module FoldableHelp
    ( safeMinimumOn
    , module Data.Foldable
    )
    where
import Data.Foldable

-- TODO check up whether and how it is necessary to specialise this
-- | A version of 'minimum' where the comparison is done on some extracted value.
--   Returns Nothing if the list is empty. Only calls the function once per element.
--
-- > safeMinimumOn id [] == Nothing
-- > safeMinimumOn length ["test","extra","a"] == Just "a"
safeMinimumOn :: (Foldable t,Ord b) => (a -> b) -> t a -> Maybe a
safeMinimumOn f = foldl' g Nothing
  where
    g (Just x) y 
        | f x < f y = Just x
        | otherwise = Just y
    g Nothing y = Just y
--{- | Partial. -}
--minimumOn :: Ord b => (a -> b) -> [a] -> a
--minimumOn f [] = error "tried to take minimumOn of empty list"
--minimumOn f (x:xs) = go x xs
--  where
--    go x (y:xs) | f x < f y = go x xs
--                | otherwise = go y xs
--    go x [] = x