Re: Callbacks in the abstact data types and extra contextual data

[Bruno Haible]

Hi Marc,

> > > a number of modules (like the hash module or the list module) allow
> > > the user to specify callbacks (e.g. a comparison function).
> > > Unfortunately, these procedures do not take a context parameter, which
> > > can be a problem because C lacks closures.
> >
> > Is this a practical, actual problem, or only a theoretical one?
> 
> It is a practical one; in a computer algebra application, I have lots
> of small entities (two words each), whose sort order is determined by
> the extra data.

OK. Then let's take the problem seriously.

I think it's time to solve the problem once and for all. I propose to add
a module that defines a function 'partial_function_last' such that, when
you have a function pointer

   int (*cmp3) (void *arg1, void *arg2, void *context)

then

   cmp2 = partial_function_last (cmp3, context);

produces a function pointer

   int (*cmp2) (void *arg1, void *arg2)

that invokes cmp3 with the given context.

Wikipedia calls it "partial function application".

The module will also have a function 'partial_function_free' that frees
a function that was constructed in this way.

Give me a couple of days to implement that.

Bruno