Add the "echo" command

src/Streamly/Coreutils/Echo.hs

@@ -0,0 +1,65 @@
+-- |
+-- Module      : Streamly.Coreutils.Echo
+-- Copyright   : (c) 2021 Composewell Technologies
+-- License     : Apache-2.0
+-- Maintainer  : streamly@composewell.com
+-- Stability   : experimental
+-- Portability : GHC
+--
+
+module Streamly.Coreutils.Echo
+    (
+    )
+where
+
+import Data.Word (Word8)
+import Streamly.Data.Array.Foreign (Array)
+import Streamly.Prelude (SerialT)
+
+-- | A convenient way to convert data types to bytes that can be displayed on
+-- console, sent to network or written to files. Basically serialize the data
+-- types to byte stream.
+--
+class IsBytes a where
+    bytes :: a -> IO ()
+
+-- XXX Move this to Console.Stdio?
+-- | A convenient way to display data types on console.
+class IsEcho a where
+    echo :: a -> IO ()
+
+-- instances for all serializable types?
+-- Any type that can be converted to a byte stream
+
+-- Char sequences
+instance IsEcho String where
+    echo = putStrLn
+
+instance IsEcho (SerialT m Char) where
+    echo = undefined
+
+instance IsEcho (Array Char) where
+    echo = undefined
+
+-- Word8 sequences
+instance IsEcho [Word8] where
+    echo = undefined
+
+instance IsEcho (SerialT m Word8) where
+    echo = undefined
+
+instance IsEcho (Array Word8) where
+    echo = undefined
+
+-- Nested
+instance IsEcho [String] where
+    echo = putStrLn . concat
+
+instance IsEcho (SerialT m (Array Word8)) where
+    echo = undefined
+
+instance IsEcho (SerialT m (SerialT m Word8)) where
+    echo = undefined
+
+instance IsEcho [SerialT m Word8] where
+    echo = undefined