These monad implemented in Elm

These.elm

type These e a
    = Both ( e, a )
    | Left e
    | Right a


map : (a -> b) -> These e a -> These e b
map f t =
    bimap ( identity, f ) t


mapLeft : (e -> g) -> These e a -> These g a
mapLeft f t =
    bimap ( f, identity ) t


bimap : ( e -> g, a -> b ) -> These e a -> These g b
bimap ( g, f ) t =
    case t of
        Left e ->
            Left (g e)

        Right a ->
            Right (f a)

        Both ( e, a ) ->
            Both ( g e, f a )


andThen : (a -> These e b) -> These e a -> These e b
andThen f t =
    case t of
        Left e ->
            Left e

        Right a ->
            f a

        Both ( e, a ) ->
            case f a of
                Left _ ->
                    Left e

                Right aa ->
                    Right aa

                Both ( ee, aa ) ->
                    Both ( ee, aa )


swap : These e a -> These a e
swap t =
    case t of
        Left e ->
            Right e

        Right a ->
            Left a

        Both ( e, a ) ->
            Both ( a, e )


leftOnly : These e a -> Maybe e
leftOnly t =
    unwrapLeft Nothing Just <| t


rightOnly : These e a -> Maybe a
rightOnly t =
    unwrap Nothing Just <| t


unwrap : b -> (a -> b) -> These e a -> b
unwrap default f t =
    case t of
        Left _ ->
            default

        Right a ->
            f a

        Both ( _, a ) ->
            f a


unwrapLeft : g -> (e -> g) -> These e a -> g
unwrapLeft default f t =
    case t of
        Left e ->
            f e

        Right _ ->
            default

        Both ( e, _ ) ->
            f e