S.D.Mechveliani writes:
> Tom Pledger <[EMAIL PROTECTED]> writes
>
> > May we please have a partial order class in Haskell at some stage? It
> > can be done outside the prelude (see below), but would be nicer inside
> > (see below, removing all the tildes and primes).
> >
> [...]
>
>
> Year and half ago all these things, and many others, were suggested
> in
> http://www.botik.ru/pub/local/Mechveliani/basAlgPropos/
>
> There the partial ordering was compare_m :: a -> a -> Maybe Ordering
Hi.
Thanks for the pointer. I apologise for not remembering it.
For the partial comparison function, Maybe Ordering is a better return
type than PartialOrdering.
An afterthought:
The partial order class should be derivable, if it becomes the home
of (<=). The derivation should be like Ord's, except that when a
comparison of constituent values returns Nothing, the overall
comparison also returns Nothing. That way, there's the least
disruption to existing types which derive Ord.
Regards,
Tom
