Given a sort function that works on numbers, you can sort in reverse order by first negating the list, then sorting, then negating again: Prelude Data.List> let revsort = (map negate) . sort . (map negate) The function sort takes any list of Ord; negate works on any Num. Prelude Data.List> :t negate negate :: (Num a) => a -> a Prelude Data.List> :t sort sort :: (Ord a) => [a] -> [a] I'd therefore expect my revsort function to work on any type that is both an Ord and a Num. However: Prelude Data.List> :t revsort revsort :: [Integer] -> [Integer] I was expecting something like revsort :: (Ord a, Num a) => [a] -> [a] instead. Question: Why did the GHCI's type inference mechanism jump to the conclusion that revsort should only work with Integer lists?