On 15-Jul-1998, Alex Ferguson <[EMAIL PROTECTED]> wrote:
>
> Mark Jones:
> > Looking at the `size' of types is, of course, a generalization of Simon's
> > suggestion that contexts might be allowed to constrain only proper sub-
> > expressions of types in the context. (Proper sub-expressions are, by
> > definition, smaller in size.) However, it is still too strict to allow
> > useful things like: instance Monad m => Functor m where ..., so there is
> > still some work to be done ...
>
> I haven't looked at this in great detail, so forgive me if I'm talking
> through my hat, as seems quite likely: is it workable to require only
> that contexts be non-increasing in size, and that there be no manifest
> circularity where the sizes are non-decreasing? That is, rule out having
> both instance Monad m => Functor m and instance Functor m => Monad m.
> Or is that not sufficient to ensure termination of CR?
There will still be problems with things like
instance Foo [m] => Foo m
> Fergus Henderson writes of:
> > the monomorphism restriction (which exists for a similar reason,
> > to ensure termination of type inference).
>
> Is this true?
Well, I had assumed that was the reason. I could be wrong.
> The rationale normally given for it by its advocates
> (boo, hiss) seems invariably the "no re-evaluation of CAFs" mantra.
Could you elaborate about how the "no re-evaluation of CAFs" mantra
justifies the monomorphism restriction?
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger [EMAIL PROTECTED] | -- the last words of T. S. Garp.
