unison/unison-src/errors/handle-inference.u

--handle inference
effect State s where
  get : ∀ s . () -> {State s} s
  set : ∀ s . s -> {State s} ()
state : ∀ a s . s -> Effect (State s) a -> a
state s e = case e of
  {a} -> a
  {State.get _ -> k} -> handle state s in k s
  {State.set s -> k} -> handle state s in k ()
-- modify : ∀ s . (s -> s) -> {State s} ()
-- modify f = State.set (f (State.get()))
ex : () -> {State UInt64} UInt64
ex blah =
  State.get() UInt64.+ 42
-- note this currently succeeds, the handle block
-- gets an inferred type of ∀ a . a, it appears that
-- the existential `a` which gets instantiated for the
-- state call never gets refined, most likely due to
-- missing a subtype check in handle
y : Text
y = handle state 5 in ex ()
()