In Haskell, sortBy takes a comparison function: sortBy :: (a -> a -> Ordering) -> [a] -> [a] And Ordering is defined as: data Ordering = LT | EQ | GT If

In Haskell, sortBy takes a comparison function: sortBy :: (a -> a -> Ordering) -> [a] -> [a] And Ordering is defined as: data Ordering = LT | EQ | GT If you want to provide a "key" function rather than a comparison function, you can use sortOn: sortOn :: Ord b => (a -> b) -> [a] -> [a] Which is basically just a sortBy with a function that applies the key function and then compares as normal.