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.