contrived.hs

contrived value input =
  let parsed = read input in  -- parsed implements Read
                              -- read takes a String, therefore input must be String
    if parsed == value        -- parsed & value are same type, which implements Eq
      then Just parsed
      else Nothing

{-
    If type bindings are still ambiguous after the function is defined, it's
    preferred to leave them unresolved rather than forcing them. This keeps your
    function as generic as possible (think duck-typing). Therefore:

        contrived :: (Read a, Eq a) => a -> String -> Maybe a

    ...which you can prove with ghci:

        $ ghci contrived.hs
        *Main> :t contrived
        contrived :: (Read a, Eq a) => a -> String -> Maybe a
        *Main> contrived True "True"
        Just True
        *Main> contrived True "False"
        Nothing
        *Main> contrived True "1"
        *** Exception: Prelude.read: no parse
        *Main> contrived 1 "1"
        Just 1

    * magic *
-}