[boothead]

Excellent write up. One point though: watch out for WriterT and RWST leaking memory:

http://stackoverflow.com/questions/25280852/space-leak-in-pi...

It's often better to just include the writer part you need in a larder State type (lenses make this really seamless)/

[tikhonj]

That's a good point, but it was also mentioned in the article, with a link to a useful mailing list thread[1] on the matter.

It seems to be a matter of laziness. If you're writing some small piece of state like an Int, this can be a real problem. If you're writing out a lazy data structure like a list (as happens in the article), I think the extra laziness should be fine or even desired. (I've never used Writer in practice, so take my comments with a grain of salt!)

[1]: http://comments.gmane.org/gmane.comp.lang.haskell.libraries/...

[boothead]

It was? I still can't see it...

AFAIR the problem with WriterT was the use of mappend within is always lazy, regardless of the strictness of the w in WriterT w m a.

[wyager]

Yep, I mentioned this at the bottom of "monad it".