[mercury-users] passing user-defined types between Mercury and C

[Fergus Henderson]

Terrence Brannon <princepawn at earthlink.net> wrote:

> Says the manual:
> 
>  Mercury types are converted to C types according to the rules in
>  section Passing data to and from C.
> 
> I looked at that section, but it didn't say or give examples on how to
> pass and return user-defined datatypes.

That section says:

| Mercury variables of any other type are passed
| as a `MR_Word', which in the current implementation is a typedef for an
| unsigned type whose size is the same size as a pointer. (Note: ...)

Mercury variables with user-defined types should be treated as
abstract data types when you are accessing them from C.
You should treat `MR_Word' as an opaque type.  Any operations that you
want to perform on the Mercury type should be done in Mercury code.
If you want to construct or deconstruct the type from C code, then you
should write Mercury procedures to do the construction or deconstruction
and export them to C (using `pragma export').

P.S. Terrance, your computer's clock is not set correctly, if the "Date:"
field in your email is any indication.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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