Add replicateM operation

Changelog.md

@@ -1,7 +1,7 @@
 ## Unreleased
 
 ### Enhancements
-* Add the `scan` operation for left scan of a stream
+* Add `replicateM`, `scan` stream operations
 * Improve performance of some stream operations (`foldl`, `dropWhile`)
 
 ### Bug Fixes

src/Streamly/Prelude.hs

@@ -59,6 +59,7 @@ module Streamly.Prelude
     , mapM
     , mapM_
     , sequence
+    , replicateM
 
     -- * Zipping
     , zipWith
@@ -431,6 +432,14 @@ sequence m = fromStream $ go (toStream m)
             yield a (Just x) = a >>= \b -> yld b (Just (go x))
          in (runStream m1) Nothing stop yield
 
+replicateM :: (Streaming t, Monad m) => Int -> m a -> t m a
+replicateM n m = fromStream $ go n
+    where
+    go cnt = Stream $ \_ stp yld ->
+        if cnt <= 0
+        then stp
+        else m >>= \a -> yld a (Just $ go (cnt - 1))
+
 ------------------------------------------------------------------------------
 -- Serially Zipping Streams
 ------------------------------------------------------------------------------

test/Main.hs

@@ -4,6 +4,7 @@
 module Main (main) where
 
 import Control.Concurrent (threadDelay)
+import Control.Monad (replicateM)
 import Data.Foldable (forM_)
 import Data.List (sort)
 import Test.Hspec
@@ -637,6 +638,14 @@ mixedOps = do
 streamOperations :: Streaming t => (t IO Int, [Int], Int) -> Spec
 streamOperations (stream, list, len) = do
 
+    -- Generation
+    it "replicateM" $ do
+            let x = return (1 :: Int)
+            str <- A.toList . serially $ A.replicateM len x
+            lst <- replicateM len x
+            return $ str == lst
+        `shouldReturn` True
+
     -- Filtering
     it "filter all out" $ transform (A.filter (> len)) (filter (> len))
     it "filter all in"  $ transform (A.filter (<= len)) (filter (<= len))