Interpolate environment variables in config

[djbe]

For a current project, I'd like to point to the generated core data files, which are located in
.../Library/Developer/Xcode/DerivedData/MyProject-ejmewwsnckjibvevfqzrcrvqxxxv/Build/Intermediates.noindex/My Project.build/Development-iphoneos/My Project.build/DerivedSources
(or some variation thereof). For now, I can run sourcery via command line --sources $DERIVED_SOURCES_DIR parameters, but it'd be nice to do this using a yml config file.

Docker implemented variable substitution in this PR:
docker/compose#1765
It's implemented in Python, but might be a good basis to start from.

[krzysztofzablocki]

@djbe I can see how that could be useful for other people, do you want to take a try at implementing that?

[djbe]

Sure, I can try to whip something up next week.

I was thinking of keeping it simple, and only supporting stuff ${STUFF} stuff syntax (with curly braces). And only interpolate with environment variables, as I don't think there's anything else to interpolate with?

[krzysztofzablocki]

works for me, thoughts @Antondomashnev @ilyapuchka @Liquidsoul @lucabartoletti ?

[Antondomashnev]

Sounds ok for me 👌

[mylemans]

Any update on this? I just ran into an issue that this would solve.

[Lutzifer]

We (@djbe and me) recently added the possibility to automatically expand environment variables in yml files to swiftgen (SwiftGen/SwiftGen#564)
If this is still of interest for the sourcery project also, I would be willing to start the implementation of that.

For my projects, this would allow me to reuse the yml files across projects.

[krzysztofzablocki]

That would be awesome @Lutzifer

[CatalinMustata]

Hey guys!

This works great if the env variables exist. However, I noticed that if they don't exist, they're not expanded to nothing (as it happens in a shell script) which breaks my use case. Is there a way around this I might have missed?