Add flag for specifying config path location

[justinwoo]

Description of the change

fixes #329, specify config path globally if desired for specific usages

e.g.

spago.dhall

-- ...
, sources =
    [ "codegen/**/*.purs", "src/**/*.purs", "test/**/*.purs" ]
}

i don't want to build with all the sources.

codegen.dhall

./spago.dhall // {
  sources = [
	"src/**/*.purs"
  ]
}

$ spago -x codegen.dhall build
# works fine

other uses:

"optional app separate from library"

"test config overrides and modifications"

"some other config modifications"

Checklist:

• Added the change to the "Unreleased" section of the changelog
• Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

P.S.: the above checks are not compulsory to get a change merged, so you may skip them. However, taking care of them will result in less work for the maintainers and will be much appreciated 😊

[justinwoo]

/bin/sh: psa: command not found

in osx build

[f-f]

@justinwoo that error there is because of the test harness and is a normal (I'll refactor the tests so it doesn't happen), but the tests fail because spago update-set is flaky on Travis machine (I'll put a PR up for this)

I'm working through my backlog, I'll review this later today

[f-f]

This is cool by me, but I'm concerned about some edge cases that might be challenging UX-wise. A non-comprehensive list:

• when we search for local packages we hardcode the spago.dhall. I think we should not change this, but it might be weird in some usecase I'm not considering
• We have a few more mentions of the spago.dhall that we might want to fix too in Spago.Messages here and here and in Spago.Dhall here
• when we encourage things like ./spago.dhall // { dependencies = [ "prelude" ] }, we have to make sure stuff that manipulates the AST directly still works (like spago install)
  So here I would like a test that makes sure that spago -x foo.dhall install simple-json works properly and:
  • actually adds the package if foo.dhall is ./spago.dhall // { dependencies = [ "prelude" ] }
  • and is a no-op if foo.dhall is ./spago.dhall // { sources = [ "src/**/*.purs" ] }

[justinwoo]

I think it would also be good to just disallow automatic editing for anything that doesn't conform to the expected Expr structure, and instead just print warnings on what the users should do manually with their frankenstein-ass configs

[f-f]

@justinwoo that is the current behaviour: if stuff doesn't conform to the expected structure then the editing is not performed. However we do not have the infrastructure to detect when this is the case and print warnings

TL;DR: I believe if you add the tests I mentioned they will just pass

[f-f]

This should now be good to go. @justinwoo can you take a look?