Implement the Monoid ability in Unison

scratch.u

ability Semigroup m where
  assoc : m -> m -> m

-- Without unique, Monoid and Ask have the same hash
unique ability Monoid m where
  identity : m

use Semigroup assoc
use Monoid identity

runMonoidSum : '{Semigroup Nat, Monoid Nat} b -> b
runMonoidSum p =
  h : Request {Semigroup Nat, Monoid Nat} b -> b
  h = cases
    {assoc m n -> k} ->
      handle k (m + n) with h
    {identity -> k} ->
      handle k 0 with h
    {e} ->
      e
  handle !p with h

> runMonoidSum 'let assoc 3 identity
> runMonoidSum 'let assoc identity 3

foldMap : (a ->{Semigroup m, Monoid m} m) -> List a ->{Semigroup m, Monoid m} m
foldMap f xs =
  List.foldr (a acc -> assoc (f a) acc) identity xs

> runMonoidSum 'let foldMap id [0, 1, 2]