Safe `maximum` in Haskell Standard Library?

Question

Is there a safe equivalent of maximum in Haskell's Standard Library?

*Main Control.Monad.State Data.List> maximum []
*** Exception: Prelude.maximum: empty list

I tried to find one, (Ord a, Foldable t) => t a -> Maybe a with hoogle, but found none.

Answer 1

For the sake of completeness: loosening your "Standard Library" requirement to "some very commonly used library", the safe package provides the Safe.Foldable module, which includes a maximumMay function:

maximumMay :: (Foldable t, Ord a) => t a -> Maybe a

Answer 2

You can code one up yourself for any Foldable, by applying foldMap to a suitable choice of Monoid.

The Option monoid takes an existing Semigroup and lifts it into a Monoid by adjoining an empty element (Option Nothing), which'll be returned by foldMap if the input Foldable is empty. The Max newtype lifts any instance of Ord into a Semigroup by making <> pick the larger of its arguments.

So by foldMapping the input Foldable through the composition of Option and Max, we get your desired behaviour.

safeMaximum :: (Foldable t, Ord a) => t a -> Maybe a
safeMaximum = fmap getMax . getOption . foldMap (Option . Just . Max)

ghci> safeMaximum "wowzers"
Just 'z'
ghci> safeMaximum ""
Nothing