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Diffstat (limited to 'compiler/GHC/Utils/Monad.hs')
| -rw-r--r-- | compiler/GHC/Utils/Monad.hs | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/compiler/GHC/Utils/Monad.hs b/compiler/GHC/Utils/Monad.hs index c5e24794a4..d91570223c 100644 --- a/compiler/GHC/Utils/Monad.hs +++ b/compiler/GHC/Utils/Monad.hs @@ -19,6 +19,8 @@ module GHC.Utils.Monad , anyM, allM, orM , foldlM, foldlM_, foldrM , whenM, unlessM + , filterOutM + , partitionM ) where ------------------------------------------------------------------------------- @@ -226,6 +228,19 @@ 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 []) + +-- | Monadic version of @partition@ +partitionM :: Monad m => (a -> m Bool) -> [a] -> m ([a], [a]) +partitionM _ [] = pure ([], []) +partitionM f (x:xs) = do + res <- f x + (as,bs) <- partitionM f xs + pure ([x | res]++as, [x | not res]++bs) + {- Note [The one-shot state monad trick] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Summary: many places in GHC use a state monad, and we really want those |