@@ -1,13 +1,14 @@
 package aqua.parser.expr.func
 
-import aqua.parser.{ArrowReturnError, Ast, Expr, ParserError}
+import aqua.parser.lexer.{ArrowTypeToken, BasicTypeToken, TypeToken, ValueToken}
-import aqua.parser.lexer.{ArrowTypeToken, DataTypeToken, TypeToken, ValueToken}
 import aqua.parser.lift.LiftParser
+import aqua.parser.lift.Span
+import aqua.parser.lift.Span.{P0ToSpan, PToSpan}
+import aqua.parser.{ArrowReturnError, Ast, Expr, ParserError}
+
 import cats.Comonad
 import cats.parse.Parser
 import cats.~>
-import aqua.parser.lift.Span
-import aqua.parser.lift.Span.{P0ToSpan, PToSpan}
 
 case class ArrowExpr[F[_]](arrowTypeExpr: ArrowTypeToken[F])
     extends Expr[F](ArrowExpr, arrowTypeExpr) {

parser/src/main/scala/aqua/parser/expr/func/DeclareStreamExpr.scala

@@ -3,14 +3,15 @@ package aqua.parser.expr.func
 import aqua.parser.Expr
 import aqua.parser.expr.func.DeclareStreamExpr
 import aqua.parser.lexer.Token.*
-import aqua.parser.lexer.{DataTypeToken, Name, Token, TypeToken}
+import aqua.parser.lexer.{BasicTypeToken, Name, Token, TypeToken}
 import aqua.parser.lift.LiftParser
-import cats.parse.Parser as P
-import cats.{Comonad, ~>}
 import aqua.parser.lift.Span
 import aqua.parser.lift.Span.{P0ToSpan, PToSpan}
 
-case class DeclareStreamExpr[F[_]](name: Name[F], `type`: DataTypeToken[F])
+import cats.parse.Parser as P
+import cats.{Comonad, ~>}
+
+case class DeclareStreamExpr[F[_]](name: Name[F], `type`: BasicTypeToken[F])
     extends Expr[F](DeclareStreamExpr, name) {
 
   override def mapK[K[_]: Comonad](fk: F ~> K): DeclareStreamExpr[K] =
@@ -20,7 +21,7 @@ case class DeclareStreamExpr[F[_]](name: Name[F], `type`: DataTypeToken[F])
 object DeclareStreamExpr extends Expr.Leaf {
 
   override val p: P[DeclareStreamExpr[Span.S]] =
-    ((Name.p <* ` : `) ~ DataTypeToken.`datatypedef`).map { case (name, t) =>
+    ((Name.p <* ` : `) ~ BasicTypeToken.`compositetypedef`).map { case (name, t) =>
       DeclareStreamExpr(name, t)
     }
 

parser/src/main/scala/aqua/parser/lexer/TypeToken.scala

@@ -3,39 +3,46 @@ package aqua.parser.lexer
 import aqua.parser.lexer.Token.*
 import aqua.parser.lift.LiftParser
 import aqua.parser.lift.LiftParser.*
+import aqua.parser.lift.Span
+import aqua.parser.lift.Span.{P0ToSpan, PToSpan, S}
 import aqua.types.ScalarType
+
 import cats.Comonad
+import cats.data.NonEmptyList
 import cats.parse.{Accumulator0, Parser as P, Parser0 as P0}
 import cats.syntax.comonad.*
 import cats.syntax.functor.*
 import cats.~>
-import aqua.parser.lift.Span
-import aqua.parser.lift.Span.{P0ToSpan, PToSpan, S}
-import cats.data.NonEmptyList
 
 sealed trait TypeToken[S[_]] extends Token[S] {
   def mapK[K[_]: Comonad](fk: S ~> K): TypeToken[K]
 }
 
-sealed trait DataTypeToken[S[_]] extends TypeToken[S] {
+sealed trait BasicTypeToken[S[_]] extends TypeToken[S] {
-  override def mapK[K[_]: Comonad](fk: S ~> K): DataTypeToken[K]
+  override def mapK[K[_]: Comonad](fk: S ~> K): BasicTypeToken[K]
 }
 
 case class TopBottomToken[S[_]: Comonad](override val unit: S[Unit], isTop: Boolean)
-    extends DataTypeToken[S] {
+    extends BasicTypeToken[S] {
   override def as[T](v: T): S[T] = unit.as(v)
   def isBottom: Boolean = !isTop
   override def mapK[K[_]: Comonad](fk: S ~> K): TopBottomToken[K] = copy(fk(unit), isTop)
 }
 
-case class ArrayTypeToken[S[_]: Comonad](override val unit: S[Unit], data: DataTypeToken[S])
+case class ArrayTypeToken[S[_]: Comonad](override val unit: S[Unit], data: BasicTypeToken[S])
-    extends DataTypeToken[S] {
+    extends BasicTypeToken[S] {
   override def as[T](v: T): S[T] = unit.as(v)
   override def mapK[K[_]: Comonad](fk: S ~> K): ArrayTypeToken[K] = copy(fk(unit), data.mapK(fk))
 }
 
-case class StreamTypeToken[S[_]: Comonad](override val unit: S[Unit], data: DataTypeToken[S])
+object ArrayTypeToken {
-    extends DataTypeToken[S] {
+
+  val `arraytypedef`: P[ArrayTypeToken[Span.S]] =
+    (`[]`.lift ~ BasicTypeToken.`compositetypedef`).map(ud => ArrayTypeToken(ud._1, ud._2))
+}
+
+case class StreamTypeToken[S[_]: Comonad](override val unit: S[Unit], data: BasicTypeToken[S])
+    extends BasicTypeToken[S] {
   override def as[T](v: T): S[T] = unit.as(v)
   override def mapK[K[_]: Comonad](fk: S ~> K): StreamTypeToken[K] = copy(fk(unit), data.mapK(fk))
 }
@@ -43,14 +50,12 @@ case class StreamTypeToken[S[_]: Comonad](override val unit: S[Unit], data: Data
 object StreamTypeToken {
 
   val `streamtypedef`: P[StreamTypeToken[Span.S]] =
-    ((`*`.lift <* P.not(`*`).withContext("Nested streams '**type' are prohibited"))
+    (`*`.lift ~ BasicTypeToken.`compositetypedef`).map(ud => StreamTypeToken(ud._1, ud._2))
-      ~ DataTypeToken.`withoutstreamdatatypedef`)
-      .map(ud => StreamTypeToken(ud._1, ud._2))
 
 }
 
-case class OptionTypeToken[F[_]: Comonad](override val unit: F[Unit], data: DataTypeToken[F])
+case class OptionTypeToken[F[_]: Comonad](override val unit: F[Unit], data: BasicTypeToken[F])
-    extends DataTypeToken[F] {
+    extends BasicTypeToken[F] {
   override def as[T](v: T): F[T] = unit.as(v)
 
   override def mapK[K[_]: Comonad](fk: F ~> K): OptionTypeToken[K] =
@@ -60,11 +65,11 @@ case class OptionTypeToken[F[_]: Comonad](override val unit: F[Unit], data: Data
 object OptionTypeToken {
 
   val `optiontypedef`: P[OptionTypeToken[Span.S]] =
-    (`?`.lift ~ DataTypeToken.`withoutstreamdatatypedef`).map(ud => OptionTypeToken(ud._1, ud._2))
+    (`?`.lift ~ BasicTypeToken.`compositetypedef`).map(ud => OptionTypeToken(ud._1, ud._2))
 
 }
 
-case class NamedTypeToken[F[_]: Comonad](name: F[String]) extends DataTypeToken[F] {
+case class NamedTypeToken[F[_]: Comonad](name: F[String]) extends BasicTypeToken[F] {
   override def as[T](v: T): F[T] = name.as(v)
   def asName: Name[F] = Name[F](name)
 
@@ -84,21 +89,22 @@ object NamedTypeToken {
     `Class`.repSep(`.`).string.lift.map(NamedTypeToken(_))
 }
 
-case class BasicTypeToken[F[_]: Comonad](scalarType: F[ScalarType]) extends DataTypeToken[F] {
+case class ScalarTypeToken[F[_]: Comonad](scalarType: F[ScalarType]) extends BasicTypeToken[F] {
   override def as[T](v: T): F[T] = scalarType.as(v)
 
-  override def mapK[K[_]: Comonad](fk: F ~> K): BasicTypeToken[K] =
+  override def mapK[K[_]: Comonad](fk: F ~> K): ScalarTypeToken[K] =
     copy(fk(scalarType))
+
   def value: ScalarType = scalarType.extract
 }
 
-object BasicTypeToken {
+object ScalarTypeToken {
 
-  val `basictypedef`: P[BasicTypeToken[Span.S]] =
+  val scalartypedef: P[ScalarTypeToken[Span.S]] =
     P.oneOf(
       ScalarType.all.map(n ⇒ P.string(n.name).as(n)).toList
     ).lift
-      .map(BasicTypeToken(_))
+      .map(ScalarTypeToken.apply)
 }
 
 case class ArrowTypeToken[S[_]: Comonad](
@@ -153,29 +159,20 @@ object ArrowTypeToken {
     }
 }
 
-object DataTypeToken {
+object BasicTypeToken {
-
-  val `arraytypedef`: P[ArrayTypeToken[Span.S]] =
-    (`[]`.lift ~ `withoutstreamdatatypedef`).map(ud => ArrayTypeToken(ud._1, ud._2))
 
   val `topbottomdef`: P[TopBottomToken[Span.S]] =
     `⊥`.lift.map(TopBottomToken(_, isTop = false)) |
       `⊤`.lift.map(TopBottomToken(_, isTop = true))
 
-  def `withoutstreamdatatypedef`: P[DataTypeToken[Span.S]] =
+  def `compositetypedef`: P[BasicTypeToken[Span.S]] =
     P.oneOf(
-      P.defer(`topbottomdef`) :: P.defer(`arraytypedef`) :: P.defer(
+      P.defer(`topbottomdef`) ::
-        OptionTypeToken.`optiontypedef`
+        P.defer(ArrayTypeToken.`arraytypedef`) ::
-      ) :: BasicTypeToken.`basictypedef` :: NamedTypeToken.dotted :: Nil
+        P.defer(StreamTypeToken.`streamtypedef`) ::
-    )
+        P.defer(OptionTypeToken.`optiontypedef`) ::
-
+        ScalarTypeToken.`scalartypedef` ::
-  def `datatypedef`: P[DataTypeToken[Span.S]] =
+        NamedTypeToken.dotted :: Nil
-    P.oneOf(
-      P.defer(`topbottomdef`) :: P.defer(`arraytypedef`) :: P.defer(
-        StreamTypeToken.`streamtypedef`
-      ) :: P.defer(
-        OptionTypeToken.`optiontypedef`
-      ) :: BasicTypeToken.`basictypedef` :: NamedTypeToken.dotted :: Nil
     )
 
 }
@@ -184,9 +181,8 @@ object TypeToken {
 
   val `typedef`: P[TypeToken[Span.S]] =
     P.oneOf(
-      ArrowTypeToken
+      ArrowTypeToken.`arrowdef`(BasicTypeToken.`compositetypedef`).backtrack ::
-        .`arrowdef`(DataTypeToken.`datatypedef`)
+        BasicTypeToken.`compositetypedef` :: Nil
-        .backtrack :: DataTypeToken.`datatypedef` :: Nil
     )
 
