Re: [PATCH] Change module interface to no longer use GMP objects directly.

[Paul Eggert <eggert@cs.ucla.edu>]

On 11/20/19 1:06 PM, Philipp Stephani wrote:
> GMP seems to make a very complex series of choices when selecting the
> limb datatype, which we can't and shouldn't replicate.

We don't need to replicate GMP's reasoning. We can use a 'configure' 
time test to use whatever type GMP uses. The emacs-module header could 
be quite simple, and just use that type (typically unsigned long, but 
not always) when typedeffing emacs_limb_t. I'll write the configure-time 
code, so you don't need to worry about implementing it (not that it's 
hard to write).

> copy_string_contents
> returns false if and only if it has signaled an error, which is a
> quite useful invariant.

No, copy_string_contents never returns false. If it signals an error, it 
does not return. If it returns, it always returns true. It's pretty 
weird, but there it is.

>> I found the guarantee useful in the first function I wrote that
>> attempted to use the proposed API, so that's good evidence that it's
>> worth documenting.
> 
> I don't think so, it's a very narrow guarantee that we also don't give
> for other functions.

I don't think any of the other functions need such a guarantee (but if 
they do, we should provide it).

> Each guarantee, however minor, comes with a cost:
> we need to document it and stick to it forever.

This particular guarantee has so little cost that the cost is not worth 
worrying about. It's a guarantee we'll never want to violate and there's 
essentially no cost to sticking to it "forever". I'll write the 
one-sentence documentation, if that helps.