MonadWriter in elm

Usage.elm

half : Int -> Writer Int String
half n =
  tell ("halved " ++ (toString n)) >>= \_ -> return (n // 2)


use =
  half 8 >>= half |> runReader
  
-- => (2, ["halved 4", "halved 8"])

Writer.elm

module Writer exposing (..)


type Writer w a
    = Writer (List w -> ( a, List w ))


runWriter : Writer w a -> ( a, List w )
runWriter (Writer w) =
    w []


andThen : Writer w a -> (a -> Writer w b) -> Writer w b
andThen (Writer w) f =
    Writer <|
        \l ->
            let
                ( av, al ) =
                    w l

                (Writer g) =
                    f av
            in
                g al


return : a -> Writer w a
return v =
    Writer <| \l -> ( v, l )


tell : w -> Writer w ()
tell l =
    Writer <| \ls -> ( (), l::ls )


(>>=) : Writer w a -> (a -> Writer w b) -> Writer w b
(>>=) =
    andThen