[m-dev.] for review: rewrite of check_typeclass.m

[David Glen JEFFERY]

On 21-May-1998, Peter Schachte <pets at students.cs.mu.oz.au> wrote:
> Hi DJ,
> 
> On Wed, 20 May 1998, David Glen JEFFERY wrote:
> 
> > A re-work on check_typeclass.m. Now, rather than the ad-hoc approach of 
> > checking each method of each instance declaration for *exact* type and mode
> > correctness, we generate a new predicate that gets inserted into the HLDS.
> > This predicate is checked for type, mode, uniqueness and determinism 
> > correctness in the appropriate compiler passes.
> 
> Does this mean that type class methods can now be impure or semipure?  That
> should probably be allowed.  Instances should be checked that the
> corresponding predicates are no less pure than the class specifies (but more
> pure should be ok, I think).

Hmmm. I don't think it will work at the moment, but it shouldn't be too much
work to get it working. 

Actually, I just tried it. For the following:

:- module x.

:- interface.

:- typeclass c(T) where [
        pred p(T::in) is det
].

:- instance c(int) where [
        pred(p/1) is my_p
].

:- impure pred my_p(int::in) is det.

:- implementation.

:- pragma c_code(my_p(X::in), "fprintf(stderr, ""Hello\n""); X = X;").

We get no error message. Now that I think about it, the reason would be
that the "optimisation" that I talked about before gets turned on. So... if
you fool the compiler like this:

:- module x.

:- interface.

:- typeclass c(T) where [
        pred p(T::in) is det
].

:- instance c(int) where [
        pred(p/1) is my_p
].

:- impure pred my_p(T::in) is det.

:- implementation.

:- pragma c_code(my_p(X::in), "fprintf(stderr, ""Hello\n""); X = X;").



Then we get the error message:

	In call to impure predicate `x:my_p/1':
	  error: call must be preceded by `impure' indicator.
	In predicate `x:Introduced predicate for x:c(int_0_) method x:p/1':
	  error: predicate is impure.
	  It must be declared `impure' or promised pure.


The lesson? The optimisation is evil. The source-to-source transformation is
good.


love and cuddles,
dgj
-- 
David Jeffery (dgj at cs.mu.oz.au) |  Marge: Did you just call everyone "chicken"?
MEngSc student,                 |  Homer: Noooo.  I swear on this Bible!
Department of Computer Science  |  Marge: That's not a Bible; that's a book of
University of Melbourne         |         carpet samples!
Australia                       |  Homer: Ooooh... Fuzzy.