Friend Numbers

import Data.List (nubBy)

-- SOLUTION 1
friends xs = nubBy f [(a, b) | a <- xs, b <- xs, ((a + b) `mod` 10) == 0]
  where
    f (a1, b1) (a2, b2) = (a1 == b2) && (a2 == b1)

main :: IO()
main = do
    a <- (readLn :: IO [Int])
    let frs = friends a
    putStrLn $ "Ha " ++ (show $ length frs) ++ " pares:"
    mapM_ putStrLn $ map show frs

-- SOLUTION 2 (related)
import Control.Monad (guard)
import Data.List (nubBy)

friends xs = do
    a <- xs
    b <- xs
    guard $ (a + b) `mod` 10 == 0
    return $ (a, b)

main :: IO()
main = do
    a <- (readLn :: IO [Int])
    let frs = nubBy f $ friends a
    putStrLn $ "Ha " ++ (show $ length frs) ++ " pares:"
    mapM_ putStrLn $ map show frs
  where
    f (a1, b1) (a2, b2) = (a1 == b2) && (a2 == b1)