Pipes-based logger

PipeLog.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module PipeLog where

import           Control.Applicative
import           Data.Functor.Identity
import           Pipes
import qualified Pipes.Prelude as P

newtype LogT l m a =
  LogT { runLog :: Producer l m a }
  deriving ( Functor, Applicative, Monad, MonadTrans, MonadIO, MFunctor )

type Log l a = LogT l Identity a

say :: Monad m => l -> LogT l m ()
say = LogT . yield

-- Not the recommended way to perform logging.
produceLog :: Monad m => LogT l m () -> m [l]
produceLog = P.toListM . runLog

runLog :: Monad m 
       => (l -> m ())    -- the logging function (consider m ~ IO)
       -> LogT l m a
       -> m a
runLog log (Log p) = runEffect (for p (lift . log))