Show resolver being used when stack ghci is invoked outside of a project directory

[wafelj]

Fix for #3651.

Approach: read Project from Config, get SnapshotLocation and compiler info from it and use that to get resolver using the loadResolver function. To my understanding it is theoretically possible that there is no project information in config (it's wrapped in Maybe). But I'm assuming this never actually happens because if I try to delete the ~/.stack/global-project/stack.yaml, a new one is recreated when stack ghci is run again. This is why I assumed it's safe to use fromJust. If there is a better approach, I'd love to hear about it.

Disclaimer: I'm new to Haskell and to stack, and this is my first open-source contribution. Any feedback is most appreciated!

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

[mgsloan]

Even if there is no currently known way for this to be Nothing, there could possibly be a way now or in the future, and the error thrown by fromJust is pretty frustratingly vague.

How about instead, for the Nothing case, using something like error "Invariant violated: no project". One nice thing about using error is that it will show where in stack's code the error occurred.

[wafelj]

I agree, ignoring the Nothing case is pretty ugly. I was thinking about this some more and decided to try something else: simply not print the resolver version if the project is Nothing. Would this work? I can do the error instead, but it's basically the same thing I had before, except it has a more useful error message.

[mgsloan]

The code related to resolvers and snapshots has changed quite a bit since I last looked, so I can't give direct guidance. However, it seems like loadResolver does fetching and parsing that I believe should have already happened. So, I think it'd be nice to avoid the redundant call, or cache it. I'll leave that decision up to the more active maintainers, though.

[wafelj]

@mgsloan thanks for your review! I was digging through the HasEnvConfig environment, trying to see if and where SnapshotDefinition is available, but I could not find it. SnapshotLocation inside Project inside Config was the closest, so I went with that. Definitely interested in other suggestions - my understanding of the project is not very deep yet.

[mgsloan]

@wafelj Welcome! Yeah I also dug around HasEnvConfig looking for snapshot name info. Couldn't seem to find it, which seems surprising since EnvConfig has BuildConfig, and info about the snapshot needs to be loaded to have a BuildConfig. So, yeah, not sure why that's not there ¯_(ツ)_/¯

Looks to me like bcSnapshot :: LoadedSnapshot could simply be added to BuildConfig and then populated in loadbuildConfig.

[qrilka]

@mgsloan @wafelj there was a large refactoring in #4412 and now Stack doesn' use LoadedSnapshot anymore. I think snapshot name could be added into SMWanted which later gets transformed into SourceMap - snapshot gets loaded in Stack.Config.loadBuildConfig but its name gets ignored (only compiler/dependencies get used).

[wafelj]

@mgsloan @qrilka thanks. I looked into that and changed the code - now it passes the name from SMWanted to SMActual to SourceMap. Then I load SourceMap from EnvConfig and read the name from that. Is this what you had in mind?

Also, two questions:

1. SMWanted is available in in BuildConfig. Would it make sense to read it directly from that in targetWarnings function instead of passing it through all those other types?
2. naming - is smwName etc. descriptive enough or would smwResolverName or smwSnapshotName etc. be better?

[qrilka]

1. The name should be the same in either of SMXXX or SourceMap so using BuildConfig should be OK
2. I think we should use longer field name as it's not a name of a source map itself. Also it makes sense to follow Make snapshot: a synonym for resolver: #4256 and use "snapshot".
   @snoyberg should we also switch to use "snapshot" instead of "resolver" in Stack output?

[snoyberg]

should we also switch to use "snapshot" instead of "resolver" in Stack output?

Sounds like a good idea to me.

[wafelj]

@qrilka changed to read name directly from BuildConfig and changed the field to smwSnapshotName. Also changed output to snapshot instead of resolver. Looks good?