[mrgriffin]

I'd probably think of Bool as being isomorphic to Maybe (), which would mean your map implementation looks a bit like:

    map :: Bool -> (() -> b) -> Maybe b
    map True f = Just (f ())
    map False _ = Nothing

Or, in Haskell because you've got laziness you could go with:

    map :: Bool -> b -> Maybe b

Which already exists as a combination of Control.Monad.guard and Data.Functor.(<$).

    map tf b = b <$ guard tf