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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 :: forall a. [a] -> Either String a
headEither (a
a:[a]
_) = a -> Either String a
forall a b. b -> Either a b
Right a
a
headEither []    = String -> Either String a
forall a b. a -> Either a b
Left String
"Empty list"

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

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