Abilities in Unison

state.u

ability State s where
  put : s ->{State s} ()
  get : {State s} s

state : s -> Effect (State s) a -> a
state s eff = case eff of
  { State.get -> k } -> handle (state s) in k s
  { State.put snew -> k } -> handle (state snew) in k ()
  { a } -> a

modify : (s ->{} s) ->{State s} ()
modify f = State.put (f State.get)

ex : (Text, Nat)
ex = handle (state 10) in -- starting state: 10
  modify (x -> x + 1) -- 11
  ("hello", State.get + State.get) -- 11 + 11 = 22

> ex