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-- | Auxiliary functions for working with values of type '[]'.
module Data.List.Extra where

-- | Safely extract the head of a list.
headEither :: [a] -> Either String a
headEither (a:_) = Right a
headEither []    = Left "Empty list"

-- | Apply a transformation only to the head of a list.
toHead :: (a -> a) -> [a] -> [a]
toHead f (x:xs) = f x : xs
toHead _ xs     = xs

-- | Apply a transformation only to the tail of a list.
toTail :: (a -> a) -> [a] -> [a]
toTail f (x:xs) = x : fmap f xs
toTail _ xs     = xs