-- | Utilities related to Monad and Applicative classes -- Mostly for backwards compatibility. module MonadUtils ( Applicative(..) , (<$>) , MonadFix(..) , MonadIO(..) , zipWith3M, zipWith3M_, zipWith4M, zipWithAndUnzipM , mapAndUnzipM, mapAndUnzip3M, mapAndUnzip4M, mapAndUnzip5M , mapAccumLM , mapSndM , concatMapM , mapMaybeM , fmapMaybeM, fmapEitherM , anyM, allM, orM , foldlM, foldlM_, foldrM , maybeMapM , whenM, unlessM , filterOutM ) where ------------------------------------------------------------------------------- -- Imports ------------------------------------------------------------------------------- import GhcPrelude import Control.Applicative import Control.Monad import Control.Monad.Fix import Control.Monad.IO.Class import Data.Foldable (sequenceA_, foldlM, foldrM) import Data.List (unzip4, unzip5, zipWith4) ------------------------------------------------------------------------------- -- Common functions -- These are used throughout the compiler ------------------------------------------------------------------------------- {- Note [Inline @zipWithNM@ functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The inline principle for 'zipWith3M', 'zipWith4M' and 'zipWith3M_' is the same as for 'zipWithM' and 'zipWithM_' in "Control.Monad", see Note [Fusion for zipN/zipWithN] in GHC/List.hs for more details. The 'zipWithM'/'zipWithM_' functions are inlined so that the `zipWith` and `sequenceA` functions with which they are defined have an opportunity to fuse. Furthermore, 'zipWith3M'/'zipWith4M' and 'zipWith3M_' have been explicitly rewritten in a non-recursive way similarly to 'zipWithM'/'zipWithM_', and for more than just uniformity: after [D5241](https://phabricator.haskell.org/D5241) for issue #14037, all @zipN@/@zipWithN@ functions fuse, meaning 'zipWith3M'/'zipWIth4M' and 'zipWith3M_'@ now behave like 'zipWithM' and 'zipWithM_', respectively, with regards to fusion. As such, since there are not any differences between 2-ary 'zipWithM'/ 'zipWithM_' and their n-ary counterparts below aside from the number of arguments, the `INLINE` pragma should be replicated in the @zipWithNM@ functions below as well. -} zipWith3M :: Monad m => (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m [d] {-# INLINE zipWith3M #-} -- Inline so that fusion with 'zipWith3' and 'sequenceA' has a chance to fire. -- See Note [Inline @zipWithNM@ functions] above. zipWith3M f xs ys zs = sequenceA (zipWith3 f xs ys zs) zipWith3M_ :: Monad m => (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m () {-# INLINE zipWith3M_ #-} -- Inline so that fusion with 'zipWith4' and 'sequenceA' has a chance to fire. -- See Note [Inline @zipWithNM@ functions] above. zipWith3M_ f xs ys zs = sequenceA_ (zipWith3 f xs ys zs) zipWith4M :: Monad m => (a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e] {-# INLINE zipWith4M #-} -- Inline so that fusion with 'zipWith5' and 'sequenceA' has a chance to fire. -- See Note [Inline @zipWithNM@ functions] above. zipWith4M f xs ys ws zs = sequenceA (zipWith4 f xs ys ws zs) zipWithAndUnzipM :: Monad m => (a -> b -> m (c, d)) -> [a] -> [b] -> m ([c], [d]) {-# INLINABLE zipWithAndUnzipM #-} -- See Note [flatten_many performance] in TcFlatten for why this -- pragma is essential. zipWithAndUnzipM f (x:xs) (y:ys) = do { (c, d) <- f x y ; (cs, ds) <- zipWithAndUnzipM f xs ys ; return (c:cs, d:ds) } zipWithAndUnzipM _ _ _ = return ([], []) {- Note [Inline @mapAndUnzipNM@ functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The inline principle is the same as 'mapAndUnzipM' in "Control.Monad". The 'mapAndUnzipM' function is inlined so that the `unzip` and `traverse` functions with which it is defined have an opportunity to fuse, see Note [Inline @unzipN@ functions] in Data/OldList.hs for more details. Furthermore, the @mapAndUnzipNM@ functions have been explicitly rewritten in a non-recursive way similarly to 'mapAndUnzipM', and for more than just uniformity: after [D5249](https://phabricator.haskell.org/D5249) for