 }

parser/src/test/scala/aqua/AquaSpec.scala

@@ -3,25 +3,25 @@ package aqua
 import aqua.AquaSpec.spanToId
 import aqua.parser.expr.*
 import aqua.parser.expr.func.*
-import aqua.parser.lexer.InfixToken.Op as InfixOp
-import aqua.parser.lexer.PrefixToken.Op as PrefixOp
-import aqua.parser.lexer.InfixToken.Op.*
-import aqua.parser.lexer.PrefixToken.Op.*
 import aqua.parser.head.FromExpr.NameOrAbAs
 import aqua.parser.head.{FromExpr, UseFromExpr}
 import aqua.parser.lexer.*
+import aqua.parser.lexer.InfixToken.Op.*
+import aqua.parser.lexer.InfixToken.Op as InfixOp
+import aqua.parser.lexer.PrefixToken.Op.*
+import aqua.parser.lexer.PrefixToken.Op as PrefixOp
 import aqua.parser.lexer.Token.LiftToken
 import aqua.parser.lift.LiftParser.Implicits.idLiftParser
-import aqua.types.LiteralType.{bool, number, signed, string, unsigned}
-import aqua.types.{LiteralType, ScalarType}
-import cats.{~>, Id}
-import org.scalatest.EitherValues
 import aqua.parser.lift.Span
 import aqua.parser.lift.Span.{P0ToSpan, PToSpan}
-import cats.~>
+import aqua.types.LiteralType.{bool, number, signed, string, unsigned}
-import cats.syntax.bifunctor.*
+import aqua.types.{LiteralType, ScalarType}
-import cats.data.NonEmptyList
 
+import cats.data.NonEmptyList
+import cats.syntax.bifunctor.*
+import cats.{Id, ~>}
+import cats.~>
+import org.scalatest.EitherValues
 import scala.collection.mutable
 import scala.language.implicitConversions
 
@@ -71,14 +71,14 @@ object AquaSpec {
   def toArrayType(str: String): ArrayTypeToken[Id] = ArrayTypeToken[Id]((), str)
 
   def toArrowType(
-    args: List[DataTypeToken[Id]],
+    args: List[BasicTypeToken[Id]],
-    res: Option[DataTypeToken[Id]]
+    res: Option[BasicTypeToken[Id]]
   ): ArrowTypeToken[Id] =
     ArrowTypeToken[Id]((), args.map(None -> _), res.toList)
 
   def toNamedArrow(
     args: List[(String, TypeToken[Id])],
-    res: List[DataTypeToken[Id]]
+    res: List[BasicTypeToken[Id]]
   ): ArrowTypeToken[Id] =
     ArrowTypeToken[Id]((), args.map(ab => Some(Name[Id](ab._1)) -> ab._2), res)
 
@@ -90,15 +90,15 @@ object AquaSpec {
   def toArgSc(str: String, scalarType: ScalarType): Arg[Id] =
     Arg[Id](str, scToBt(scalarType))
 
-  def scToBt(sc: ScalarType): BasicTypeToken[Id] = BasicTypeToken[Id](sc)
+  def scToBt(sc: ScalarType): ScalarTypeToken[Id] = ScalarTypeToken[Id](sc)
 
-  val boolSc: BasicTypeToken[Id] = BasicTypeToken[Id](ScalarType.bool)
+  val boolSc: ScalarTypeToken[Id] = ScalarTypeToken[Id](ScalarType.bool)
-  val stringSc: BasicTypeToken[Id] = BasicTypeToken[Id](ScalarType.string)
+  val stringSc: ScalarTypeToken[Id] = ScalarTypeToken[Id](ScalarType.string)
 
   given Conversion[String, Name[Id]] = toName
   given Conversion[String, NamedTypeToken[Id]] = toNamedType
   given Conversion[Int, LiteralToken[Id]] = toNumber
-  given Conversion[ScalarType, BasicTypeToken[Id]] = scToBt
+  given Conversion[ScalarType, ScalarTypeToken[Id]] = scToBt
 }
 
 trait AquaSpec extends EitherValues {

parser/src/test/scala/aqua/parser/FuncExprSpec.scala

@@ -7,21 +7,20 @@ import aqua.parser.lexer.*
 import aqua.parser.lift.Span
 import aqua.types.ScalarType.*
 
-import cats.{Eval, Id}
 import cats.data.Chain.*
 import cats.data.Validated.{Invalid, Valid}
 import cats.data.{Chain, NonEmptyList}
 import cats.free.Cofree
 import cats.syntax.foldable.*
+import cats.{Eval, Id}
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers
 import org.scalatest.{Inside, Inspectors}
-
 import scala.collection.mutable
 import scala.language.implicitConversions
 
 class FuncExprSpec extends AnyFlatSpec with Matchers with Inside with Inspectors with AquaSpec {
-  import AquaSpec.{*, given}
+  import AquaSpec.{given, *}
 
   private val parser = Parser.spanParser
 
@@ -31,7 +30,7 @@ class FuncExprSpec extends AnyFlatSpec with Matchers with Inside with Inspectors
     )
 
     val arrowToken =
-      ArrowTypeToken[Id]((), List(None -> BasicTypeToken[Id](u8)), List(BasicTypeToken[Id](bool)))
+      ArrowTypeToken[Id]((), List(None -> ScalarTypeToken[Id](u8)), List(ScalarTypeToken[Id](bool)))
     arrowExpr("(peer: PeerId, other: u8 -> bool)") should be(
       ArrowExpr[Id](
         toNamedArrow(("peer" -> toNamedType("PeerId")) :: ("other" -> arrowToken) :: Nil, Nil)
@@ -41,8 +40,8 @@ class FuncExprSpec extends AnyFlatSpec with Matchers with Inside with Inspectors
     val arrowToken2 =
       ArrowTypeToken[Id](
         (),
-        List(None -> BasicTypeToken[Id](u32), None -> BasicTypeToken[Id](u64)),
+        List(None -> ScalarTypeToken[Id](u32), None -> ScalarTypeToken[Id](u64)),
-        List(BasicTypeToken[Id](bool))
+        List(ScalarTypeToken[Id](bool))
       )
     arrowExpr("(peer: PeerId, other: u32, u64 -> bool)") should be(
       ArrowExpr[Id](
@@ -50,12 +49,12 @@ class FuncExprSpec extends AnyFlatSpec with Matchers with Inside with Inspectors
       )
     )
 
-    val arrowToken3 = ArrowTypeToken[Id]((), List(None -> BasicTypeToken[Id](u32)), Nil)
+    val arrowToken3 = ArrowTypeToken[Id]((), List(None -> ScalarTypeToken[Id](u32)), Nil)
     arrowExpr("(peer: PeerId, ret: u32 -> ()) -> string, u32") should be(
       ArrowExpr[Id](
         toNamedArrow(
           ("peer" -> toNamedType("PeerId")) :: ("ret" -> arrowToken3) :: Nil,
-          BasicTypeToken[Id](string) :: BasicTypeToken[Id](u32) :: Nil
+          ScalarTypeToken[Id](string) :: ScalarTypeToken[Id](u32) :: Nil
         )
       )
     )

parser/src/test/scala/aqua/parser/lexer/TypeTokenSpec.scala

@@ -2,40 +2,52 @@ package aqua.parser.lexer
 
 import aqua.parser.lift.LiftParser.Implicits.idLiftParser
 import aqua.types.ScalarType
-import aqua.types.ScalarType.u32
+
 import cats.Id
 import cats.parse.Parser
+import cats.syntax.option.*
 import org.scalatest.EitherValues
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers
 
-import scala.language.implicitConversions
-
 class TypeTokenSpec extends AnyFlatSpec with Matchers with EitherValues {
 
   import aqua.AquaSpec._
 
-  implicit def strToBt(st: ScalarType): BasicTypeToken[Id] = BasicTypeToken[Id](st)
+  def stToStt(st: ScalarType): ScalarTypeToken[Id] = ScalarTypeToken(st)
 
-  "Basic type" should "parse" in {
+  "basic type token" should "parse scalar types" in {
-    BasicTypeToken.`basictypedef`.parseAll("u32").value.mapK(spanToId) should be(strToBt(u32))
+    ScalarType.all.foreach(st =>
-    BasicTypeToken.`basictypedef`.parseAll("()").isLeft should be(true)
+      ScalarTypeToken.`scalartypedef`
+        .parseAll(st.name)
+        .value
+        .mapK(spanToId) should be(stToStt(st))
+    )
   }
 
-  "Return type" should "parse" in {
+  it should "not parse empty brackets" in {
+    ScalarTypeToken.`scalartypedef`
+      .parseAll("()")
+      .isLeft should be(true)
+  }
+
+  "arrow type token" should "parse type def" in {
 
     def typedef(str: String) =
       ArrowTypeToken.typeDef().parseAll(str).value.mapK(spanToId)
 
-    def returndef(str: String) =
-      ArrowTypeToken.returnDef().parseAll(str).value.map(_.mapK(spanToId))
-
     typedef("(A -> ())") should be(
       ArrowTypeToken[Id]((), List((None, NamedTypeToken[Id]("A"))), Nil)
     )
+
     typedef("(A -> B)") should be(
       ArrowTypeToken[Id]((), List((None, NamedTypeToken[Id]("A"))), List(NamedTypeToken[Id]("B")))
     )
+  }
+
+  it should "parse return def" in {
+    def returndef(str: String) =
+      ArrowTypeToken.returnDef().parseAll(str).value.map(_.mapK(spanToId))
 
     returndef("(A -> B), (C -> D)") should be(
       List(
@@ -69,11 +81,16 @@ class TypeTokenSpec extends AnyFlatSpec with Matchers with EitherValues {
     )
   }
 
