[mercury-users] using univ with C

[Mark Brown]

On 05-Jun-2008, QRL_Charles QRL_Gretton <g12.charles.gretton at gmail.com> wrote:
> I'm not trying to store a list (or array) of words in C. The C macro
> that gives me different "pushes" for different types looks as follows
> -- i.e., I want the C code to be able to reason about the type of the
> contents.
> 
> 
> #define __IMPLEMENT__POLYMORPHIC_LIST_IN_C__PUSH(TYPE_NAME,
> FUNCTION_NAME) struct polymorphic_list_in_c* \
>     FUNCTION_NAME(struct polymorphic_list_in_c* list, \
> 				     void* _data_element){		\
>   TYPE_NAME* data_element = (TYPE_NAME*)_data_element; \
>   TYPE_NAME* list_data;	       \
> ...			
> 
> 
> In the end, I expect the code I'm looking for will occure something like:
> 
> 
> :- pragma foreign_proc("C",
>      push_back_c_vector(Univ::in, Vector_In::in,S0::mdi, S::muo),
>      [promise_pure],
> "
>                      Vector_In->push_back(Vector_In, Univ->the_actual_data);
>                      S = S0;
>  ").

Simply make push_back_c_vector polymorphic:

:- pred push_back_c_vector(T::in, c_vector::in, int_store::mdi,
	int_store::muo) is det.

When calling it, access the value of the univ with univ_value.

To interpret the *type* stored in the univ, use univ_type and the predicates
and functions in the type_desc library module.  Alternatively, you can use
univ_to_type to pick out the types you know about.

Cheers,
Mark.

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