Idris2/tests/idris2/misc/docs004/List.idr

module List

data List a = Nil | (::) a (List a)

infixr 5 ::
infixr 5 ++

interface Monoid ty where
  ||| Users can hand-craft their own monoid implementations
  constructor MkMonoid
  neutral : ty
  (++) : ty -> ty -> ty

Monoid (List a) where
  neutral = []

  xs ++ ys = case xs of
    [] => ys
    (x :: xs) => let ih = xs ++ ys in x :: ih