-  "Arrow type" should "parse" in {
+  it should "parse arrow def" in {
     def arrowdef(str: String) =
-      ArrowTypeToken.`arrowdef`(DataTypeToken.`datatypedef`).parseAll(str).value.mapK(spanToId)
+      ArrowTypeToken
+        .`arrowdef`(BasicTypeToken.`compositetypedef`)
+        .parseAll(str)
+        .value
+        .mapK(spanToId)
+
     def arrowWithNames(str: String) = ArrowTypeToken
-      .`arrowWithNames`(DataTypeToken.`datatypedef`)
+      .`arrowWithNames`(BasicTypeToken.`compositetypedef`)
       .parseAll(str)
       .value
       .mapK(spanToId)
@@ -81,6 +98,7 @@ class TypeTokenSpec extends AnyFlatSpec with Matchers with EitherValues {
     arrowdef("-> B") should be(
       ArrowTypeToken[Id]((), Nil, List(NamedTypeToken[Id]("B")))
     )
+
     arrowdef("A -> B") should be(
       ArrowTypeToken[Id](
         (),
@@ -147,9 +165,11 @@ class TypeTokenSpec extends AnyFlatSpec with Matchers with EitherValues {
     arrowWithNames("{SomeAb, SecondAb}(a: A) -> B") should be(
       ArrowTypeToken[Id](
         (),
-        (Some(Name[Id]("SomeAb")) -> NamedTypeToken[Id]("SomeAb")) :: (Some(Name[Id](
+        (Some(Name[Id]("SomeAb")) -> NamedTypeToken[Id]("SomeAb")) :: (Some(
-          "SecondAb"
+          Name[Id](
-        )) -> NamedTypeToken[Id]("SecondAb")) :: (
+            "SecondAb"
+          )
+        ) -> NamedTypeToken[Id]("SecondAb")) :: (
           Some(Name[Id]("a")) -> NamedTypeToken[Id]("A")
         ) :: Nil,
         List(NamedTypeToken[Id]("B"))
@@ -159,25 +179,28 @@ class TypeTokenSpec extends AnyFlatSpec with Matchers with EitherValues {
     arrowdef("u32 -> Boo") should be(
       ArrowTypeToken[Id](
         (),
-        (None -> strToBt(u32)) :: Nil,
+        (None -> stToStt(ScalarType.u32)) :: Nil,
         List(NamedTypeToken[Id]("Boo"))
       )
     )
+
     TypeToken.`typedef`.parseAll("u32 -> ()").value.mapK(spanToId) should be(
-      ArrowTypeToken[Id]((), (None -> strToBt(u32)) :: Nil, Nil)
+      ArrowTypeToken[Id]((), (None -> stToStt(ScalarType.u32)) :: Nil, Nil)
     )
+
     arrowdef("A, u32 -> B") should be(
       ArrowTypeToken[Id](
         (),
-        (None -> NamedTypeToken[Id]("A")) :: (None -> strToBt(u32)) :: Nil,
+        (None -> NamedTypeToken[Id]("A")) :: (None -> stToStt(ScalarType.u32)) :: Nil,
         List(NamedTypeToken[Id]("B"))
       )
     )
+
     arrowdef("[]Absolutely, u32 -> B, C") should be(
       ArrowTypeToken[Id](
         (),
         (Option.empty[Name[Id]] -> ArrayTypeToken[Id]((), NamedTypeToken[Id]("Absolutely"))) ::
-          (Option.empty[Name[Id]] -> strToBt(u32)) :: Nil,
+          (Option.empty[Name[Id]] -> stToStt(ScalarType.u32)) :: Nil,
         NamedTypeToken[Id]("B") ::
           NamedTypeToken[Id]("C") :: Nil
       )
@@ -185,18 +208,46 @@ class TypeTokenSpec extends AnyFlatSpec with Matchers with EitherValues {
 
   }
 
-  "Array type" should "parse" in {
+  "data type token" should "parse nested types" in {
-    def typedef(str: String) = TypeToken.`typedef`.parseAll(str).value.mapK(spanToId)
+    def typedef(str: String): BasicTypeToken[Id] =
+      BasicTypeToken.`compositetypedef`.parseAll(str).value.mapK(spanToId)
 
-    typedef("[]Something") should be(
+    val baseTypes: List[(String, BasicTypeToken[Id])] = List(
-      ArrayTypeToken[Id]((), NamedTypeToken[Id]("Something"))
+      "u32" -> stToStt(ScalarType.u32),
+      "string" -> stToStt(ScalarType.string),
+      "Named" -> NamedTypeToken[Id]("Named")
     )
-    typedef("[]u32") should be(
+
-      ArrayTypeToken[Id]((), strToBt(u32))
+    val modifiers: List[(String, BasicTypeToken[Id] => BasicTypeToken[Id])] = List(
-    )
+      "[]" -> ((t: BasicTypeToken[Id]) => ArrayTypeToken[Id]((), t)),
-    typedef("[][]u32") should be(
+      "?" -> ((t: BasicTypeToken[Id]) => OptionTypeToken[Id]((), t)),
-      ArrayTypeToken[Id]((), ArrayTypeToken[Id]((), strToBt(u32)))
+      "*" -> ((t: BasicTypeToken[Id]) => StreamTypeToken[Id]((), t))
     )
+
+    LazyList
+      // Generate all cartesian products of modifiers
+      .unfold(modifiers)(prod =>
+        (
+          prod,
+          for {
+            m <- modifiers
+            (sm, mt) = m
+            p <- prod
+            (sp, pt) = p
+          } yield (sm + sp, mt.compose(pt))
+        ).some
+      )
+      .take(6)
+      .foreach { mods =>
+        for {
+          base <- baseTypes
+          (bs, bt) = base
+          mod <- mods
+          (ms, mt) = mod
+          // Apply modifiers to base type
+          (st, t) = (ms + bs, mt(bt))
+        } typedef(st) should be(t)
+      }
   }
 
 }

semantics/src/main/scala/aqua/semantics/expr/AliasSem.scala

@@ -4,14 +4,15 @@ import aqua.parser.expr.AliasExpr
 import aqua.raw.{Raw, TypeRaw}
 import aqua.semantics.Prog
 import aqua.semantics.rules.types.TypesAlgebra
-import cats.syntax.functor.*
+
-import cats.Monad
 import cats.Applicative
+import cats.Monad
 import cats.syntax.flatMap.*
+import cats.syntax.functor.*
 
 class AliasSem[S[_]](val expr: AliasExpr[S]) extends AnyVal {
 
-  def program[Alg[_]: Monad](implicit T: TypesAlgebra[S, Alg]): Prog[Alg, Raw] =
+  def program[Alg[_]: Monad](using T: TypesAlgebra[S, Alg]): Prog[Alg, Raw] =
     T.resolveType(expr.target).flatMap {
       case Some(t) => T.defineAlias(expr.name, t) as (TypeRaw(expr.name.value, t): Raw)
       case None => Applicative[Alg].pure(Raw.error("Alias type unresolved"))

semantics/src/main/scala/aqua/semantics/expr/func/ArrowSem.scala

@@ -58,13 +58,13 @@ class ArrowSem[S[_]](val expr: ArrowExpr[S]) extends AnyVal {
         // TODO: wrap with local on...via...
         val retsAndArgs = retValues zip funcArrow.codomain.toList
 
-        val dataArgsNames = funcArrow.domain.labelledData.map { case (name, _) => name }
+        val streamArgNames = funcArrow.domain.labelledStreams.map { case (name, _) => name }
         val streamsThatReturnAsStreams = retsAndArgs.collect {
           case (VarRaw(n, StreamType(_)), StreamType(_)) => n
         }.toSet
 
         // Remove arguments, and values returned as streams
-        val localStreams = streamsInScope -- dataArgsNames -- streamsThatReturnAsStreams
+        val localStreams = streamsInScope -- streamArgNames -- streamsThatReturnAsStreams
 
         // process stream that returns as not streams and all Apply*Raw
         retsAndArgs.traverse {

semantics/src/main/scala/aqua/semantics/expr/func/DeclareStreamSem.scala

@@ -1,48 +1,39 @@
 package aqua.semantics.expr.func
 
-import aqua.raw.ops.DeclareStreamTag
+import aqua.helpers.syntax.optiont.*
 import aqua.parser.expr.func.DeclareStreamExpr
 import aqua.raw.Raw
+import aqua.raw.ops.DeclareStreamTag
 import aqua.raw.value.VarRaw
 import aqua.semantics.Prog
 import aqua.semantics.rules.names.NamesAlgebra
 import aqua.semantics.rules.types.TypesAlgebra
-import aqua.types.{ArrayType, OptionType, StreamType}
+import aqua.types.*
+
 import cats.Monad
 import cats.data.Chain
+import cats.data.OptionT
 import cats.syntax.applicative.*
 import cats.syntax.flatMap.*
 import cats.syntax.functor.*
 
 class DeclareStreamSem[S[_]](val expr: DeclareStreamExpr[S]) {
 
-  def program[Alg[_]: Monad](implicit
+  def program[Alg[_]: Monad](using
     N: NamesAlgebra[S, Alg],
     T: TypesAlgebra[S, Alg]
-  ): Prog[Alg, Raw] =
+  ): Prog[Alg, Raw] = Prog.leaf {
-    Prog.leaf(
+    val sem = for {
-      T.resolveType(expr.`type`)
+      streamType <- OptionT(
-        .flatMap {
+        T.resolveStreamType(expr.`type`)
-          case Some(t: StreamType) =>
+      )
-            N.define(expr.name, t).map(b => Option.when(b)(t))
+      _ <- OptionT.withFilterF(
-          case Some(t: OptionType) =>
+        N.define(expr.name, streamType)
-            val streamType = StreamType(t.element)
+      )
-            N.define(expr.name, streamType).map(b => Option.when(b)(streamType))
+      valueModel = VarRaw(expr.name.value, streamType)
-          case Some(at @ ArrayType(t)) =>
+    } yield DeclareStreamTag(valueModel).funcOpLeaf: Raw
-            val streamType = StreamType(t)
+
-            T.ensureTypeMatches(expr.`type`, streamType, at).map(b => Option.when(b)(streamType))
+    sem.getOrElse(Raw.error(s"Name `${expr.name.value}` not defined"))
-          case Some(t) =>
+  }
-            val streamType = StreamType(t)
-            T.ensureTypeMatches(expr.`type`, streamType, t).map(b => Option.when(b)(streamType))
-          case None =>
-            None.pure[Alg]
-        }
-        .map {
-          case Some(streamType) =>
-            val valueModel = VarRaw(expr.name.value, streamType)
-            DeclareStreamTag(valueModel).funcOpLeaf: Raw
-          case None => Raw.error(s"Name `${expr.name.value}` not defined")
-        }
-    )
 
 }

semantics/src/main/scala/aqua/semantics/expr/func/ForSem.scala

@@ -1,27 +1,27 @@
 package aqua.semantics.expr.func
 
-import aqua.raw.Raw
 import aqua.parser.expr.func.ForExpr
+import aqua.parser.expr.func.ForExpr.Mode
 import aqua.parser.lexer.{Name, ValueToken}
-import aqua.raw.value.ValueRaw
+import aqua.raw.Raw
 import aqua.raw.ops.*
 import aqua.raw.ops.ForTag
+import aqua.raw.value.ValueRaw
 import aqua.semantics.Prog
+import aqua.semantics.expr.func.FuncOpSem
 import aqua.semantics.rules.ValuesAlgebra
 import aqua.semantics.rules.abilities.AbilitiesAlgebra
 import aqua.semantics.rules.names.NamesAlgebra
 import aqua.semantics.rules.types.TypesAlgebra
-import aqua.types.{ArrayType, BoxType, StreamType}
+import aqua.types.*
-import aqua.semantics.expr.func.FuncOpSem
 
 import cats.Monad
-import cats.data.Chain
+import cats.data.{Chain, OptionT}
 import cats.syntax.applicative.*
 import cats.syntax.apply.*
 import cats.syntax.flatMap.*
 import cats.syntax.functor.*
 import cats.syntax.option.*
-import aqua.parser.expr.func.ForExpr.Mode
 
 class ForSem[S[_]](val expr: ForExpr[S]) extends AnyVal {
 
@@ -74,19 +74,18 @@ class ForSem[S[_]](val expr: ForExpr[S]) extends AnyVal {
 
 object ForSem {
 
-  def beforeFor[S[_], F[_]: Monad](item: Name[S], iterable: ValueToken[S])(implicit
+  def beforeFor[S[_], F[_]: Monad](
+    item: Name[S],
+    iterable: ValueToken[S]
+  )(using
     V: ValuesAlgebra[S, F],
     N: NamesAlgebra[S, F],
     T: TypesAlgebra[S, F]
-  ): F[Option[ValueRaw]] =
+  ): F[Option[ValueRaw]] = (for {
-    V.valueToRaw(iterable).flatMap {
+    value <- V.valueToIterable(iterable)
-      case Some(vm) =>
+    (raw, typ) = value
-        vm.`type` match {
+    _ <- OptionT.liftF(
-          case t: BoxType =>
+      N.define(item, typ.element)
-            N.define(item, t.element).as(vm.some)
+    )
-          case dt =>
+  } yield raw).value
-            T.ensureTypeMatches(iterable, ArrayType(dt), dt).as(none)
-        }
-      case _ => none.pure
-    }
 }

semantics/src/main/scala/aqua/semantics/expr/func/FuncOpSem.scala

@@ -1,17 +1,17 @@
 package aqua.semantics.expr.func
 
+import aqua.raw.Raw
+import aqua.raw.ops.{RawTag, RestrictionTag}
+import aqua.semantics.rules.names.NamesAlgebra
+
 import cats.Monad
 import cats.syntax.functor.*
 
