The lazy Writer monad

I found an interesting example of the lazy State monad that uses "head recursion".

I tried to adapt the example for the lazy Writer monad but it proved difficult. If you naively consume the head of the infinite accumulator, it will hang, even with the lazy version of the monad:

	import Control.Monad.Writer.Lazy
	import Data.Monoid
	lazywr :: Writer [()] ()
	lazywr = lazywr >> tell [()]
	-- hangs
	main :: IO ()
	main = putStrLn
	. show
	. take 1
	. snd
	. runWriter
	$ lazywr
	import Control.Monad.Writer.Lazy
	import Data.Monoid
	lazywr :: Writer (Dual [()]) ()
	lazywr = lazywr >> tell (Dual [()])
	-- works ok
	main :: IO ()
	main = putStrLn
	. show
	. take 1
	. getDual
	. snd
	. runWriter
	$ lazywr

The trick is to use Dual from Data.Monoid:

	import Control.Monad.Writer.Lazy
	import Data.Monoid
	lazywr :: Writer [()] ()
	lazywr = lazywr >> tell [()]
	-- hangs
	main :: IO ()
	main = putStrLn
	. show
	. take 1
	. snd
	. runWriter
	$ lazywr
	import Control.Monad.Writer.Lazy
	import Data.Monoid
	lazywr :: Writer (Dual [()]) ()
	lazywr = lazywr >> tell (Dual [()])
	-- works ok
	main :: IO ()
	main = putStrLn
	. show
	. take 1
	. getDual
	. snd
	. runWriter
	$ lazywr

Maybe I suffer from a failure of imagination, but I can't think of any practical problem in which the lazy Writer monad offers an advantage over the strict version.

process-streaming 0.3.0.0

I have released version 0.3.0.0 of process-streaming, my library of pipes-based helpers built on top of the process package.

It contains breaking changes. Some functions have new names, and a few newtypes have been introduced in order to hide unnecessarily complex signatures. Hopefully the changes make the library a bit more intuitive.

You might find this library useful if:

• You want an easy way to consume the standard streams of an external process using the tools provided by the pipes ecosystem.
• You want concurrent, streaming access to both the stdout and stderr of an external process, without fear of deadlocks caused by full output buffers.
• You want to consume stdout and stderr combined in a single stream.
• You want to be relieved of tedious bookkeeping like: automatically killing the external process on the face of errors and exceptions, ensuring the termination of threads spawned for concurrent reads, and so on.