Tail recursion example

Partition.hs

{-# LANGUAGE LambdaCase #-}
module Partition (main) where

-- original implementation
partitionMaybe' :: (a -> Maybe b) -> [a] -> ([(a, b)], [a])
partitionMaybe' f = go
    where
      go = \case
           [] -> ([], [])
           (head : tail) ->
               let (withs, withouts) = go tail
               in case f head of
                    Nothing -> (withs, head : withouts)
                    Just b -> ((head, b) : withs, withouts)


-- tail recursive variant
partitionMaybe :: (a -> Maybe b) -> [a] -> ([(a, b)], [a])
partitionMaybe f = go ([], [])
    where
      go (withs, withouts) = \case
           [] -> (reverse withs, reverse withouts)
           (head : tail) ->
               case f head of
                    Nothing -> go (withs, head : withouts) tail
                    Just b -> go ((head, b) : withs, withouts) tail
main = do
  let res1 = partitionMaybe' f xs
  let res2 = partitionMaybe f xs

  putStrLn $ show $ res1 == res2
  putStrLn "--"
  putStrLn $ show $ res1
  putStrLn "--"
  putStrLn $ show $ res2

  where
    xs = [1..100]
    f a =
       if a `mod` 2 == 0
       then Just "huray!"
       else Nothing