-import aqua.semantics.rules.names.NamesAlgebra
-import aqua.raw.Raw
-import aqua.raw.ops.{RawTag, RestrictionTag}
-
 object FuncOpSem {
 
-  def restrictStreamsInScope[S[_], Alg[_]: Monad](tree: RawTag.Tree)(using
+  def restrictStreamsInScope[S[_], Alg[_]: Monad](
-    N: NamesAlgebra[S, Alg]
+    tree: RawTag.Tree
-  ): Alg[RawTag.Tree] = N
+  )(using N: NamesAlgebra[S, Alg]): Alg[RawTag.Tree] = N
     .streamsDefinedWithinScope()
     .map(streams =>
       streams.toList

semantics/src/main/scala/aqua/semantics/expr/func/OnSem.scala

@@ -1,23 +1,23 @@
 package aqua.semantics.expr.func
 
-import aqua.raw.ops.{FuncOp, OnTag}
 import aqua.parser.expr.func.OnExpr
 import aqua.parser.lexer.ValueToken
 import aqua.raw.Raw
+import aqua.raw.ops.{FuncOp, OnTag}
 import aqua.raw.value.ValueRaw
 import aqua.semantics.Prog
 import aqua.semantics.rules.ValuesAlgebra
 import aqua.semantics.rules.abilities.AbilitiesAlgebra
 import aqua.semantics.rules.types.TypesAlgebra
-import aqua.types.{BoxType, OptionType, ScalarType}
+import aqua.types.{CollectionType, OptionType, ScalarType}
 
 import cats.data.Chain
 import cats.data.OptionT
 import cats.syntax.applicative.*
 import cats.syntax.apply.*
 import cats.syntax.flatMap.*
-import cats.syntax.traverse.*
 import cats.syntax.functor.*
+import cats.syntax.traverse.*
 import cats.{Monad, Traverse}
 
 class OnSem[S[_]](val expr: OnExpr[S]) extends AnyVal {
@@ -64,7 +64,7 @@ object OnSem {
       .traverse(v =>
         OptionT(V.valueToRaw(v)).filterF { vm =>
           val expectedType = vm.`type` match {
-            case _: BoxType => OptionType(ScalarType.string)
+            case _: CollectionType => OptionType(ScalarType.string)
             case _ => ScalarType.string
           }
 

semantics/src/main/scala/aqua/semantics/expr/func/ParSeqSem.scala

@@ -1,24 +1,24 @@
 package aqua.semantics.expr.func
 
-import aqua.raw.Raw
 import aqua.parser.expr.func.ParSeqExpr
-import aqua.raw.value.ValueRaw
+import aqua.raw.Raw
 import aqua.raw.ops.*
 import aqua.raw.ops.ForTag
+import aqua.raw.value.ValueRaw
 import aqua.semantics.Prog
 import aqua.semantics.rules.ValuesAlgebra
 import aqua.semantics.rules.abilities.AbilitiesAlgebra
 import aqua.semantics.rules.names.NamesAlgebra
 import aqua.semantics.rules.types.TypesAlgebra
-import aqua.types.{ArrayType, BoxType, StreamType}
+import aqua.types.{ArrayType, CollectionType, StreamType}
 
 import cats.Monad
 import cats.data.Chain
-import cats.syntax.option.*
 import cats.syntax.applicative.*
 import cats.syntax.apply.*
 import cats.syntax.flatMap.*
 import cats.syntax.functor.*
+import cats.syntax.option.*
 
 class ParSeqSem[S[_]](val expr: ParSeqExpr[S]) extends AnyVal {
 

semantics/src/main/scala/aqua/semantics/expr/func/PushToStreamSem.scala

@@ -1,15 +1,19 @@
 package aqua.semantics.expr.func
 
-import aqua.raw.ops.{Call, PushToStreamTag}
+import aqua.helpers.syntax.optiont.*
 import aqua.parser.expr.func.PushToStreamExpr
 import aqua.parser.lexer.Token
 import aqua.raw.Raw
+import aqua.raw.ops.{Call, PushToStreamTag}
 import aqua.semantics.Prog
 import aqua.semantics.rules.ValuesAlgebra
 import aqua.semantics.rules.names.NamesAlgebra
 import aqua.semantics.rules.types.TypesAlgebra
-import aqua.types.{ArrayType, StreamType, Type}
+import aqua.types.*
+import aqua.types.TopType
+
 import cats.Monad
+import cats.data.OptionT
 import cats.syntax.applicative.*
 import cats.syntax.apply.*
 import cats.syntax.flatMap.*
@@ -22,24 +26,14 @@ class PushToStreamSem[S[_]](val expr: PushToStreamExpr[S]) extends AnyVal {
     elementToken: Token[S],
     stream: Type,
     element: Type
-  )(implicit
+  )(using T: TypesAlgebra[S, Alg]): Alg[Boolean] = (
-    T: TypesAlgebra[S, Alg]
+    T.typeToStream(streamToken, stream),
-  ): Alg[Boolean] =
+    T.typeToCollectible(elementToken, element)
-    stream match {
+  ).merged.semiflatMap { case (st, et) =>
-      case StreamType(st) =>
+    T.ensureTypeMatches(elementToken, st.element, et)
-        T.ensureTypeMatches(elementToken, st, element)
+  }.getOrElse(false)
-      case _ =>
-        T.ensureTypeMatches(
-          streamToken,
-          StreamType(element match {
-            case StreamType(e) => ArrayType(e)
-            case _ => element
-          }),
-          stream
-        )
-    }
 
-  def program[Alg[_]: Monad](implicit
+  def program[Alg[_]: Monad](using
     N: NamesAlgebra[S, Alg],
     T: TypesAlgebra[S, Alg],
     V: ValuesAlgebra[S, Alg]

semantics/src/main/scala/aqua/semantics/rules/ValuesAlgebra.scala

@@ -1,6 +1,9 @@
 package aqua.semantics.rules
 
+import aqua.errors.Errors.internalError
+import aqua.helpers.syntax.optiont.*
 import aqua.parser.lexer.*
+import aqua.parser.lexer.InfixToken.value
 import aqua.parser.lexer.InfixToken.{BoolOp, CmpOp, EqOp, MathOp, Op as InfOp}
 import aqua.parser.lexer.PrefixToken.Op as PrefOp
 import aqua.raw.value.*
@@ -9,7 +12,6 @@ import aqua.semantics.rules.names.NamesAlgebra
 import aqua.semantics.rules.report.ReportAlgebra
 import aqua.semantics.rules.types.TypesAlgebra
 import aqua.types.*
-import aqua.helpers.syntax.optiont.*
 
 import cats.Monad
 import cats.data.{NonEmptyList, OptionT}
@@ -151,26 +153,22 @@ class ValuesAlgebra[S[_], Alg[_]: Monad](using
             raws
               .zip(values)
               .traverse { case (raw, token) =>
-                T.ensureTypeIsCollectible(token, raw.`type`)
+                T.typeToCollectible(token, raw.`type`).map(raw -> _)
-                  .map(Option.when(_)(raw))
               }
-              .map(_.sequence)
+              .value
           )
           raw = valuesRawChecked.map(raws =>
             NonEmptyList
               .fromList(raws)
               .fold(ValueRaw.Nil) { nonEmpty =>
-                val element = raws.map(_.`type`).reduceLeft(_ `∩` _)
+                val (values, types) = nonEmpty.unzip
-                // In case we mix values of uncomparable types, intersection returns bottom, meaning "uninhabited type".
+                val element = CollectionType.elementTypeOf(types.toList)
-                // But we want to get to TopType instead: this would mean that intersection is empty, and you cannot
-                // make any decision about the structure of type, but can push anything inside
-                val elementNotBottom = if (element == BottomType) TopType else element
                 CollectionRaw(
-                  nonEmpty,
+                  values,
                   ct.mode match {
-                    case CollectionToken.Mode.StreamMode => StreamType(elementNotBottom)
+                    case CollectionToken.Mode.StreamMode => StreamType(element)
-                    case CollectionToken.Mode.ArrayMode => ArrayType(elementNotBottom)
+                    case CollectionToken.Mode.ArrayMode => ArrayType(element)
-                    case CollectionToken.Mode.OptionMode => OptionType(elementNotBottom)
+                    case CollectionToken.Mode.OptionMode => OptionType(element)
                   }
                 )
               }
@@ -323,14 +321,19 @@ class ValuesAlgebra[S[_], Alg[_]: Monad](using
       }
     )
 
