solver: Interaction of independent-goals and constraints is not clear

[mpickering]

It would be worthwhile to improve the documentation about how independent-goals is supposed to interact with user specified constraints in a cabal.project or given on the command line.

For example if I have a package A which depends on B, if I give the solver the goal A and enable --independent-goals should a constraint --constraint= B == 2 apply inside the independent goal of A?

Reasons to consider:

• The constraint does not apply inside other qualified contexts (for example, setup stanzas), so why does it apply inside a qualified goal.
• If you want a constraint to apply within ALL goals then you can already write any.A rather than simply A.
• The goal of A is supposed to be solved "indepedently" of the rest of the goals/context, this constraint is applied at a global rather than per-goal level.

Therefore it seems like a bug to me that the constraint affects the solver when deciding which version of B to pick when solving the independent goal for A.

cc @grayjay

This situation is witnessed by this test, the result is A == 1, B == 2.

 testIndepGoals8 :: String -> SolverTest                                         
 testIndepGoals8 name =                                                          
   constraints [ExVersionConstraint (scopeTopLevel "B") (V.thisVersion (V.mkVersion [2,0,0]))]  $
     independentGoals $                                                          
         mkTest db name ["A"] $                                                  
           solverSuccess [("A", 1), ("B", 1)]                                    
   where                                                                         
     db :: ExampleDb                                                             
     db =                                                                        
       [ Right $ exAv "A" 1 [ExAny "B"]                                          
       , Right $ exAv "B" 1 []                                                   
       , Right $ exAv "B" 2 []                                                   
       , Right $ exAv "B" 3 []                                                   
       ]    

[grayjay]

I think that the issue is that the independent goals feature was never designed to be used for anything except testing qualified goals or possibly acting as a workaround when another issue prevents cabal from finding a solution. I don't think that the feature is complete. Qualified constraints also isn't complete. #3502 tracks the remaining work, including constraints for build tool dependencies and qualified constraints in freeze files. If we wanted to fully support independent goals, I think we would need to allow qualifying constraints with the name of the target.

What is your use case for independent goals? Are you planning to build something on top of the feature?

[mpickering]

@grayjay I am implementing #4035 and perhaps also an extension which generalises the base shim logic.

There are some tests which fail after I have performed some refactoring of qualified goals, so I need to know what the specification is for this feature in order to know how to fix them.

[grayjay]

Okay, I think that the part of independent goals that is implemented is well defined; it just hasn't been made into a useful feature or added to the documentation yet. It looks like qualified constraints for independent goals also hasn't been implemented yet.

#3220 has the best description of independent goals that I have found. I believe that the feature just gives each build target a unique namespace (part of the PackagePath) in the dependency solver, and that namespace is inherited by all dependencies, including setup and build tool dependencies. Ignoring the rules of inheritance, PackagePaths with different namespaces are treated similarly to PackagePaths with different qualifiers.

I found an example of a use case for independent goals, in case that helps: #4295

Is there anything that I can read about the implementation of private dependencies? I'm wondering how it would affect the dependency solver. I'm especially interested in the effects on performance, since qualified goals have a big impact on solver run time, as in #6309.

[mpickering]

@grayjay Back to the original question:

For example if I have a package A which depends on B, if I give the solver the goal A and enable --independent-goals should a constraint --constraint= B == 2 apply inside the independent goal of A?

It seems you are suggesting that it shouldn't (which is the conclusion I came to).