aqua-book/language/closures.md

Closures

In Aqua, you can create an arrow within the function, enclosing its context.

service Hello:
  say_hello(to_name: string, peer: string)

func bar(callback: string -> ()):
   callback("Fish")

func foo(peer: string):
  on peer:
    -- Capture service resolution
    Hello "world"
    -- Create a closure named "closure"
    closure = (name: string) -> string:
      -- Use a value that's available on the definition site
      -- To call a service that's resolved on the definition site
      Hello.say_hello(name, peer)
      -- Return a value from the closure; syntax is the same as in functions
      <- name
  -- Pass this closure to another function
  bar(closure)

Closures can be created anywhere in the function, starting with Aqua 0.4.1, and then used just like any other arrow (argument of arrow type, function, or service method): passed to another function as an argument, or called right there.

Closures enclose over three domains:

• Values in scope,
• Service resolutions,
• Topology: place where the closure is defined should be the place where it's executed.

Comparing with functions, closures have one important difference: functions are detached from topology by default. func keyword can be used to bring this behavior to closures, if needed.

service Hello:
  say_hello()

func foo():
  on HOST_PEER_ID:
    Hello "hello"
    
    -- This closure will execute on HOST_PEER_ID
    closure = () -> ():
      Hello.say_hello() 
      
    fn = func () -> ():
      Hello.say_hello()
      
  -- Will go to HOST_PEER_ID, where Hello service is resolved, and call say_hello     
  closure()        
  
  -- Will be called on current peer, probably INIT_PEER_ID, and may fail
  -- in case Hello service is not defined or has another ID
  fn()

It is not yet possible to return an arrow from an arrow.