How one might implement a Workflow (Monad) library in some future version of Elm.

Workflow.elm

module Workflow where

import List
import Maybe as M
import Either as E

type Workflow container =
  { return : forall a. a -> container a
  , bind : forall a. container a -> (a -> container b) -> container b
  }

maybe : Workflow Maybe
maybe =
  { return = M.Just
  , bind m f = M.maybe M.Nothing f m
  }

either : Workflow Either
either =
  { return = E.Right
  , bind e f = E.either E.Left f e
  }

sequence : Workflow container -> [container a] -> container [a]
sequence wf list =
    let cons w ws =
          wf.bind w  <| \x ->
          wf.bind ws <| \xs -> wf.return (x::xs)
    in
        List.foldr cons (wf.return []) list

map : Workflow container -> (a -> container b) -> [a] -> container [b]
map workflow f list =
    sequence workflow (List.map f list)

WorkflowUsage.elm

  
import open Workflow
import Maybe
import String

ns : Maybe.Maybe [Int]
ns = map maybe String.toInt ["123","42","56"]

main = asText ns