From bbfe463f0aa754a036439808d3b289d74f46a8bf Mon Sep 17 00:00:00 2001
From: Adithya Kumar <adithya@composewell.com>
Date: Fri, 22 Jan 2021 07:22:15 +0530
Subject: [PATCH] Add Streamly.Internal.Data.Fold.Tee

---
 Changelog.md                           |  2 +
 src/Streamly/Internal/Data/Fold/Tee.hs | 51 ++++++++++++++++++++++++++
 streamly.cabal                         |  1 +
 3 files changed, 54 insertions(+)
 create mode 100644 src/Streamly/Internal/Data/Fold/Tee.hs

diff --git a/Changelog.md b/Changelog.md
index 1efe36cab..0318b6730 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -84,6 +84,8 @@
 * A bug was fixed in the conversion of MicroSecond64 and MilliSecond64
   (commit e5119626)
 * Bug fix: classifySessionsBy now flushes sessions at the end and terminates.
+* `Streamly.Internal.Data.Fold.Tee` module added for folds with tee-like
+  applicative behaviour.
 
 ## 0.7.2
 
diff --git a/src/Streamly/Internal/Data/Fold/Tee.hs b/src/Streamly/Internal/Data/Fold/Tee.hs
new file mode 100644
index 000000000..a73c4f787
--- /dev/null
+++ b/src/Streamly/Internal/Data/Fold/Tee.hs
@@ -0,0 +1,51 @@
+-- |
+-- Module      : Streamly.Internal.Data.Fold.Tee
+-- Copyright   : (c) 2020 Composewell Technologies
+-- License     : BSD-3-Clause
+-- Maintainer  : streamly@composewell.com
+-- Stability   : experimental
+-- Portability : GHC
+--
+module Streamly.Internal.Data.Fold.Tee
+    ( Tee(..)
+    , fromFold
+    , toFold
+    )
+where
+
+import Data.Coerce (coerce)
+import Streamly.Internal.Data.Fold.Types (Fold)
+
+import qualified Streamly.Internal.Data.Fold.Types as Fold
+
+-- | The type @Tee m a b@ represents a left fold over an input stream of values
+-- of type @a@ to a single value of type @b@ in 'Monad' @m@.
+--
+-- @Tee@ is a wrapper over 'Fold' that uses 'teeWith' to define the applicative
+-- instance.
+--
+-- /Internal/
+newtype Tee m a b =
+    Tee { runTee :: Fold m a b }
+    deriving (Functor)
+
+-- | Convert a 'Tee' to 'Fold'.
+{-# INLINE toFold #-}
+toFold :: Tee m a b -> Fold m a b
+toFold = coerce
+
+-- | Convert a 'Fold' to 'Tee'.
+{-# INLINE fromFold #-}
+fromFold :: Fold m a b -> Tee m a b
+fromFold = coerce
+
+-- | The 'Tee' resulting from '<*>' distributes its input to both the argument
+-- 'Tee's and combines their output using function application.
+--
+instance Monad m => Applicative (Tee m a) where
+
+    {-# INLINE pure #-}
+    pure a = fromFold (Fold.yield a)
+
+    {-# INLINE (<*>) #-}
+    (<*>) a b = fromFold (Fold.teeWith ($) (toFold a) (toFold b))
diff --git a/streamly.cabal b/streamly.cabal
index bb60e4717..b2f7b785d 100644
--- a/streamly.cabal
+++ b/streamly.cabal
@@ -468,6 +468,7 @@ library
 
                      -- streamly-core
                      , Streamly.Internal.Data.Unfold
+                     , Streamly.Internal.Data.Fold.Tee
                      , Streamly.Internal.Data.Fold
                      , Streamly.Internal.Data.Sink
                      , Streamly.Internal.Data.Parser