unison/unison-src/tests/map-traverse.u

--map/traverse
structural ability Noop where
  noop : ∀ a . a -> {Noop} a

structural ability Noop2 where
  noop2 : ∀ a . a -> a -> {Noop2} a

structural type List a = Nil | Cons a (List a)

map : ∀ a b e . (a -> {e} b) -> List a -> {e} (List b)
map f = cases
  List.Nil -> List.Nil
  List.Cons h t -> List.Cons (f h) (map f t)

c = List.Cons
z : ∀ a . List a
z = List.Nil

ex = (c 1 (c 2 (c 3 z)))

pureMap : List Text
pureMap = map (a -> "hello") ex

-- `map` is ability polymorphic
zappy : () -> {Noop} (List Nat)
zappy u = map (zap -> (Noop.noop (zap Nat.+ 1))) ex

-- mixing multiple abilitys in a call to `map` works fine
zappy2 : () -> {Noop, Noop2} (List Nat)
zappy2 u = map (zap -> Noop.noop (zap Nat.+ Noop2.noop2 2 7)) ex