+  def valueToIterable(v: ValueToken[S]): OptionT[Alg, (ValueRaw, CollectionType)] =
+    for {
+      raw <- OptionT(valueToRaw(v))
+      typ <- T.typeToIterable(v, raw.`type`)
+    } yield raw -> typ
+
   def valueToTypedRaw(v: ValueToken[S], expectedType: Type): Alg[Option[ValueRaw]] =
-    OptionT(valueToRaw(v))
+    (for {
-      .flatMap(raw =>
+      raw <- OptionT(valueToRaw(v))
-        OptionT.whenM(
+      _ <- OptionT.withFilterF(
-          T.ensureTypeMatches(v, expectedType, raw.`type`)
+        T.ensureTypeMatches(v, expectedType, raw.`type`)
-        )(raw.pure)
       )
-      .value
+    } yield raw).value
 
   def valueToStringRaw(v: ValueToken[S]): Alg[Option[ValueRaw]] =
     valueToTypedRaw(v, LiteralType.string)

semantics/src/main/scala/aqua/semantics/rules/types/TypeResolution.scala

@@ -0,0 +1,112 @@
+package aqua.semantics.rules.types
+
+import aqua.parser.lexer.*
+import aqua.types.*
+
+import cats.data.ValidatedNec
+import cats.syntax.apply.*
+import cats.syntax.bifunctor.*
+import cats.syntax.either.*
+import cats.syntax.functor.*
+import cats.syntax.option.*
+import cats.syntax.traverse.*
+import cats.syntax.validated.*
+
+final case class TypeResolution[S[_], +T](
+  `type`: T,
+  definitions: List[(Token[S], NamedTypeToken[S])]
+)
+
+object TypeResolution {
+
+  final case class TypeResolutionError[S[_]](
+    token: Token[S],
+    hint: String
+  )
+
+  type Res[S[_], A] = ValidatedNec[
+    TypeResolutionError[S],
+    TypeResolution[S, A]
+  ]
+
+  private def resolveCollection[S[_]](
+    tt: TypeToken[S],
+    collectionName: String,
+    collectionType: DataType => Type
+  )(state: TypesState[S]): Res[S, Type] =
+    resolveTypeToken(tt)(state).andThen {
+      case TypeResolution(it: DataType, t) =>
+        TypeResolution(collectionType(it), t).validNec
+      case TypeResolution(it, _) =>
+        TypeResolutionError(
+          tt,
+          s"$collectionName could not contain values of type $it"
+        ).invalidNec
+    }
+
+  def resolveTypeToken[S[_]](
+    tt: TypeToken[S]
+  )(state: TypesState[S]): Res[S, Type] =
+    tt match {
+      case TopBottomToken(_, isTop) =>
+        val `type` = if (isTop) TopType else BottomType
+
+        TypeResolution(`type`, Nil).validNec
+      case ArrayTypeToken(_, dtt) =>
+        resolveCollection(dtt, "Array", ArrayType.apply)(state)
+      case StreamTypeToken(_, dtt) =>
+        resolveCollection(dtt, "Stream", StreamType.apply)(state)
+      case OptionTypeToken(_, dtt) =>
+        resolveCollection(dtt, "Option", OptionType.apply)(state)
+      case ntt: NamedTypeToken[S] =>
+        val defs = state
+          .getTypeDefinition(ntt.value)
+          .toList
+          .map(ntt -> _)
+
+        state
+          .getType(ntt.value)
+          .map(typ => TypeResolution(typ, defs))
+          .toValidNec(
+            TypeResolutionError(
+              ntt,
+              s"Type ${ntt.value} is not defined"
+            )
+          )
+      case stt: ScalarTypeToken[S] =>
+        TypeResolution(stt.value, Nil).validNec
+      case att: ArrowTypeToken[S] =>
+        resolveArrowDef(att)(state)
+    }
+
+  def resolveArrowDef[S[_]](
+    arrowTypeToken: ArrowTypeToken[S]
+  )(state: TypesState[S]): Res[S, ArrowType] = {
+    val res = arrowTypeToken.res
+      .traverse(typeToken => resolveTypeToken(typeToken)(state).toEither)
+    val args = arrowTypeToken.args.traverse { case (argName, typeToken) =>
+      resolveTypeToken(typeToken)(state)
+        .map(argName.map(_.value) -> _)
+        .toEither
+    }
+
+    (args, res).mapN { (args, res) =>
+      val (argsLabeledTypes, argsTokens) =
+        args.map { case lbl -> TypeResolution(typ, tkn) =>
+          (lbl, typ) -> tkn
+        }.unzip.map(_.flatten)
+      val (resTypes, resTokens) =
+        res.map { case TypeResolution(typ, tkn) =>
+          typ -> tkn
+        }.unzip.map(_.flatten)
+
+      val typ = ArrowType(
+        ProductType.maybeLabelled(argsLabeledTypes),
+        ProductType(resTypes)
+      )
+      val defs = (argsTokens ++ resTokens)
+
+      TypeResolution(typ, defs)
+    }.toValidated
+  }
+}

semantics/src/main/scala/aqua/semantics/rules/types/TypesAlgebra.scala

@@ -3,14 +3,18 @@ package aqua.semantics.rules.types
 import aqua.parser.lexer.*
 import aqua.raw.value.{PropertyRaw, ValueRaw}
 import aqua.types.*
+import aqua.types.Type.*
 
-import cats.data.NonEmptyMap
 import cats.data.NonEmptyList
+import cats.data.NonEmptyMap
+import cats.data.OptionT
 
 trait TypesAlgebra[S[_], Alg[_]] {
 
   def resolveType(token: TypeToken[S]): Alg[Option[Type]]
-  
+
+  def resolveStreamType(token: TypeToken[S]): Alg[Option[StreamType]]
+
   def resolveNamedType(token: TypeToken[S]): Alg[Option[AbilityType | StructType]]
 
   def getType(name: String): Alg[Option[Type]]
@@ -56,7 +60,11 @@ trait TypesAlgebra[S[_], Alg[_]] {
 
   def ensureTypeMatches(token: Token[S], expected: Type, givenType: Type): Alg[Boolean]
 
-  def ensureTypeIsCollectible(token: Token[S], givenType: Type): Alg[Boolean]
+  def typeToCollectible(token: Token[S], givenType: Type): OptionT[Alg, CollectibleType]
+
+  def typeToStream(token: Token[S], givenType: Type): OptionT[Alg, StreamType]
+
+  def typeToIterable(token: Token[S], givenType: Type): OptionT[Alg, CollectionType]
 
   def ensureTypeOneOf[T <: Type](
     token: Token[S],

semantics/src/main/scala/aqua/semantics/rules/types/TypesInterpreter.scala

@@ -1,35 +1,28 @@
 package aqua.semantics.rules.types
 
 import aqua.parser.lexer.*
-import aqua.raw.value.{
+import aqua.raw.value.*
-  FunctorRaw,
-  IntoArrowRaw,
-  IntoCopyRaw,
-  IntoFieldRaw,
-  IntoIndexRaw,
-  PropertyRaw,
-  ValueRaw
-}
-import aqua.semantics.rules.locations.LocationsAlgebra
 import aqua.semantics.rules.StackInterpreter
+import aqua.semantics.rules.locations.LocationsAlgebra
 import aqua.semantics.rules.report.ReportAlgebra
-import aqua.semantics.rules.types.TypesStateHelper.{TypeResolution, TypeResolutionError}
+import aqua.semantics.rules.types.TypeResolution.TypeResolutionError
 import aqua.types.*
+import aqua.types.Type.*
 
 import cats.data.Validated.{Invalid, Valid}
 import cats.data.{Chain, NonEmptyList, NonEmptyMap, OptionT, State}
 import cats.syntax.applicative.*
 import cats.syntax.apply.*
 import cats.syntax.flatMap.*
-import cats.syntax.functor.*
-import cats.syntax.traverse.*
 import cats.syntax.foldable.*
-import cats.{~>, Applicative}
+import cats.syntax.functor.*
 import cats.syntax.option.*
+import cats.syntax.traverse.*
+import cats.{Applicative, ~>}
 import monocle.Lens
 import monocle.macros.GenLens
-
 import scala.collection.immutable.SortedMap
+import scala.reflect.TypeTest
 
 class TypesInterpreter[S[_], X](using
   lens: Lens[X, TypesState[S]],
@@ -49,15 +42,22 @@ class TypesInterpreter[S[_], X](using
     getState.map(st => st.strict.get(name))
 
   override def resolveType(token: TypeToken[S]): State[X, Option[Type]] =
-    getState.map(TypesStateHelper.resolveTypeToken(token)).flatMap {
+    getState.map(TypeResolution.resolveTypeToken(token)).flatMap {
-      case Some(TypeResolution(typ, tokens)) =>
+      case Valid(TypeResolution(typ, tokens)) =>
         val tokensLocs = tokens.map { case (t, n) => n.value -> t }
         locations.pointLocations(tokensLocs).as(typ.some)
-      case None =>
+      case Invalid(errors) =>
-        // TODO: Give more specific error message
+        errors.traverse_ { case TypeResolutionError(token, hint) =>
-        report.error(token, s"Unresolved type").as(None)
+          report.error(token, hint)
+        }.as(none)
     }
 
+  override def resolveStreamType(token: TypeToken[S]): State[X, Option[StreamType]] =
+    OptionT(resolveType(token)).flatMapF {
+      case st: StreamType => st.some.pure[ST]
+      case t => report.error(token, s"Expected stream type, got $t").as(none)
+    }.value
+
   def resolveNamedType(token: TypeToken[S]): State[X, Option[AbilityType | StructType]] =
     resolveType(token).flatMap(_.flatTraverse {
       case t: (AbilityType | StructType) => Option(t).pure
@@ -65,7 +65,7 @@ class TypesInterpreter[S[_], X](using
     })
 
   override def resolveArrowDef(arrowDef: ArrowTypeToken[S]): State[X, Option[ArrowType]] =
-    getState.map(TypesStateHelper.resolveArrowDef(arrowDef)).flatMap {
+    getState.map(TypeResolution.resolveArrowDef(arrowDef)).flatMap {
       case Valid(TypeResolution(tt, tokens)) =>
         val tokensLocs = tokens.map { case (t, n) => n.value -> t }
         locations.pointLocations(tokensLocs).as(tt.some)
@@ -142,11 +142,7 @@ class TypesInterpreter[S[_], X](using
     ensureNameNotDefined(name.value, name, ifDefined = none)(
       fields.toList.traverse {
         case (field, (fieldName, t: DataType)) =>
-          t match {
+          (field -> t).some.pure[ST]
-            case _: StreamType =>
-              report.error(fieldName, s"Field '$field' has stream type").as(none)
-            case _ => (field -> t).some.pure[ST]
-          }
         case (field, (fieldName, t)) =>
           report
             .error(
@@ -294,7 +290,7 @@ class TypesInterpreter[S[_], X](using
           op.idx.fold(
             State.pure(Some(IntoIndexRaw(idx, ot.element)))
           )(v => report.error(v, s"Options might have only one element, use ! to get it").as(None))
-        case rt: BoxType =>
+        case rt: CollectionType =>
           State.pure(Some(IntoIndexRaw(idx, rt.element)))
         case _ =>
           report.error(op, s"Expected $rootT to be a collection type").as(None)
@@ -318,7 +314,7 @@ class TypesInterpreter[S[_], X](using
           true
         case (LiteralType.signed, rst: ScalarType) if ScalarType.number(rst) =>
           true
-        case (lbt: BoxType, rbt: BoxType) =>
+        case (lbt: CollectionType, rbt: CollectionType) =>
           isComparable(lbt.element, rbt.element)
         // Prohibit comparing abilities
         case (_: AbilityType, _: AbilityType) =>
@@ -329,7 +325,7 @@ class TypesInterpreter[S[_], X](using
         case (LiteralType(xs, _), LiteralType(ys, _)) =>
           xs.intersect(ys).nonEmpty
         case _ =>
-          lt.uniteTop(rt) != TopType
+          lt `∪` rt != TopType
       }
 
     if (isComparable(left, right)) State.pure(true)
@@ -379,35 +375,71 @@ class TypesInterpreter[S[_], X](using
             }
           }
         case _ =>
-          val notes =
+          val notes = (expected, givenType) match {
-            if (expected.acceptsValueOf(OptionType(givenType)))
+            case (_, dt: DataType) if expected.acceptsValueOf(OptionType(dt)) =>
               "note: Try converting value to optional" :: Nil
-            else if (givenType.acceptsValueOf(OptionType(expected)))
+            case (dt: DataType, _) if givenType.acceptsValueOf(OptionType(dt)) =>
               "note: You're providing an optional value where normal value is expected." ::
                 "You can extract value with `!`, but be aware it may trigger join behaviour." ::
                 Nil
-            else Nil
+            case _ => Nil
+          }
+
           report
             .error(
               token,
-              "Types mismatch." :: s"expected:   $expected" :: s"given:      $givenType" :: Nil ++ notes
+              "Types mismatch." +:
+                s"expected:   $expected" +:
+                s"given:      $givenType" +:
+                notes
             )
             .as(false)
       }
     }
 
