Hm. I hadn't considered the liftM2 version. It would need Monoid a instead
of Monoid r, and it would behave differently. The version I suggested is
motivated by a library for a Fran-like functional event-based programming
that I'm preparing to release, based on Cont. In the monoid def I gave
corresponds, mempty is the never-occurring event, and mappend is a sort of
union of two events.
I sure don't know how to resolve these situations of more than one credible
instance. I'm seeing more & more of them.
Thanks for the pointer on library submissions.
I'm with you about liftM0 (and liftA0).
Cheers, - Conal
On 9/8/07, Stefan O'Rear
On Sat, Sep 08, 2007 at 05:26:09PM -0700, Conal Elliott wrote:
I'd like to see the following addition to Control.Monad.Cont in mtl:
instance Monoid r => Monoid (Cont r a) where mempty = Cont mempty m `mappend` m' = Cont (runCont m `mappend` runCont m')
What's the process for suggesting, discussing, and making such changes?
http://haskell.org/haskellwiki/Library_submissions
(My 2 cents: Why not mempty = return mempty; mappend = liftM2 mappend ? Instances are best if unambiguous.)
(Why is there no liftM0 ?)
Stefan
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