[mercury-users] Predicate call uses the "wrong" mode

[Nicholas Nethercote]

On Mon, 6 Aug 2007, Peter Schachte wrote:

> The problem is that may not be uniquely defined.  If you have
>
> 	:- mode foo(out,out) is multi.
> 	:- mode foo(in, out) is det.
> 	:- mode foo(out, in) is det.
>
> and call it in mode (in,in), which mode "most closely matches" the state of the
> variables?  The only thing you can be sure of is that it's not the first one.
> If you instead have
>
> 	:- mode foo(out,out) is multi.
> 	:- mode foo(in, out) is nondet.
> 	:- mode foo(out, in) is semidet.
>
> would you expect Mercury to prefer the semidet mode over the nondet one, and
> choose the third?  That'd be nice.

We have a similar thing in Zinc with overloading, and solve it by requiring 
that there be a version that is the glb of all the different signatures.

Eg. if you have

     f(int, float) -> float
     f(float, int) -> float

you also have to provide

     f(int, int) -> float

which is the glb of the other two.  (Zinc has automatic int-to-float 
coercions, which is why this is required.)

The question for Mercury then becomes:  what is the glb mean over modes and 
determinisms?  For 'foo' above, I would guess:

  	:- mode foo(in, in) is det.

But maybe there are other complications I'm unaware of.

Nick
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