Generic `head`-like functions implemented with functional dependency.

HasHeadFD.hs

{-# LANGUAGE FunctionalDependencies, FlexibleInstances #-}

module HasHeadFD where


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


class HasHeadFD c e | c -> e where
    safeHead :: c -> Maybe (e)


instance HasHeadFD [a] a where
    safeHead [] = Nothing
    safeHead (x:_) = Just (x)


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


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