-  override def ensureTypeIsCollectible(token: Token[S], givenType: Type): State[X, Boolean] =
+  private def typeTo[T <: Type](
+    token: Token[S],
+    givenType: Type,
+    error: String
+  )(using tt: TypeTest[Type, T]): OptionT[State[X, *], T] =
     givenType match {
-      case _: DataType => true.pure
+      case t: T => OptionT.pure(t)
       case _ =>
-        report
+        OptionT.liftF(
-          .error(
+          report.error(token, error)
-            token,
+        ) *> OptionT.none
-            s"Value of type '$givenType' could not be put into a collection"
-          )
-          .as(false)
     }
 
+  override def typeToCollectible(
+    token: Token[S],
+    givenType: Type
+  ): OptionT[State[X, *], CollectibleType] =
+    typeTo[CollectibleType](
+      token,
+      givenType,
+      s"Value of type '$givenType' could not be put into a collection"
+    )
+
+  override def typeToStream(
+    token: Token[S],
+    givenType: Type
+  ): OptionT[State[X, *], StreamType] =
+    typeTo[StreamType](
+      token,
+      givenType,
+      s"Expected stream value, got value of type '$givenType'"
+    )
+
+  override def typeToIterable(
+    token: Token[S],
+    givenType: Type
+  ): OptionT[State[X, *], CollectionType] =
+    typeTo[CollectionType](
+      token,
+      givenType,
+      s"Value of type '$givenType' could not be iterated over"
+    )
+
   override def ensureTypeOneOf[T <: Type](
     token: Token[S],
     expected: Set[T],

semantics/src/main/scala/aqua/semantics/rules/types/TypesState.scala

@@ -1,20 +1,19 @@
 package aqua.semantics.rules.types
 
-import aqua.raw.value.{FunctorRaw, IntoIndexRaw, LiteralRaw, PropertyRaw, ValueRaw}
 import aqua.parser.lexer.*
-import aqua.types.*
 import aqua.raw.RawContext
+import aqua.raw.value.{FunctorRaw, IntoIndexRaw, LiteralRaw, PropertyRaw, ValueRaw}
+import aqua.types.*
 
 import cats.data.Validated.{Invalid, Valid}
 import cats.data.{Chain, NonEmptyChain, ValidatedNec}
 import cats.kernel.Monoid
-import cats.syntax.option.*
-import cats.syntax.traverse.*
-import cats.syntax.validated.*
 import cats.syntax.apply.*
 import cats.syntax.bifunctor.*
 import cats.syntax.functor.*
-import cats.syntax.apply.*
+import cats.syntax.option.*
+import cats.syntax.traverse.*
+import cats.syntax.validated.*
 
 case class TypesState[S[_]](
   fields: Map[String, (Name[S], Type)] = Map(),
@@ -37,97 +36,6 @@ case class TypesState[S[_]](
     definitions.get(name)
 }
 
-object TypesStateHelper {
-
-  final case class TypeResolution[S[_], +T](
-    `type`: T,
-    definitions: List[(Token[S], NamedTypeToken[S])]
-  )
-
-  final case class TypeResolutionError[S[_]](
-    token: Token[S],
-    hint: String
-  )
-
-  def resolveTypeToken[S[_]](tt: TypeToken[S])(
-    state: TypesState[S]
-  ): Option[TypeResolution[S, Type]] =
-    tt match {
-      case TopBottomToken(_, isTop) =>
-        val `type` = if (isTop) TopType else BottomType
-
-        TypeResolution(`type`, Nil).some
-      case ArrayTypeToken(_, dtt) =>
-        resolveTypeToken(dtt)(state).collect { case TypeResolution(it: DataType, t) =>
-          TypeResolution(ArrayType(it), t)
-        }
-      case StreamTypeToken(_, dtt) =>
-        resolveTypeToken(dtt)(state).collect { case TypeResolution(it: DataType, t) =>
-          TypeResolution(StreamType(it), t)
-        }
-      case OptionTypeToken(_, dtt) =>
-        resolveTypeToken(dtt)(state).collect { case TypeResolution(it: DataType, t) =>
-          TypeResolution(OptionType(it), t)
-        }
-      case ntt: NamedTypeToken[S] =>
-        val defs = state
-          .getTypeDefinition(ntt.value)
-          .toList
-          .map(ntt -> _)
-
-        state
-          .getType(ntt.value)
-          .map(typ => TypeResolution(typ, defs))
-      case btt: BasicTypeToken[S] =>
-        TypeResolution(btt.value, Nil).some
-      case att: ArrowTypeToken[S] =>
-        resolveArrowDef(att)(state).toOption
-    }
-
-  def resolveArrowDef[S[_]](arrowTypeToken: ArrowTypeToken[S])(
-    state: TypesState[S]
-  ): ValidatedNec[TypeResolutionError[S], TypeResolution[S, ArrowType]] = {
-    val res = arrowTypeToken.res.traverse(typeToken =>
-      resolveTypeToken(typeToken)(state)
-        .toValidNec(
-          TypeResolutionError(
-            typeToken,
-            "Can not resolve the result type"
-          )
-        )
-    )
-    val args = arrowTypeToken.args.traverse { case (argName, typeToken) =>
-      resolveTypeToken(typeToken)(state)
-        .toValidNec(
-          TypeResolutionError(
-            typeToken,
-            "Can not resolve the argument type"
-          )
-        )
-        .map(argName.map(_.value) -> _)
-    }
-
-    (args, res).mapN { (args, res) =>
-      val (argsLabeledTypes, argsTokens) =
-        args.map { case lbl -> TypeResolution(typ, tkn) =>
-          (lbl, typ) -> tkn
-        }.unzip.map(_.flatten)
-      val (resTypes, resTokens) =
-        res.map { case TypeResolution(typ, tkn) =>
-          typ -> tkn
-        }.unzip.map(_.flatten)
-
-      val typ = ArrowType(
-        ProductType.maybeLabelled(argsLabeledTypes),
-        ProductType(resTypes)
-      )
-      val defs = (argsTokens ++ resTokens)
-
-      TypeResolution(typ, defs)
-    }
-  }
-}
-
 object TypesState {
 
   final case class TypeDefinition[S[_]](

semantics/src/test/scala/aqua/semantics/ArrowSemSpec.scala

@@ -1,7 +1,7 @@
 package aqua.semantics
 
 import aqua.parser.expr.func.ArrowExpr
-import aqua.parser.lexer.{BasicTypeToken, Name}
+import aqua.parser.lexer.{Name, ScalarTypeToken}
 import aqua.raw.Raw
 import aqua.raw.arrow.ArrowRaw
 import aqua.raw.ops.*
@@ -12,12 +12,12 @@ import aqua.types.*
 import aqua.types.ScalarType.*
 
 import cats.Id
-import cats.syntax.applicative.*
 import cats.data.{NonEmptyList, NonEmptyMap, State}
+import cats.syntax.applicative.*
 import org.scalatest.EitherValues
+import org.scalatest.Inside
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers
-import org.scalatest.Inside
 
 class ArrowSemSpec extends AnyFlatSpec with Matchers with EitherValues with Inside {
 
@@ -84,7 +84,7 @@ class ArrowSemSpec extends AnyFlatSpec with Matchers with EitherValues with Insi
     val state = getState(seq)(program("(a: string, b: u32) -> u32"))
 
     state.errors.headOption.get shouldBe RulesViolated[Id](
-      BasicTypeToken[Id](u32),
+      ScalarTypeToken[Id](u32),
       "Types mismatch, expected: u32, given: string" :: Nil
     )
 
@@ -112,7 +112,7 @@ class ArrowSemSpec extends AnyFlatSpec with Matchers with EitherValues with Insi
 
       state.errors shouldBe empty
       inside(raw) { case ArrowRaw(_, Nil, bodyRes) =>
-        bodyRes shouldBe body
+        bodyRes.equalsOrShowDiff(body) shouldBe true
       }
     }
 

semantics/src/test/scala/aqua/semantics/SemanticsSpec.scala

@@ -1,26 +1,27 @@
 package aqua.semantics
 
-import aqua.raw.RawContext
 import aqua.parser.Ast
-import aqua.raw.ops.{Call, CallArrowRawTag, FuncOp, OnTag, ParTag, RawTag, SeqGroupTag, SeqTag}
 import aqua.parser.Parser
 import aqua.parser.lift.{LiftParser, Span}
+import aqua.raw.ConstantRaw
+import aqua.raw.RawContext
+import aqua.raw.ops.*
+import aqua.raw.ops.{Call, CallArrowRawTag, FuncOp, OnTag, ParTag, RawTag, SeqGroupTag, SeqTag}
 import aqua.raw.value.*
 import aqua.types.*
-import aqua.raw.ops.*
 
-import org.scalatest.flatspec.AnyFlatSpec
+import cats.Eval
-import org.scalatest.matchers.should.Matchers
+import cats.data.State
-import org.scalatest.Inside
+import cats.data.Validated
-import cats.~>
 import cats.data.{Chain, EitherNec, NonEmptyChain}
+import cats.free.Cofree
+import cats.syntax.foldable.*
 import cats.syntax.show.*
 import cats.syntax.traverse.*
-import cats.syntax.foldable.*
+import cats.~>
-import cats.data.Validated
+import org.scalatest.Inside
-import cats.free.Cofree
+import org.scalatest.flatspec.AnyFlatSpec
-import cats.data.State
+import org.scalatest.matchers.should.Matchers
-import cats.Eval
 
 class SemanticsSpec extends AnyFlatSpec with Matchers with Inside {
 
@@ -40,7 +41,11 @@ class SemanticsSpec extends AnyFlatSpec with Matchers with Inside {
       Any
     ]
   ): Unit = inside(parser(script)) { case Validated.Valid(ast) =>
-    val init = RawContext.blank
+    val init = RawContext.blank.copy(
+      parts = Chain
+        .fromSeq(ConstantRaw.defaultConstants())
+        .map(const => RawContext.blank -> const)
+    )
     inside(semantics.process(ast, init).value.run)(test)
   }
 
