Data.Map.lookup without Maybe

Main.hs

module Main
    where

import Control.Exception
import Data.Map as M
import Data.Maybe

-- Well, since you don't have any constraints on the type of the input argument,
-- you can't be absolutely sure that your function will be always used
-- correctly. I too would use an assertion here (or change my function to return
-- Maybe if I'm in the mood); another option is to force your clients to check
-- that the input is well-formed via the type system:

type Key = String
type Value = Int
type MyMap = M.Map Key Value
newtype MapWithAnInterestingElement = Mp MyMap

interesting_key :: Key
interesting_key = "interesting"

makeMapWithAnInterstingElement :: MyMap -> MapWithAnInterestingElement
makeMapWithAnInterstingElement m = assert (interesting_key `member` m) (Mp m)

getInterestingElement :: MapWithAnInterestingElement -> Value
getInterestingElement (Mp m) = fromJust . M.lookup interesting_key $ m

-- I admit that this solution doesn't scale.