Async ability in Unison

async.u

-- Nat is just an index into some internal store
-- provided by the handler

unique type Source a = Source Nat
unique type Sink a = Sink Nat

ability Abort where abort : a

-- You wouldn't typically program with these operations directly,
-- but you can implement `fork` in terms of these operations
ability Async where
  promise  : Either (Sink a) (Source a)
  wait     : Source a -> a
  complete : a -> Sink a -> x

fork : '{e} a ->{Async,e} '{Async} a
fork a = match Async.promise with
  Left sink -> complete !a sink
  Right src -> '(wait src)
  
-- we also considered -
ability Async where
  promise  : Either (Sink a) (Source a)
  wait     : Source a -> a
  complete : a -> Sink a -> ()

fork : '{e} a ->{Async,e} '{Async,Abort} a
fork a = match Async.promise with
  Left sink -> 
    complete !a sink
    'Abort.abort
  Right src -> '(wait src)

Credit: @dolio in Unison Slack.