@@ -839,4 +844,52 @@ class SemanticsSpec extends AnyFlatSpec with Matchers with Inside {
       }
     }
   }
+
+  it should "allow pushing `nil` to stream" in {
+    def test(quantifier: String) = {
+      val script = s"""
+                      |func test() -> []${quantifier}string:
+                      |  stream: *${quantifier}string
+                      |  stream <<- nil
+                      |  <- stream
+                      |""".stripMargin
+
+      insideBody(script) { body =>
+        matchSubtree(body) { case (PushToStreamTag(VarRaw(name, _), _), _) =>
+          name shouldEqual "nil"
+        }
+      }
+    }
+
+    test("?")
+    test("[]")
+  }
+
+  it should "allow putting stream into collection" in {
+    def test(t: String, p: String) = {
+      val script = s"""
+                      |service Srv("test-srv"):
+                      |  consume(value: ${t}[]string)
+                      |
+                      |func test():
+                      |  stream: *string
+                      |  Srv.consume(${p}[stream])
+                      |""".stripMargin
+
+      insideBody(script) { body =>
+        println(body.show)
+        matchSubtree(body) { case (CallArrowRawTag(_, ca: CallArrowRaw), _) =>
+          inside(ca.arguments) { case (c: CollectionRaw) :: Nil =>
+            c.values.exists {
+              case VarRaw(name, _) => name == "stream"
+              case _ => false
+            } should be(true)
+          }
+        }
+      }
+    }
+
+    test("[]", "")
+    test("?", "?")
+  }
 }

semantics/src/test/scala/aqua/semantics/TypeResolutionSpec.scala

@@ -0,0 +1,179 @@
+package aqua.semantics
+
+import aqua.parser.lexer.*
+import aqua.semantics.rules.types.TypeResolution.TypeResolutionError
+import aqua.semantics.rules.types.{TypeResolution, TypesState}
+import aqua.types.*
+
+import cats.Endo
+import cats.Id
+import cats.SemigroupK
+import cats.data.NonEmptyMap
+import cats.data.Validated.*
+import cats.kernel.Semigroup
+import cats.syntax.foldable.*
+import cats.syntax.option.*
+import cats.syntax.semigroup.*
+import org.scalatest.Inside
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class TypeResolutionSpec extends AnyFlatSpec with Matchers with Inside {
+
+  given [A, B]: Semigroup[(Endo[A], Endo[B])] with {
+    private val algebraA = SemigroupK[Endo].algebra[A]
+    private val algebraB = SemigroupK[Endo].algebra[B]
+
+    override def combine(x: (Endo[A], Endo[B]), y: (Endo[A], Endo[B])): (Endo[A], Endo[B]) =
+      (algebraA.combine(x._1, y._1), algebraB.combine(x._2, y._2))
+  }
+
+  def stt(st: ScalarType): ScalarTypeToken[Id] = ScalarTypeToken(st)
+
+  def ntt(name: String): NamedTypeToken[Id] = NamedTypeToken(name)
+
+  def resolve(
+    token: TypeToken[Id],
+    types: Map[String, Type]
+  ): TypeResolution.Res[Id, Type] =
+    TypeResolution.resolveTypeToken(token)(TypesState(strict = types))
+
+  val validCollectionModifiers: LazyList[
+    List[(Endo[BasicTypeToken[Id]], DataType => Type)]
+  ] = {
+    val baseModifiers: List[(Endo[BasicTypeToken[Id]], Endo[DataType])] = List(
+      (ArrayTypeToken[Id]((), _)) -> (ArrayType.apply),
+      (OptionTypeToken[Id]((), _)) -> (OptionType.apply)
+    )
+
+    val streamModifier = (dt: BasicTypeToken[Id]) => StreamTypeToken[Id]((), dt)
+
+    val dataModifiers = LazyList.unfold(baseModifiers) { mods =>
+      (
+        mods,
+        for {
+          m <- mods
+          b <- baseModifiers
+        } yield m combine b
+      ).some
+    }
+
+    dataModifiers.map { mods =>
+      mods.map { case (token, typ) =>
+        (token andThen streamModifier) -> (typ andThen StreamType.apply)
+      } ++ mods
+    }.prepended(List((streamModifier, StreamType.apply))).take(6)
+  }
+
+  val structType = StructType("Struct", NonEmptyMap.of("field" -> ScalarType.i8))
+
+  "TypeResolution resolveTypeToken" should "resolve basic types" in {
+    val baseTypes = List(
+      stt(ScalarType.u32) -> ScalarType.u32,
+      stt(ScalarType.string) -> ScalarType.string,
+      ntt("Struct") -> structType
+    )
+
+    for {
+      base <- baseTypes
+      (token, expected) = base
+    } inside(resolve(token, Map("Struct" -> structType))) {
+      case Valid(TypeResolution(result, Nil)) =>
+        result shouldEqual expected
+    }
+  }
+
+  it should "resolve nested types" in {
+    val baseTypes = List(
+      stt(ScalarType.u32) -> ScalarType.u32,
+      stt(ScalarType.string) -> ScalarType.string,
+      ntt("Struct") -> structType
+    )
+
+    validCollectionModifiers
+      .take(6)
+      .toList
+      .flatten
+      .foreach(modifier =>
+        for {
+          base <- baseTypes
+          (btoken, btype) = base
+          (mod, typ) = modifier
+        } inside(resolve(mod(btoken), Map("Struct" -> structType))) {
+          case Valid(TypeResolution(result, Nil)) =>
+            result shouldEqual typ(btype)
+        }
+      )
+  }
+
+  it should "forbid services and abilities in collections" in {
+    val arrow = NonEmptyMap.of("arrow" -> ArrowType(ProductType(Nil), ProductType(Nil)))
+
+    val serviceType = ServiceType("Srv", arrow)
+    val abilityType = AbilityType("Abl", arrow)
+
+    val types = List(
+      ntt(serviceType.name) -> serviceType,
+      ntt(abilityType.name) -> abilityType
+    )
+
+    validCollectionModifiers
+      .take(6)
+      .toList
+      .flatten
+      .foreach(modifier =>
+        for {
+          base <- types
+          (btoken, btype) = base
+          (mod, _) = modifier
+        } inside(
+          resolve(
+            mod(btoken),
+            Map(
+              serviceType.name -> serviceType,
+              abilityType.name -> abilityType
+            )
+          )
+        ) { case Invalid(errors) =>
+          errors.exists(_.hint.contains("contain")) shouldBe true
+        }
+      )
+  }
+
+  it should "forbid streams inside any collection" in {
+    val baseTypes = List(
+      stt(ScalarType.u32),
+      stt(ScalarType.string),
+      ntt("Struct")
+    )
+
+    val modifiers = validCollectionModifiers
+      .map(_.map { case (token, _) => token })
+      .take(3)
+      .toList
+      .flatten
+
+    for {
+      left <- modifiers
+      right <- identity[BasicTypeToken[Id]] +: modifiers
+      base <- baseTypes
+      t = left(StreamTypeToken[Id]((), right(base)))
+    } inside(
+      resolve(t, Map(structType.name -> structType))
+    ) { case Invalid(errors) =>
+      errors.exists(_.hint.contains("of type *")) shouldBe true
+    }
+  }
+
+  it should "forbid stream of streams through alias" in {
+    val streamType = StreamType(ScalarType.u32)
+
+    val t = StreamTypeToken[Id]((), ntt("Als"))
+
+    inside(
+      resolve(t, Map("Als" -> streamType))
+    ) { case Invalid(errors) =>
+      errors.exists(_.hint.contains("of type *")) shouldBe true
+    }
+  }
+}

semantics/src/test/scala/aqua/semantics/ValuesAlgebraSpec.scala

@@ -12,13 +12,13 @@ import aqua.semantics.rules.names.{NamesAlgebra, NamesInterpreter, NamesState}
 import aqua.semantics.rules.report.{ReportAlgebra, ReportInterpreter}
 import aqua.semantics.rules.types.{TypesAlgebra, TypesInterpreter}
 import aqua.types.*
+
 import cats.Id
 import cats.data.{NonEmptyList, NonEmptyMap, State}
 import monocle.syntax.all.*
 import org.scalatest.Inside
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers
-
 import scala.collection.immutable.SortedMap
 
 class ValuesAlgebraSpec extends AnyFlatSpec with Matchers with Inside {
@@ -137,7 +137,7 @@ class ValuesAlgebraSpec extends AnyFlatSpec with Matchers with Inside {
       val vl = variable("left")
       val vr = variable("right")
 
-      val ut = lt.uniteTop(rt)
+      val ut = lt `∪` rt
 
       val state = genState(
         vars = Map(

types/src/main/scala/aqua/types/IntersectTypes.scala

@@ -1,5 +1,7 @@
 package aqua.types
 
+import aqua.errors.Errors.internalError
+
 import cats.Monoid
 import cats.data.NonEmptyMap
 
@@ -19,6 +21,12 @@ case class IntersectTypes(scalarsCombine: ScalarsCombine.T) extends Monoid[Type]
       ap.toList.zip(bp.toList).map(combine)
     )
 
+  private def combineDataTypes(a: DataType, b: DataType): DataType =
+    (a `∩` b) match {
+      case d: DataType => d
+      case t => internalError(s"$a ∩ $b yields non-data type $t")
+    }
+
   override def combine(a: Type, b: Type): Type =
     (a, b) match {
       case _ if CompareTypes(a, b) == 0.0 => a
@@ -39,18 +47,20 @@ case class IntersectTypes(scalarsCombine: ScalarsCombine.T) extends Monoid[Type]
           combineProducts(aa.codomain, bb.codomain)
         )
 
-      case (ac: OptionType, bc: BoxType) =>
+      case (ac: OptionType, bc: CollectionType) =>
-        OptionType(ac.element `∩` bc.element)
+        OptionType(combineDataTypes(ac.element, bc.element))
+      case (ac: CollectionType, bc: OptionType) =>
+        OptionType(combineDataTypes(ac.element, bc.element))
 
-      case (ac: BoxType, bc: OptionType) =>
+      case (ac: ArrayType, bc: CollectionType) =>
-        OptionType(ac.element `∩` bc.element)
+        ArrayType(combineDataTypes(ac.element, bc.element))
+      case (ac: CollectionType, bc: ArrayType) =>
+        ArrayType(combineDataTypes(ac.element, bc.element))
 
-      case (ac: ArrayType, bc: BoxType) =>
-        ArrayType(ac.element `∩` bc.element)
-      case (ac: BoxType, bc: ArrayType) =>
-        ArrayType(ac.element `∩` bc.element)
       case (ac: StreamType, bc: StreamType) =>
-        StreamType(ac.element `∩` bc.element)
+        StreamType(combineDataTypes(ac.element, bc.element))
+      case (ac: StreamMapType, bc: StreamMapType) =>
+        StreamMapType(combineDataTypes(ac.element, bc.element))
 
       case (a: ScalarType, b: ScalarType) =>
         scalarsCombine(a, b)

types/src/main/scala/aqua/types/Type.scala

@@ -1,33 +1,39 @@
 package aqua.types
 
-import cats.PartialOrder
+import aqua.errors.Errors.internalError
-import cats.data.NonEmptyMap
+import aqua.types.Type.*
+
 import cats.Eval
-import cats.syntax.traverse.*
+import cats.PartialOrder
+import cats.data.NonEmptyList
+import cats.data.NonEmptyMap
 import cats.syntax.applicative.*
 import cats.syntax.option.*
+import cats.syntax.partialOrder.*
+import cats.syntax.traverse.*
 
 sealed trait Type {
 
-  def acceptsValueOf(incoming: Type): Boolean = {
+  def acceptsValueOf(incoming: Type): Boolean =
-    import Type.typesPartialOrder
-    import cats.syntax.partialOrder._
     this >= incoming
-  }
 
   def isInhabited: Boolean = true
 
-  infix def `∩`(other: Type): Type = intersectBottom(other)
+  infix def `∩`[T <: Type](other: T): Type = intersectBottom(other)
 
