Conditional Dependencies

[00vareladavid]

Crazy idea. Support "conditional dependencies"/"features" in a "sandbox"-analogous manner. (Note this is just meant to prod the search space, not as a serious/complete solution)

Foo
├── feature
│   ├── A
│   │   ├── Project.toml
│   │   └── src
│   │       └── A.jl
│   └── B
│       ├── Project.toml
│       └── src
│           └── B.jl
├── Project.toml
└── src
    └── Foo.jl

When someone runs add --feature=A Foo, Pkg will sandbox Foo in Foo/feature/A and add the result to the active dependency graph.

So Foo/src/Foo.jl will contain the core logic. Foo/feature/A/src/A.jl will contain the "A feature" logic.

Advantages:

• No need to query for which features are activated, it is apparent from the filesystem location
• Conceptually reinforces "environments" as an organizational unit.

[StefanKarpinski]

I like it. This seems like a promising idea.

[aaowens]

If this were to happen it could help with JuliaStats/Distributions.jl#702 without needing Distributions to require Optim.

[cscherrer]

Doesn't Flux have some trick to handle this problem? I don't see CuArrays as a dependency, but...
https://fluxml.ai/Flux.jl/stable/gpu/

[KristofferC]

It uses Requires.jl, see #1285

[KristofferC]

I think #1285 is a bit more recent take on this.

[00vareladavid]

For the purpose of history, this proposal more closely resembles #1233, not #1285.