From 814570f739399a92cf181e12660e610679a2d801 Mon Sep 17 00:00:00 2001
From: David Feuer <David.Feuer@gmail.com>
Date: Tue, 30 Dec 2014 11:42:39 -0500
Subject: [PATCH] Add filterM

Thus far, this proposal has gotten good reception on the libraries
list.
---
 Data/Sequence.hs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Data/Sequence.hs b/Data/Sequence.hs
index 0a64c3e85..094046b40 100644
--- a/Data/Sequence.hs
+++ b/Data/Sequence.hs
@@ -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
@@ -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