Parametric attributes

[Timon Gehr]

On 10/2/24 02:43, Manu wrote:
> I feel like we really need parametric attributes...
> ...

I think so, too.

> I constantly have situations like this:
>    void f(CallbackFun cb) { cb(); }
> Where the attribute-ness of a function depends on the attribute-ness of 
> the callback.
> 
> I want my function to be @nogc, but I don't want to require the user 
> supply a @nogc callback to use my API.
> 
> If we're going to have all these attributes, then we really need 
> something like this:
>    void f(CallbackFun cb) @nogc(is(cb : @nogc))
> ...

Basing the syntax on that of compile-time introspection seems like an 
interesting idea. However, in terms of implementation, passing around 
attribute parameters is perhaps simpler to pull off.

> This applies to the classic suite of attributes.
> 
> The trouble with this, obviously, is; how do you mangle this function? 
> It's not a template... so...?
> ...

A simple way involves encoding bound variables by index.
https://en.wikipedia.org/wiki/De_Bruijn_index

> That problem feels loosely related to `inout`, which is an innovative 
> hack... I wonder if some generalisation of that concept could be 
> satisfying here, while also satisfying the actual input case (allowing 
> to delete `inout` from the language)...?
> 
> It's a whole category of problem that needs some expression... where the 
> attribute-ness of a declaration is dependent on the attribute-ness of 
> some facet of the declaration; and where the declaration is not a template.

Yes. It would even be nice to have the option of passing entire type 
parameters at runtime. (Like the old druntime hooks do, but in a more 
user-friendly and type safe way.) But I think the most pressing issue is 
indeed function attributes.