Trac ticket #14037, all @unzipN@ functions fuse, meaning 'mapAndUnzip3M', 'mapAndUnzip4M' and 'mapAndUnzip5M' now behave like 'mapAndUnzipM' with regards to fusion. As such, since there are not any differences between 2-ary 'mapAndUnzipM' and its n-ary counterparts below aside from the number of arguments, the `INLINE` pragma should be replicated in the @mapAndUnzipNM@ functions below as well. -} -- | mapAndUnzipM for triples mapAndUnzip3M :: Monad m => (a -> m (b,c,d)) -> [a] -> m ([b],[c],[d]) {-# INLINE mapAndUnzip3M #-} -- Inline so that fusion with 'unzip3' and 'traverse' has a chance to fire. -- See Note [Inline @mapAndUnzipNM@ functions] above. mapAndUnzip3M f xs = unzip3 <$> traverse f xs mapAndUnzip4M :: Monad m => (a -> m (b,c,d,e)) -> [a] -> m ([b],[c],[d],[e]) {-# INLINE mapAndUnzip4M #-} -- Inline so that fusion with 'unzip4' and 'traverse' has a chance to fire. -- See Note [Inline @mapAndUnzipNM@ functions] above. mapAndUnzip4M f xs = unzip4 <$> traverse f xs mapAndUnzip5M :: Monad m => (a -> m (b,c,d,e,f)) -> [a] -> m ([b],[c],[d],[e],[f]) {-# INLINE mapAndUnzip5M #-} -- Inline so that fusion with 'unzip5' and 'traverse' has a chance to fire. -- See Note [Inline @mapAndUnzipNM@ functions] above. mapAndUnzip5M f xs = unzip5 <$> traverse f xs -- | Monadic version of mapAccumL mapAccumLM :: Monad m => (acc -> x -> m (acc, y)) -- ^ combining function -> acc -- ^ initial state -> [x] -- ^ inputs -> m (acc, [y]) -- ^ final state, outputs mapAccumLM _ s [] = return (s, []) mapAccumLM f s (x:xs) = do (s1, x') <- f s x (s2, xs') <- mapAccumLM f s1 xs return (s2, x' : xs') -- | Monadic version of mapSnd mapSndM :: Monad m => (b -> m c) -> [(a,b)] -> m [(a,c)] mapSndM _ [] = return [] mapSndM f ((a,b):xs) = do { c <- f b; rs <- mapSndM f xs; return ((a,c):rs) } -- | Monadic version of concatMap concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b] concatMapM f xs = liftM concat (mapM f xs) -- | Applicative version of mapMaybe mapMaybeM :: Applicative m => (a -> m (Maybe b)) -> [a] -> m [b] mapMaybeM f = foldr g (pure []) where g a = liftA2 (maybe id (:)) (f a) -- | Monadic version of fmap fmapMaybeM :: (Monad m) => (a -> m b) -> Maybe a -> m (Maybe b) fmapMaybeM _ Nothing = return Nothing fmapMaybeM f (Just x) = f x >>= (return . Just) -- | Monadic version of fmap fmapEitherM :: Monad m => (a -> m b) -> (c -> m d) -> Either a c -> m (Either b d) fmapEitherM fl _ (Left a) = fl a >>= (return . Left) fmapEitherM _ fr (Right b) = fr b >>= (return . Right) -- | Monadic version of 'any', aborts the computation at the first @True@ value anyM :: Monad m => (a -> m Bool) -> [a] -> m Bool anyM _ [] = return False anyM f (x:xs) = do b <- f x if b then return True else anyM f xs -- | Monad version of 'all', aborts the computation at the first @False@ value allM :: Monad m => (a -> m Bool) -> [a] -> m Bool allM _ [] = return True allM f (b:bs) = (f b) >>= (\bv -> if bv then allM f bs else return False) -- | Monadic version of or orM :: Monad m => m Bool -> m Bool -> m Bool orM m1 m2 = m1 >>= \x -> if x then return True else m2 -- | Monadic version of foldl that discards its result foldlM_ :: (Monad m, Foldable t) => (a -> b -> m a) -> a -> t b -> m () foldlM_ = foldM_ -- | Monadic version of fmap specialised for Maybe maybeMapM :: Monad m => (a -> m b) -> (Maybe a -> m (Maybe b)) maybeMapM _ Nothing = return Nothing maybeMapM m (Just x) = liftM Just $ m x -- | Monadic version of @when@, taking the condition in the monad whenM :: Monad m => m Bool -> m () -> m () whenM mb thing = do { b <- mb ; when b thing } -- | Monadic version of @unless@, taking the condition in the monad unlessM :: Monad m => m Bool -> m () -> m () unlessM condM acc = do { cond <- condM ; unless cond acc } -- | Like 'filterM', only it reverses the sense of the test. filterOutM :: (Applicative m) => (a -> m Bool) -> [a] -> m [a] filterOutM p = foldr (\ x -> liftA2 (\ flg -> if flg then id else (x:)) (p x)) (pure [])