Speed up MonadUtils.mapMaybeM

Summary: This version is nearly 2x faster according to a few small benchmarks.

Reviewers: bgamari, monoidal

Reviewed By: monoidal

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5344

1 files changed, 4 insertions, 5 deletions

diff --git a/compiler/utils/MonadUtils.hs b/compiler/utils/MonadUtils.hs
index e86bc49708..8f40f88ba9 100644
--- a/compiler/utils/MonadUtils.hs
+++ b/compiler/utils/MonadUtils.hs
@@ -30,8 +30,6 @@ module MonadUtils
 
 import GhcPrelude
 
-import Maybes
-
 import Control.Applicative
 import Control.Monad
 import Control.Monad.Fix
@@ -144,9 +142,10 @@ mapSndM f ((a,b):xs) = do { c <- f b; rs <- mapSndM f xs; return ((a,c):rs) }
 concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b]
 concatMapM f xs = liftM concat (mapM f xs)
 
--- | Monadic version of mapMaybe
-mapMaybeM :: (Monad m) => (a -> m (Maybe b)) -> [a] -> m [b]
-mapMaybeM f = liftM catMaybes . mapM f
+-- | 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)