* Edward Z. Yang <ezy...@mit.edu> [2012-12-08 14:18:38-0800]
> Excerpts from Roman Cheplyaka's message of Sat Dec 08 14:00:52 -0800 2012:
> > * Edward Z. Yang <ezy...@mit.edu> [2012-12-08 11:19:01-0800]
> > > The monoid instance is necessary to ensure adherence to the monad laws.
> >
> > This doesn't make any sense to me. Are you sure you're talking about the
> > MonadWriter class and not about the Writer monad?
>
> Well, I assume the rules for Writer generalize for MonadWriter, no?
>
> Here's an example. Haskell monads have the associativity law:
>
> (f >=> g) >=> h === f >=> (g >=> h)
>
> From this, we can see that
>
> (m1 >> m2) >> m3 === m1 >> (m2 >> m3)
>
> Now, consider tell. We'd expect it to obey a law like this:
>
> tell w1 >> tell w2 === tell (w1 <> w2)
First of all, I don't see why two tells should be equivalent to one
tell. Imagine a MonadWriter that additionally records the number of
times 'tell' has been called. (You might argue that your last equation
should be a MonadWriter class law, but that's a different story — we're
talking about the Monad laws here.)
Second, even *if* the above holds (two tells are equivalent to one
tell), then there is *some* function f such that
tell w1 >> tell w2 == tell (f w1 w2)
It isn't necessary that f coincides with mappend, or even that the type
w is declared as a Monoid at all. The only thing we can tell from the
Monad laws is that that function f should be associative.
Roman
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