8 Dec 19:59 2012

## mtl: Why there is "Monoid w" constraint in the definition of class MonadWriter?

Petr P <petr.mvd <at> gmail.com>

2012-12-08 18:59:25 GMT

2012-12-08 18:59:25 GMT

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.

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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