Generalizing `head`-like functions with TypeFamilies.

HasHead.hs

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, FlexibleInstances #-}

module HasHead where


import qualified Data.Text as T
import qualified Data.ByteString.Char8 as BSC


-- Usage example
--
-- ghci> (safeHead [], safeHead [1], safeHead (T.pack ""), safeHead (T.pack "foo"), safeHead (BSC.pack ""), safeHead (BSC.pack "bar"))
-- (Nothing,Just 1,Nothing,Just 'f',Nothing,Just 'b')
-- ghci> 
--


class HasHead c where
    type Element c :: *
    safeHead :: c -> Maybe (Element c)


instance HasHead [a] where
    type Element [a] = a
    safeHead [] = Nothing
    safeHead (x:_) = Just (x)


instance HasHead T.Text where
    type Element T.Text = Char
    safeHead t | T.null t = Nothing
               | otherwise = Just (T.head t)


instance HasHead BSC.ByteString where
    type Element BSC.ByteString = Char
    safeHead s | BSC.null s = Nothing
               | otherwise = Just (BSC.head s)