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Add filterM #120

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7 changes: 7 additions & 0 deletions Data/Sequence.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -130,6 +130,7 @@ module Data.Sequence (
-- * Transformations
mapWithIndex, -- :: (Int -> a -> b) -> Seq a -> Seq b
reverse, -- :: Seq a -> Seq a
filterM, -- :: Applicative f => (a -> f Bool) -> Seq a -> f (Seq a)
-- ** Zips
zip, -- :: Seq a -> Seq b -> Seq (a, b)
zipWith, -- :: (a -> b -> c) -> Seq a -> Seq b -> Seq c
Expand Down Expand Up @@ -488,6 +489,12 @@ thin12 (Deep s pr m (Three a b c)) = Deep s pr (thin $ m `snocTree` node2 a b) (
thin12 (Deep s pr m (Four a b c d)) = Deep s pr (thin $ m `snocTree` node2 a b) (Two c d)
thin12 _ = error "thin12 expects a Deep FingerTree."

filterM :: Applicative f => (a -> f Bool) -> Seq a -> f (Seq a)
filterM p = foldr go (pure empty)
where
go x r = f <$> p x <*> r
where
f flg ys = if flg then x <| ys else ys

instance MonadPlus Seq where
mzero = empty
Expand Down