Hi Petr,
On Sun, Dec 9, 2012 at 7:59 AM, Petr P
The class is defined as
class (Monoid w, Monad m) => MonadWriter w m | m -> w where ...
What is the reason for the Monoid constrait? It seems superfluous to me. I recompiled the whole package without it, with no problems.
How I see it, the MTL classes are there to lift operations automatically through layers of transformers. They're just a hack to avoid having to call `lift` all the time, and aren't really designed to be used on monads other than the original WriterT. With this interpretation, the constraint makes sense -- it is simply reflecting the constraints already on the concrete monad. Chris
Of course, the Monoid constraint is necessary for most _instances_, like in
instance (Monoid w, Monad m) => MonadWriter w (Lazy.WriterT w m) where ...
but this is a different thing - it depends on how the particular instance is implemented.
I encountered the problem when I needed to define an instance where the monoidal structure is fixed (Last) and I didn't want to expose it to the user. I wanted to spare the user of of having to write Last/getLast everywhere. (I have an instance of MonadWriter independent of WriterT, its 'tell' saves values to a MVar. Functions 'listen' and 'pass' create a new temporary MVar. I can post the detail, if anybody is interested.)
Would anything break by removing the constraint? I think the type class would get a bit more general this way.
Thanks for help, Petr Pudlak
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