NoMonomorphismRestriction

[Ravi Nanavati]

On Fri, Aug 7, 2009 at 1:04 PM, Simon Peyton-Jones<simonpj at microsoft.com> wrote:
> | . Understanding how to respond to type inference and error messages is
> | hard enough without having additional differences in innocent-looking
> | code.  Do you think my hope is reasonable that not-generalizing could
> | lead to better error messages?
>
> I don't think it's obvious one way or the other. We'll have to see.

The Bluespec experience with local bindings (complicated by our
"imperative desugaring", but my opinion is that's mostly an irrelevant
complication) is that automatically generalized local bindings create
all sorts of user-level havoc: poor error messages for one, programs
that surprisingly typecheck or have other surprising behaviors and so
on. And I wouldn't blame these issues on Bluespec's desire to hide the
type system details from our users - when some sort of
polymorphism-related problem is discovered it is common for people at
Bluespec itself to be confused and have trouble isolating the
underlying issue.

Based on that experience, I'd expect that removing the automatic
generalization of local bindings would lead to many positive
user-level side-effects that are independent of any implementation
simplifications that result. In fact, when I first heard the
suggestion (via Joe Stoy) I started looking for an excuse to end (or
at least limit) Bluespec's generalization of local bindings, since I
think the backwards compatibility issues that result will be more than
worth it in the long run.

 - Ravi