[mercury-users] number of args for bag.init exported to C

[Paul Bone]

On Tue, Jun 07, 2011 at 04:38:49PM +0200, Guillaume Yziquel wrote:
> Hi.
> 
> I'm exporting the bag.init function with the following pragma:
> 
> 	:- pragma foreign_export("C", bag.init(out), "no_props").
> 
> And the C function prototype that is exported is:
> 
> 	void no_props(MR_Word, MR_Word *);
> 
> What is the meaning of the first argument? There is no 'in' argument in
> the Mercury code, and yet it shows in the C function prototype. More
> importantly: what should I set it to?
> 
> I tried investigation the C code, but the way Mercury generates code
> with kind of virtual registers doesn't lend itself to an immediate
> understanding of the C code for bag.init.

Most likly the first argument is used to pass a type info structure.

You're creating a bag that will have the type bag(T) where T is a variable.
When this code is compiled an implicit argument is added.

bag.init(TypeInfoForT, Bag) :- ...

The easiest way to work around this is to know the type of the bag that you're
creating and create a predicate that creates that type of bag.  This way
Mercury will provide the first argument for you.  For example, I can construct
a bag of ints.

    % Note that I specify bag(int) as the type, this way the polymorphism
    % transformation is not necessary.
    %
:- pred bag_of_ints_init(bag(int)::out) is det.
:- pragma foreign_export("C", bag_of_ints_init(out), "no_props").

bag_of_ints_init(Bag) :-
    bag.init(Bag).

Hope this helps.

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