Platform specific configuration

[baszalmstra]

Multi-platform packages will require the need to have platform-specific dependencies or configurations.

Cargo supports this through the [target] section.

How to format them in TOML?

[dependencies]
bla = "3.1.*"
windows_specific = { version = "3.1.*", only_windows = true }

or cargos approach:

[dependencies]
bla = "3.1.*"

[target.'sel(win)'.dependencies]
windows_specific = "3.1"

I like the latter the most because its clearer that this is an exception that only applies in a certain scenario.

Selectors or platforms?

Cargo uses the cfg(..) syntax in its selectors. I think this is pretty cool because it allows you to do more complex things like selecting for two platforms or for specific architectures (arm vs x86).

We could also simplify things a little bit and only use platform names as selectors e.g. osx-arm64 vs sel(osx). Cargo also supports specifying the target directly.

I would opt to go for the full selector experience because it provides a lot of flexibility.

Todo

• Figure out our approach
• Implement it

[baszalmstra]

In light of the recent work on conda/ceps#54 maybe we can adopt a similar style if statement.

[dependencies]
bla = { matchspec = "3.1.*", if = "win or osx" }

[wolfv]

I think I like this. It would only work for single dependencies, though. Should we come up with something for a list?

[baszalmstra]

Ah this method:

[dependencies]
bla = { matchspec = "3.1.*", if = "win or osx" }

falls apart if you would want to have different matchspecs based on the os:

[dependencies]
bla = { matchspec = "3.1.*", if = "win" }
bla = { matchspec = "4.1.*", if = "osx" }

The key bla is defined twice which is illegal.

However, using the cargo method this is possible:

[target.'if(win)'.dependencies]
bla = "3.1.*"

[target.'if(osx)'.dependencies]
bla = "4.1.*"

Alternatively, we could also accept minijinja syntax in the dependencies list but it makes parsing a lot harder (because we have to post-process the expressions:

[dependencies]
bla = '${{ "3.1.*" if win else "4.1.*" }}'

its also a bit weird with the to types of quotes..

[baszalmstra]

Target specific dependencies have been implemented in:

• feat: simple platform specific dependencies #111

With that you can provide per platform specific dependencies but you cannot easily add a dependency for an architecture or os.

We could add a simple expression language to do that. E.g. target.if(os == win).dependencies or target.'if(os != win)'.dependencies. WDYT @tdejager ?