Сравнивая элементы двух списков вернуть пару первых не равных элементов

f1 :: Ord a => [a] -> [a] -> (a,a)
f1 [] _ = error "First argument is empty list."
f1 _ [] = error "Second argument is empty list."
f1 (x:xs) (y:ys) = if x /= y then (x,y) else f1 xs ys

>> f1 [1,1,1] [1,1,1]
*** Exception: First argument is empty list.
>>

import Data.List 
 
f :: Eq a => [a] -> [a] -> Maybe (a,a)
f = 
  (find (uncurry (/=)) .) . zip

f1 :: (Eq a) => [a] -> [a] -> Maybe (a,a)
f1 [] _ = Nothing
f1 _ [] = Nothing
f1 (x:xs) (y:ys) = if x /= y then Just(x,y) else f1 xs ys

import Data.List
 
f :: Eq a => [a] -> [a] -> Maybe (a,a)
f = ((find (uncurry (/=)) . reverse) . ) . zip

import Data.List
import Data.Maybe
 
f1 :: (Eq a) => [a] -> [a] -> Maybe (a,a)
f1 [] _ = Nothing
f1 _ [] = Nothing
f1 (x:xs) (y:ys) =
    let b = f1 xs ys in
    if isJust b then b else
    if x /= y then Just(x,y) else Nothing

import Data.List  ( find )
import Data.Maybe ( fromJust, isJust )
import Data.Word  ( Word8 )
 
f1 :: Eq a => [a] -> [a] -> Maybe (a,a)
f1 = ((find (uncurry (/=)) . reverse) . ) . zip
 
less :: (Eq a, Ord a) => [a] -> [a] -> Bool
less xs ys = let b = isJust (f1 xs ys) in
                 if   b
                 then uncurry (<) $ fromJust (f1 xs ys)
                 else b
 
-- младший разряд по младшему индексу!
less1 :: [Word8] -> [Word8] -> Bool
less1 []  [] = False
less1 [0] [] = False
less1 [] [0] = False
less1 xs  [] = False
less1 []  ys = True
less1 xs  ys | (length xs) < (length ys) = True
             | (length xs) > (length ys) = False
             | otherwise = less xs ys

cmpNum =
    let prep = dropWhile (==0) . reverse in
    (. prep) . (cmpNum' . prep)
    where cmpNum' xs ys =
        let b = compare (length xs) (length ys) in
        if b /= EQ then b else compare xs ys
        
lessNum = ((==LT) . ) . cmpNum

parse error (possibly incorrect indentation)

{-# LANGUAGE NoMonomorphismRestriction #-}
 
import Data.Function (on)
import Data.Monoid (mappend)
import Control.Monad (ap)
import Control.Arrow ()
 
f1 xs ys = (compare `on` length) xs ys `mappend` compare xs ys
 
f2 = ap (ap . (mappend .) . (compare `on` length)) compare

f1 = (compare `on` length) `mappend` compare

>> max 4 5 + max 6 7
12

on (+) (uncurry max) (4,5) (6,7)

-- bokunopico
f1 xs ys = (compare `on` length) xs ys `mappend` compare xs ys 
f2 = ap (ap . (mappend .) . (compare `on` length)) compare
 
-- calabi-yau
f1 = (compare `on` length) `mappend` compare
 
-- а вот эта функция (Ctrl) правильный:
cmpNum =
    let prep = dropWhile (==0) . reverse in
    (. prep) . (cmpNum' . prep)
    where cmpNum' xs ys =
        let b = compare (length xs) (length ys) in
        if b /= EQ then b else compare xs ys
        
lessNum = ((==LT) . ) . cmpNum

>> f1 [4,2,1] [2,2,3]
GT
it :: Ordering
(0.00 secs, 521836 bytes)
>>