[m-dev.] discussion about the implementation of compact type representations

[Julien Fischer]

Hi,

On Tue, 31 Oct 2017, Zoltan Somogyi wrote:

>> > - Specifying a reserve tag pragma for them should be an error.
>> 
>> Never used reserved tags myself.
>
> They were added for HAL and then for G12. Does G12 still use them?
> Julien?

G12 has *never* used them.  (HAL did, but that's no longer an issue.)
I would be quite happy to get rid of them.

>> > - Specifying a foreign_type tag pragma for them should be an error.
>> 
>> A small number of types in the standard library will need to be
>> trivially changed (io.file_id, dir.stream, etc.).
>> User code should be less likely to be broken by this change.
>
> If a type has a foreign_type pragma specified for it on *all* backends,
> then the language reference manual allows the programmer to omit
> giving a Mercury definition at all.
>
> If there are backends on which these types do *not* have a foreign type,
> then we should fix that omission, since a dummy Mercury type clearly
> won't work for them.

See my response you original post; to my mind there is a reasonable
use case for such types (i.e. where you have code that only has
to be functional for some target languages but compile in all of them.)

Julien.