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src/FoldableHelp.hs

@@ -5,6 +5,12 @@ module FoldableHelp
     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
@@ -12,3 +18,11 @@ safeMinimumOn f = foldl' g Nothing
         | 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