Add --deps-only flag to spago build

[justinwoo]

Many times you will change deps in your project, and want to then interactively fix your codebase.

[f-f]

@justinwoo could you detail an example?

[justinwoo]

$ psc-package build --help
Usage: psc-package build [-d|--only-dependencies] [`purs compile`-options]
                         [--jobs ARG]
  Install dependencies and compile the current package

Available options:
  -d,--only-dependencies   Compile only the package's dependencies

if you remove deps you don't want in your project, your project doesn't build anymore. however, you still want to actually develop interactively at the leaf modules of your codebase.

[f-f]

I'm not sure I understand how this can happen. Assuming you have dependencies a, b, c and your project builds using all of them:

• if you remove a then your project stops building. It doesn't make sense to rebuild only dependencies because the dependencies have already been built once
• if you add d then your project should rebuild just fine even if you include project code

[justinwoo]

This is also a problem when you upgrade packages, which is the equivalent of removing a package to add another.

[justinwoo]

Another crucial detail is that it's not like all non-project sources will be built guaranteed, especially if module headers are parsed first and the compiler determines that there are no valid build plans (for good reason)

[justinwoo]

Really, the only actual workaround for this missing feature is to rm -rf src test && spago build && git checkout .

[f-f]

We had a IRL chat with @justinwoo and it turns out the reason why you'd want to compile deps-only is for cases in which you:

• switch branches and there's a significant diff of deps
• update dependencies that introduce breakage

..and still want your purs ide to give you meaningful suggestions.

So you'd do spago build --deps-only and then start purs ide. Then you'd have relevant suggestions to fix your type errors.