Condensers and Shading

[Ron Pressler]

There is no general way to know whether multiple instances of a library can coexist in the same process or in what mysterious ways attempting that will break (the instances could write to the same files, bind to the same port etc.). This is true for all languages. However, to the extent that is possible, modules already support this with layers; i.e. you already have the best possible “language level shading” you can hope for. The reason layers can only be configured programmatically is that the best possible is not good enough to recommend for wide use, but there are tools out there (e.g. https://github.com/moditect/layrry) that offer configuring layers with configuration files.

— Ron

> On 13 Jun 2023, at 10:16, Ethan McCue <ethan at mccue.dev> wrote:
> 
> Hi All,
> 
> I just read through the early discussion on the condenser concept.
> 
> A brain-worm I can't dispose of[1] is that the concept of time shifting could be used to allow language level shading.
> 
> That is, if a user can say that none of the classes in a module will be reflected upon using dynamically constructed values.[2]
> 
>      Class.forName(f()); // X
> 
> Then it would be a "safe" operation to change the package name of all of those classes and bundle them inside of a module which requires them.[3]
> 
>     shadeable module org.example {
>         exports org.example;
>         exports org.example.abc;
>     }
> 
>     module org.other {
>       requires shaded org.example;
>   }
> 
> Now, this is not in Leyden's mission statement. Nothing about startup time is improved by renaming packages and classes. But the set of information you'd need to allow shading seems to overlap with the set of information you'd need to allow for making a static executable.
> 
> So in the same way Valhalla is converging towards T! + value + implicit constructor = inline-ability there might be a "can make static exe" that falls out of other features like this.
> 
> [1]: And one I feel I should bring up considering that the set of condensers might start out or remain closed to keep a consistent platform definition.
> [2]: + some other restrictions I am sure
> [3]: There might be a place other than modules which could work, but module-info declarations are currently the only place where users explicitly mark reflect-ability constraints today, so it 
>