unison/unison-src/tests/handler-stacking.u

use State get put
use Writer tell

> handle
    handle replicate 5 main
    with writerHandler []
  with stateHandler "hello"


main = '(tell get)

replicate : Nat -> '{e} () -> {e} ()
replicate n x =
  if Nat.eq n 0 then () else
    !x
    replicate (drop n 1) x

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

structural ability Writer w where
  tell : w -> {Writer w} ()

stateHandler : s -> Request {State s} a -> (s, a)
stateHandler s = cases
  { State.get -> k } -> handle k s with stateHandler s
  { State.put s -> k } -> handle k () with stateHandler s
  { a } -> (s, a)

writerHandler : [w] -> Request {Writer w} a -> ([w], a)
writerHandler ww = cases
  { Writer.tell w -> k } -> handle k () with writerHandler (snoc ww w)
  { a } -> (ww, a)