From d5e433ecd3857ed0d5ed71c0769e4caf7b84aed8 Mon Sep 17 00:00:00 2001
From: Ranjeet Kumar Ranjan <ranjeetr@composewell.com>
Date: Fri, 15 Jul 2022 17:20:45 +0530
Subject: [PATCH 1/2] Add Tail module

---
 src/Streamly/Coreutils/Tail.hs | 39 ++++++++++++++++++++++++++++++++++
 streamly-coreutils.cabal       |  2 ++
 2 files changed, 41 insertions(+)
 create mode 100644 src/Streamly/Coreutils/Tail.hs

diff --git a/src/Streamly/Coreutils/Tail.hs b/src/Streamly/Coreutils/Tail.hs
new file mode 100644
index 00000000..f8db98fe
--- /dev/null
+++ b/src/Streamly/Coreutils/Tail.hs
@@ -0,0 +1,39 @@
+-- |
+-- Module      : Streamly.Coreutils.Tail
+-- Copyright   : (c) 2022 Composewell Technologies
+-- License     : Apache-2.0
+-- Maintainer  : streamly@composewell.com
+-- Stability   : experimental
+-- Portability : GHC
+--
+-- Output the last part of files.
+
+module Streamly.Coreutils.Tail
+    (tail, tailN)
+where
+
+import Control.Monad.IO.Class (MonadIO(liftIO))
+import Data.Function ((&))
+import Streamly.Prelude (SerialT)
+import System.IO (IOMode(..), openFile)
+
+import qualified Streamly.FileSystem.Handle as File
+import qualified Streamly.Internal.Data.Array as Array
+import qualified Streamly.Internal.Data.Stream.IsStream as Stream
+import qualified Streamly.Internal.Data.Fold as FL
+import qualified Streamly.Internal.Unicode.Stream as Unicode
+
+import Prelude hiding (tail)
+
+tailN :: Int -> FilePath -> SerialT IO [Char]
+tailN n fp = do
+    fh <- liftIO $ openFile fp ReadMode
+    Stream.unfold File.read fh
+        & Unicode.decodeUtf8
+        & Unicode.lines FL.toList
+        & Stream.fold (Array.writeLastN n)
+        & Stream.map Array.toStream
+        & Stream.concat
+
+tail :: FilePath -> SerialT IO [Char]
+tail = tailN 10
diff --git a/streamly-coreutils.cabal b/streamly-coreutils.cabal
index fa31b395..06aadff5 100644
--- a/streamly-coreutils.cabal
+++ b/streamly-coreutils.cabal
@@ -92,6 +92,7 @@ library
     , unix >= 2.7.0 && < 2.8
     , directory >= 1.2.2 && < 1.4
     , data-default-class >= 0.1 && < 0.2
+    , monad-control    >= 1.0.3 && < 1.0.4
   hs-source-dirs:      src
   exposed-modules:
       Streamly.Coreutils
@@ -99,6 +100,7 @@ library
     , Streamly.Coreutils.Cp
     , Streamly.Coreutils.FileTest
     , Streamly.Coreutils.ShellWords
+    , Streamly.Coreutils.Tail
     , Streamly.Coreutils.Uniq
     , Streamly.Coreutils.Which
 

From be9bef5c530d83376963b6308c5d142955eb477c Mon Sep 17 00:00:00 2001
From: Ranjeet Kumar Ranjan <ranjeetr@composewell.com>
Date: Fri, 15 Jul 2022 18:11:09 +0530
Subject: [PATCH 2/2] Add follow option

---
 src/Streamly/Coreutils/Tail.hs | 45 +++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 6 deletions(-)

diff --git a/src/Streamly/Coreutils/Tail.hs b/src/Streamly/Coreutils/Tail.hs
index f8db98fe..edcc8fee 100644
--- a/src/Streamly/Coreutils/Tail.hs
+++ b/src/Streamly/Coreutils/Tail.hs
@@ -9,7 +9,14 @@
 -- Output the last part of files.
 
 module Streamly.Coreutils.Tail
-    (tail, tailN)
+
+    ( tail
+
+    -- * Options
+    , Tail
+    , follow
+    , tailN
+    )
 where
 
 import Control.Monad.IO.Class (MonadIO(liftIO))
@@ -24,16 +31,42 @@ import qualified Streamly.Internal.Data.Fold as FL
 import qualified Streamly.Internal.Unicode.Stream as Unicode
 
 import Prelude hiding (tail)
+import Streamly.Coreutils.Common (Switch(..))
+
+newtype Tail = Tail{tailFollow :: Switch}
+
+defaultConfig :: Tail
+defaultConfig = Tail Off
 
-tailN :: Int -> FilePath -> SerialT IO [Char]
-tailN n fp = do
+follow :: Switch -> Tail -> Tail
+follow opt cfg = cfg {tailFollow = opt}
+
+tailForEver :: Int -> FilePath -> SerialT IO [Char]
+tailForEver n fp = do
     fh <- liftIO $ openFile fp ReadMode
-    Stream.unfold File.read fh
+    Stream.concatMap Stream.fromList
+        $ Stream.repeatM
+        $ Stream.toList
+        $ Stream.unfold File.read fh
         & Unicode.decodeUtf8
         & Unicode.lines FL.toList
         & Stream.fold (Array.writeLastN n)
         & Stream.map Array.toStream
         & Stream.concat
 
-tail :: FilePath -> SerialT IO [Char]
-tail = tailN 10
+tailN :: (Tail -> Tail) ->  Int -> FilePath -> SerialT IO [Char]
+tailN f n fp = do
+    fh <- liftIO $ openFile fp ReadMode
+    let opt = f defaultConfig
+    case tailFollow opt of
+        On -> tailForEver n fp
+        Off ->
+            Stream.unfold File.read fh
+                & Unicode.decodeUtf8
+                & Unicode.lines FL.toList
+                & Stream.fold (Array.writeLastN n)
+                & Stream.map Array.toStream
+                & Stream.concat
+
+tail :: (Tail -> Tail) -> FilePath -> SerialT IO [Char]
+tail f = tailN f 10