[PATCH] generalize filterM, mapAndUnzipM, zipWithM, zipWithM_, replicateM, replicateM_
M Farkas-Dyck
strake888 at gmail.com
Mon Mar 16 23:29:19 UTC 2015
---
libraries/base/Control/Monad.hs | 37 ++++++++++++++++---------------------
1 file changed, 16 insertions(+), 21 deletions(-)
diff --git a/libraries/base/Control/Monad.hs b/libraries/base/Control/Monad.hs
index 6fa4a07..02eabd1 100644
--- a/libraries/base/Control/Monad.hs
+++ b/libraries/base/Control/Monad.hs
@@ -75,9 +75,9 @@ module Control.Monad
, (<$!>)
) where
-import Data.Foldable ( Foldable, sequence_, msum, mapM_, foldlM, forM_ )
-import Data.Functor ( void )
-import Data.Traversable ( forM, mapM, sequence )
+import Data.Functor ( void, (<$>) )
+import Data.Foldable ( Foldable, sequence_, sequenceA_, msum, mapM_, foldlM, forM_ )
+import Data.Traversable ( forM, mapM, traverse, sequence, sequenceA )
import GHC.Base hiding ( mapM, sequence )
import GHC.List ( zipWith, unzip, replicate )
@@ -94,13 +94,8 @@ guard False = empty
-- | This generalizes the list-based 'filter' function.
{-# INLINE filterM #-}
-filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
-filterM p = foldr go (return [])
- where
- go x r = do
- flg <- p x
- ys <- r
- return (if flg then x:ys else ys)
+filterM :: (Applicative m) => (a -> m Bool) -> [a] -> m [a]
+filterM p = foldr (\ x -> liftA2 (\ flg -> if flg then (x:) else id) (p x)) (pure [])
infixr 1 <=<, >=>
@@ -125,19 +120,19 @@ forever a = let a' = a >> a' in a'
-- | The 'mapAndUnzipM' function maps its first argument over a list, returning
-- the result as a pair of lists. This function is mainly used with complicated
-- data structures or a state-transforming monad.
-mapAndUnzipM :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
+mapAndUnzipM :: (Applicative m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
{-# INLINE mapAndUnzipM #-}
-mapAndUnzipM f xs = sequence (map f xs) >>= return . unzip
+mapAndUnzipM f xs = unzip <$> traverse f xs
--- | The 'zipWithM' function generalizes 'zipWith' to arbitrary monads.
-zipWithM :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
+-- | The 'zipWithM' function generalizes 'zipWith' to arbitrary applicative functors.
+zipWithM :: (Applicative m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
{-# INLINE zipWithM #-}
-zipWithM f xs ys = sequence (zipWith f xs ys)
+zipWithM f xs ys = sequenceA (zipWith f xs ys)
-- | 'zipWithM_' is the extension of 'zipWithM' which ignores the final result.
-zipWithM_ :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m ()
+zipWithM_ :: (Applicative m) => (a -> b -> m c) -> [a] -> [b] -> m ()
{-# INLINE zipWithM_ #-}
-zipWithM_ f xs ys = sequence_ (zipWith f xs ys)
+zipWithM_ f xs ys = sequenceA_ (zipWith f xs ys)
{- | The 'foldM' function is analogous to 'foldl', except that its result is
encapsulated in a monad. Note that 'foldM' works from left-to-right over
@@ -175,18 +170,18 @@ foldM_ f a xs = foldlM f a xs >> return ()
-- | @'replicateM' n act@ performs the action @n@ times,
-- gathering the results.
-replicateM :: (Monad m) => Int -> m a -> m [a]
+replicateM :: (Applicative m) => Int -> m a -> m [a]
{-# INLINEABLE replicateM #-}
{-# SPECIALISE replicateM :: Int -> IO a -> IO [a] #-}
{-# SPECIALISE replicateM :: Int -> Maybe a -> Maybe [a] #-}
-replicateM n x = sequence (replicate n x)
+replicateM n x = sequenceA (replicate n x)
-- | Like 'replicateM', but discards the result.
-replicateM_ :: (Monad m) => Int -> m a -> m ()
+replicateM_ :: (Applicative m) => Int -> m a -> m ()
{-# INLINEABLE replicateM_ #-}
{-# SPECIALISE replicateM_ :: Int -> IO a -> IO () #-}
{-# SPECIALISE replicateM_ :: Int -> Maybe a -> Maybe () #-}
-replicateM_ n x = sequence_ (replicate n x)
+replicateM_ n x = sequenceA_ (replicate n x)
-- | The reverse of 'when'.
unless :: (Applicative f) => Bool -> f () -> f ()
--
2.3.1
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