Blaze.ByteString.Builder and Writer Monad

Blaze-ByteString-Builder-Example.hs

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Blaze.ByteString.Builder (toLazyByteString, Builder)
import Blaze.ByteString.Builder.ByteString (fromByteString)
import Data.String (IsString)
import Control.Monad.IO.Class (liftIO)
import Control.Monad.Writer (execWriterT, execWriter, WriterT, Writer, MonadWriter, tell)

import qualified Data.ByteString.Char8      as BS (getLine)
import qualified Data.ByteString.Lazy.Char8 as BL (putStrLn)

main :: IO ()
main = do
  BL.putStrLn "Writer"
  putBuilder $ execWriter logging

  BL.putStrLn "Writer IO"
  s <- execWriterT loggingIO
  putBuilder s

putBuilder :: Builder -> IO ()
putBuilder = BL.putStrLn . toLazyByteString

type MyWriter   = Writer  Builder
type MyWriterIO = WriterT Builder IO

logging :: MyWriter ()
logging = do
  tellLn "hoge 1"
  tellLn "hoge 2"
  tellLn "hoge 3"
  return ()

loggingIO :: MyWriterIO ()
loggingIO = do
  tellLn "hoge 1"

  liftIO $ BL.putStrLn "hihi"
  line <- liftIO $ fromByteString <$> BS.getLine
  tellLn line

  tellLn "hoge 2"
  tellLn "hoge 3"
  return ()

tellLn :: (IsString w, MonadWriter w m) => w -> m ()
tellLn w = do
  tell w
  tell "\n"
  return ()