-  def intersectTop(other: Type): Type = IntersectTypes.top.combine(this, other)
+  private final def intersectTop(other: Type): Type =
+    IntersectTypes.top.combine(this, other)
 
-  def intersectBottom(other: Type): Type = IntersectTypes.bottom.combine(this, other)
+  private final def intersectBottom(other: Type): Type =
+    IntersectTypes.bottom.combine(this, other)
 
-  infix def `∪`(other: Type): Type = uniteTop(other)
+  infix def `∪`[T <: Type](other: T): Type = uniteTop(other)
 
-  def uniteTop(other: Type): Type = UniteTypes.top.combine(this, other)
+  private final def uniteTop(other: Type): Type =
+    UniteTypes.top.combine(this, other)
 
-  def uniteBottom(other: Type): Type = UniteTypes.bottom.combine(this, other)
+  private final def uniteBottom(other: Type): Type =
+    UniteTypes.bottom.combine(this, other)
 
   def properties: Map[String, Type] = Map.empty
 
@@ -73,8 +79,7 @@ sealed trait ProductType extends Type {
    */
   def toLabelledList(prefix: String = "arg", index: Int = 0): List[(String, Type)] = this match {
     case LabeledConsType(label, t, pt) => (label -> t) :: pt.toLabelledList(prefix, index + 1)
-    case UnlabeledConsType(t, pt) =>
+    case UnlabeledConsType(t, pt) => (s"$prefix$index" -> t) :: pt.toLabelledList(prefix, index + 1)
-      (s"$prefix$index" -> t) :: pt.toLabelledList(prefix, index + 1)
     case _ => Nil
   }
 
@@ -85,6 +90,15 @@ sealed trait ProductType extends Type {
       pt.labelledData
     case _ => Nil
   }
+
+  lazy val labelledStreams: List[(String, StreamType)] = this match {
+    case LabeledConsType(label, t: StreamType, pt) =>
+      (label -> t) :: pt.labelledStreams
+    case ConsType(_, pt) =>
+      pt.labelledStreams
+    case _ => Nil
+  }
+
 }
 
 object ProductType {
@@ -247,42 +261,73 @@ object LiteralType {
   def forInt(n: Long): LiteralType = if (n < 0) signed else unsigned
 }
 
-sealed trait BoxType extends DataType {
+sealed trait CollectionType extends Type {
   def isStream: Boolean
 
-  def element: Type
+  def element: DataType
 
-  def withElement(t: Type): BoxType
+  def withElement(t: DataType): CollectionType
 
   override def properties: Map[String, Type] =
     Map("length" -> ScalarType.u32)
 }
 
-case class CanonStreamType(element: Type) extends BoxType {
+object CollectionType {
 
-  override def isStream: Boolean = false
+  def elementTypeOf(types: List[CollectibleType]): DataType =
+    NonEmptyList
+      .fromList(types)
+      .fold(BottomType)(
+        _.map {
+          case StreamType(el) => ArrayType(el)
+          case dt: DataType => dt
+        }.reduce[Type](_ `∩` _) match {
+          // In case we mix values of uncomparable types, intersection returns bottom, meaning "uninhabited type".
+          // But we want to get to TopType instead: this would mean that intersection is empty, and you cannot
+          // make any decision about the structure of type, but can push anything inside
+          case BottomType => TopType
+          case dt: DataType => dt
+          case t =>
+            internalError(
+              s"Expected data type from " +
+                s"intersection of ${types.mkString(", ")}; " +
+                s"got $t"
+            )
+        }
+      )
+}
+
+case class CanonStreamType(
+  override val element: DataType
+) extends DataType with CollectionType {
+
+  override val isStream: Boolean = false
 
   override def toString: String = "#" + element
 
-  override def withElement(t: Type): BoxType = copy(element = t)
+  override def withElement(t: DataType): CollectionType = copy(element = t)
 }
 
-case class ArrayType(element: Type) extends BoxType {
+case class ArrayType(
+  override val element: DataType
+) extends DataType with CollectionType {
 
-  override def isStream: Boolean = false
+  override val isStream: Boolean = false
 
   override def toString: String = "[]" + element
 
-  override def withElement(t: Type): BoxType = copy(element = t)
+  override def withElement(t: DataType): CollectionType = copy(element = t)
 }
 
-case class OptionType(element: Type) extends BoxType {
+case class OptionType(
+  override val element: DataType
+) extends DataType with CollectionType {
 
-  override def isStream: Boolean = false
+  override val isStream: Boolean = false
 
   override def toString: String = "?" + element
 
-  override def withElement(t: Type): BoxType = copy(element = t)
+  override def withElement(t: DataType): CollectionType = copy(element = t)
 }
 
 sealed trait NamedType extends Type {
@@ -374,7 +419,13 @@ case class StructType(name: String, fields: NonEmptyMap[String, Type])
     s"$fullName{${fields.map(_.toString).toNel.toList.map(kv => kv._1 + ": " + kv._2).mkString(", ")}}"
 }
 
-case class StreamMapType(element: Type) extends DataType {
+sealed trait MutableStreamType extends Type with CollectionType
+
+case class StreamMapType(override val element: DataType) extends MutableStreamType {
+
+  override val isStream: Boolean = true
+
+  override def withElement(t: DataType): CollectionType = copy(element = t)
 
   override def toString: String = s"%$element"
 }
@@ -383,6 +434,15 @@ object StreamMapType {
   def top(): StreamMapType = StreamMapType(TopType)
 }
 
+case class StreamType(override val element: DataType) extends MutableStreamType {
+
+  override val isStream: Boolean = true
+
+  override def toString: String = s"*$element"
+
+  override def withElement(t: DataType): CollectionType = copy(element = t)
+}
+
 case class ServiceType(name: String, fields: NonEmptyMap[String, ArrowType]) extends NamedType {
 
   override val specifier: String = "service"
@@ -424,17 +484,13 @@ case class ArrowType(domain: ProductType, codomain: ProductType) extends Type {
     s"$domain -> $codomain"
 }
 
-case class StreamType(element: Type) extends BoxType {
-
-  override def isStream: Boolean = true
-
-  override def toString: String = s"*$element"
-
-  override def withElement(t: Type): BoxType = copy(element = t)
-}
-
 object Type {
 
-  implicit lazy val typesPartialOrder: PartialOrder[Type] =
+  /**
+   * `StreamType` is collectible with canonicalization
+   */
+  type CollectibleType = DataType | StreamType
+
+  given PartialOrder[Type] =
     CompareTypes.partialOrder
 }

types/src/main/scala/aqua/types/UniteTypes.scala

@@ -1,8 +1,9 @@
 package aqua.types
 
+import aqua.errors.Errors.internalError
+
 import cats.Monoid
 import cats.data.NonEmptyMap
-
 import scala.annotation.tailrec
 
 /**
@@ -28,6 +29,12 @@ case class UniteTypes(scalarsCombine: ScalarsCombine.T) extends Monoid[Type]:
     step(a.toList, b.toList, Nil)
   }
 
+  def combineDataTypes(a: DataType, b: DataType): DataType =
+    (a `∪` b) match {
+      case d: DataType => d
+      case t => internalError(s"$a ∪ $b yields non-data type $t")
+    }
+
   override def combine(a: Type, b: Type): Type =
     (a, b) match {
       case (ap: ProductType, bp: ProductType) =>
@@ -52,18 +59,19 @@ case class UniteTypes(scalarsCombine: ScalarsCombine.T) extends Monoid[Type]:
         )
 
       case (ac: OptionType, bc: ArrayType) =>
-        ArrayType(ac.element `∪` bc.element)
+        ArrayType(combineDataTypes(ac.element, bc.element))
-
       case (ac: ArrayType, bc: OptionType) =>
-        ArrayType(ac.element `∪` bc.element)
+        ArrayType(combineDataTypes(ac.element, bc.element))
 
       case (ac: ArrayType, bc: ArrayType) =>
-        ArrayType(ac.element `∪` bc.element)
+        ArrayType(combineDataTypes(ac.element, bc.element))
       case (ac: OptionType, bc: OptionType) =>
-        OptionType(ac.element `∪` bc.element)
+        OptionType(combineDataTypes(ac.element, bc.element))
 
       case (ac: StreamType, bc: StreamType) =>
-        StreamType(ac.element `∩` bc.element)
+        StreamType(combineDataTypes(ac.element, bc.element))
+      case (ac: StreamMapType, bc: StreamMapType) =>
+        StreamMapType(combineDataTypes(ac.element, bc.element))
 
       case (a: ScalarType, b: ScalarType) =>
         scalarsCombine(a, b)

types/src/test/scala/aqua/types/TypeSpec.scala

@@ -1,6 +1,5 @@
 package aqua.types
 
-import aqua.types.Type.typesPartialOrder
 import cats.data.NonEmptyMap
 import cats.kernel.PartialOrder
 import cats.syntax.partialOrder._
@@ -15,7 +14,7 @@ class TypeSpec extends AnyFlatSpec with Matchers {
 
   def `?`(t: DataType): DataType = OptionType(t)
 
-  def `*`(t: DataType): DataType = StreamType(t)
+  def `*`(t: DataType): StreamType = StreamType(t)
 
   def accepts(recv: Type, incoming: Type) =
     recv >= incoming
@@ -76,7 +75,8 @@ class TypeSpec extends AnyFlatSpec with Matchers {
 
   "structs of scalars with literals" should "be variant" in {
     val one: Type = StructType("one", NonEmptyMap.of("field" -> u64))
-    val two: Type = StructType("two", NonEmptyMap.of("field" -> LiteralType.number, "other" -> string))
+    val two: Type =
+      StructType("two", NonEmptyMap.of("field" -> LiteralType.number, "other" -> string))
 
     accepts(one, two) should be(true)
     accepts(two, one) should be(false)

utils/helpers/src/main/scala/aqua/syntax/optiont.scala

@@ -1,8 +1,10 @@
 package aqua.helpers.syntax
 
-import cats.{Functor, Monad}
 import cats.data.OptionT
+import cats.syntax.apply.*
+import cats.syntax.flatMap.*
 import cats.syntax.functor.*
+import cats.{Functor, Monad}
 
 object optiont {
 
@@ -28,4 +30,20 @@ object optiont {
     )(using F: Monad[F]): OptionT[F, B] =
       o.flatTransform(f.andThen(_.value))
   }
+
+  extension [F[_]: Monad, A, B](
+    t: Tuple2[OptionT[F, A], OptionT[F, B]]
+  ) {
+
+    /**
+     * Merges `OptionT`s into `OptionT` of a tuple,
+     * **executing both effects**.
+     */
+    def merged: OptionT[F, (A, B)] = OptionT(
+      for {
+        a <- t._1.value
+        b <- t._2.value
+      } yield (a, b).tupled
+    )
+  }
 }