A simple exception monad in Elm

ExceptionMonad.elm

import Html exposing (text)


type Result a
    = Return a
    | Raise Exception


type alias Exception =
    String


unit : a -> Result a
unit a =
    Return a


-- Create a "bind" operation to combine computations.
(>>=) : Result a -> (a -> Result b) -> Result b
(>>=) m k =
    case m of
        Raise e ->
            Raise e

        Return a ->
            k a

-- The operation has low precedence to avoid death-by-parens.
infixr 0 >>=




{- Evaluate a simple integer-division expression. -}

type Term = Div Term Term | Const Int

eval : Term -> Result Int
eval t = case t of
  Const a -> Return a
  Div t1 t2 -> 
    eval t1 >>= \a -> eval t2 >>= \b -> 
      if b == 0 then Raise "division by zero!"
      else Return (a // b)


main = text <| toString <| eval <| Div (Const 4) (Const 0)