Cont as Monoid

Conal Elliott conal at
Sun Sep 9 00:42:56 EDT 2007

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 <stefanor at> wrote:
> 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?
> (My 2 cents: Why not mempty = return mempty; mappend = liftM2 mappend ?
> Instances are best if unambiguous.)
> (Why is there no liftM0 ?)
> Stefan
> Version: GnuPG v1.4.6 (GNU/Linux)
> iD8DBQFG40G1FBz7OZ2P+dIRAiIgAJ0bhthqmBvcpoqLnNF+RcbccUBKYACeO4QO
> sJ9sUvnkvkfZsgIBbgq6r9U=
> =cCFe
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