Haskell code is often pleasingly elegant, but I particularly liked this code which uses the View Patterns1 extension to good effect: 

{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE OverloadedStrings #-}

import qualified Data.Map.Strict as M
import qualified Data.Text       as T

....

r :: T.Text -> T.Text
r (T.stripPrefix "inc:"  -> Just k) = handleInc k
r (T.stripPrefix "h1:"   -> Just k) = header    k
r ((`M.lookup` snippets) -> Just h) = h
r k                                 = unknown k

I suppose the key insight is that in the context of View Patterns functions of type u -> Maybe v are great to optionally match against some u and extract a v.

Some Haskell libraries e.g. Data.Text2 have functions3 specifically to enable this sort of thing.

It struck me as a more principled version of this sort of Perl code: 

if ($foo =~ /^h1:(.*)/) { header($1) }

You can do much more with View Patterns: the GHC wiki lists some examples4.