Display information on why a snapshot was rejected?

[snoyberg]

Currently, stack init doesn't tell you why a snapshot won't work, at least not by default. If you run with --verbose, you get much more information. So the discussion point is: should we display that information by default? The downside would be possibly overwhelming levels of output, but I think we could manage that somehow.

[rvion]

I think we should.

• stack init will not be run a lot by new users.
• Haskell users tend not to like magic too much
• people for whom stack init fails won't probably rerun the command with --verbose
• Failing without error messages is just annoying more generally

[snoyberg]

OK, I've implemented this, sample output:

Basing on cabal files:
- /home/vagrant/Desktop/stack/stack.cabal
- /home/vagrant/Desktop/stack/new-template/new-template.cabal

Checking against build plan lts-2.9

* Build plan did not match your requirements:
    optparse-simple version 0.0.2 found
    - stack requires >=0.0.3

    path not found
    - stack requires -any && -any && >=0.5.1


Checking against build plan lts-1.9

* Build plan did not match your requirements:
    attoparsec version 0.12.1.3 found
    - stack requires >=0.12.1.5

    bifunctors version 4.2 found
    - stack requires >=4.2.1

    conduit-combinators version 0.3.0.6 found
    - stack requires >=0.3.1

    conduit-extra version 1.1.6.2 found
    - stack requires -any && -any && >=1.1.7.1

    exceptions version 0.6.1 found
    - stack requires -any && -any && >=0.8.0.2

    fast-logger version 2.2.3 found
    - stack requires >=2.3.1

    hashable version 1.2.3.1 found
    - stack requires -any && >=1.2.3.2

[rvion]

nice :)

2 minor things:

• Should we add a final sentence: Try changing dependencies in your *.cabal files or add extra-deps in your stack.yml file?
• should we be a bit more explicit at the cost of being a bit more verbose ?
  • fast-logger version 2.2.3 found in snapshot
  • cabal files require -any && -any && >=0.8.0.2

It might help new users to understand what is going one.

[snoyberg]

I think the recommendations already given at the end (which I didn't include in this copy-paste since they haven't changed) are best. I'd be surprised if a user was uncertain about where the dependency ranges are coming from, since that's the only place it can come from and we already mentioned the .cabal files explicitly earlier. And "in snapshot" is more likely to confuse than anything else; we already stated which build plan it is (lts-2.9, etc) just before giving the mismatches.

[rvion]

ok 👍