more-monads/src/Control/Monad/Extras.hs

{-# LANGUAGE ExistentialQuantification #-}

module Control.Monad.Extras (
  ifM,
  unlessM,
  whenM,
  intercalateM
  )where

import Control.Monad

-- | A version of if with monadic test.
ifM :: Monad m => m Bool -> m a -> m a -> m a
ifM i t e = do
  i' <- i
  if i' then t else e

-- | A version of unless with monadic test
unlessM :: Monad m => m Bool -> m () -> m ()
unlessM p me = do
  p' <- p
  unless p' me

-- | A version of when with monadic test
whenM :: Monad m => m Bool -> m () -> m ()
whenM p me = do
  p' <- p
  when p' me

{- |
Folds the second list with the function applied to the first,
intercalating the evaluation. That is:

intercalateM f -> [a00, a10, a20] -> [b1, b2] = do
  a01 <- f a00 b1
  a11 <- f a10 b1
  a21 <- f a20 b1
  a02 <- f a11 b2
  a12 <- f a21 b2
  a22 <- f a31 b2
  return [a02, a12, a22]

Usefull for consuming lazy sequences.
-}
intercalateM :: Monad m => (a -> b -> m a) -> [a] -> [b] -> m [a]
intercalateM _ aa [] = return aa
intercalateM f aa (b:bb) = do
  aa' <- sequence $ f <$> aa <*> [b]
  intercalateM f aa' bb