I'm starting to get the hang of certain aspects of typeclasses, particularly with Maybe and list types. For instance I needed to write a function as follows:

Ord k => k -> Map k [a] -> Maybe a

which evaluates to "Nothing" if there is no such key in the map, or Just the first element of [a] if there is such a key, or Nothing if there is such a key but [a] is null.

So I could write

import qualified Data.Map as M

import Control.Monad

import Data.Maybe

f k m = case M.lookup k m of

Nothing -> Nothing

Just xs -> listToMaybe xs

But the case "Nothing -> Nothing" is suspicious... seems like that's always a clue some typeclass could simplify it. Eventually I figured out

f k = join . fmap listToMaybe . M.lookup k