unison/unison-src/tests/delay.u

structural type Foo a = Foo a

(+) = (Nat.+)

-- The type 'a is sugar for `() -> a`.
-- The term 'a is sugar for `() -> a`.
-- !a forces a delayed expression (equivalent to `a()`)

woot : 'Nat
woot = '42

-- A 'a can also be created by prefixing `let` with a '
woot2 : 'Nat
woot2 = 'let
  x = 1
  y = 2
  x + y

-- ' has higher precedence than -> in type signatures
--   and a lower precedence than type application
woot3 : 'Nat -> Nat
woot3 x = !x + 1

woot4 : ∀ a . 'Foo a -> Foo a
woot4 foo = !foo

woot4Usage = woot4 '(Foo.Foo 19)

woot4Usage2 =
  foo = 'let
    x : Nat
    x = 99
    Foo.Foo (x + x)
  woot4 foo

> woot4Usage2