Generated GenericPackageDescription is not strictly equal to cabal one

[Javier Neira]

Hi, in the process of integrate dhall with etlas (a cabal fork suited for eta), we've found that the GenericPackageDescription generated by dhall-to-cabal is not equal to the generated by cabal itself using parseGenericPackageDescription.
Although it is similar enough to generate the same output using showGenericPackageDescription (to be fair, the main goal of the package), i am afraid that you cant use it directly against the cabal api.
I've setup a branch adding a strict comparation to analyze the changes between them:

• options, profOptions, sharedOptions, staticOptions are an empty list in cabal and a list of (Compiler,[]) in dhall-to-cabal: i guess this probably would be not a problem but most cases fail with it
• In projects with executables cabal has a CondNode {condTreeData = Executable {exeName = UnqualComponentName "dhall-to-cabal" and dhall-to-cabal CondNode {condTreeData = Executable {exeName = UnqualComponentName "". Likely it could cause problems using against the api.
• In projects with dependencies cabal has condTreeConstraints filled for each component and dhall-to-cabal empty: this one caused the first (and unique for now) error when trying to use it against cabal api
• In dhall-to-cabal.dhall there is a diff in the license: licenseRaw = Left (License (ELicense (ELicenseId MIT) Nothing)) for cabal and licenseRaw = Right MIT for dhall-to-cabal

I think some sort of strict comparation can be useful to detect and analyze possible bugs. I will open a pr to discuss the relevance of and possible changes over the strict tests.

[quasicomputational]

RE (3), I independently discovered that omitting condTreeConstraints isn't a good idea if the GenericPackageDescription is being passed into the solver. Fortunately, the value of condTreeConstraints is completely determined by the contents of the corresponding node, so it can be done as a post-processing step. Here's some code that does just that:

fixGPDConstraints
  :: GenericPackageDescription
  -> GenericPackageDescription
fixGPDConstraints
  = over ( Lenses.condBenchmarks . traverse . _2 ) fixCondTreeConstraints
  . over ( Lenses.condExecutables . traverse . _2 ) fixCondTreeConstraints
  . over ( Lenses.condForeignLibs . traverse . _2 ) fixCondTreeConstraints
  . over ( Lenses.condLibrary . traverse ) fixCondTreeConstraints
  . over ( Lenses.condSubLibraries . traverse . _2 ) fixCondTreeConstraints
  . over ( Lenses.condTestSuites . traverse . _2 ) fixCondTreeConstraints

fixCondTreeConstraints
  :: ( Lenses.HasBuildInfo a )
  => CondTree v cs a
  -> CondTree v [Dependency] a
fixCondTreeConstraints ( CondNode a _ branches ) =
  CondNode a deps ( fixCondBranchConstraints <$> branches )
  where
  deps = view ( Lenses.buildInfo . Lenses.targetBuildDepends ) a

fixCondBranchConstraints
  :: ( Lenses.HasBuildInfo a )
  => CondBranch v cs a
  -> CondBranch v [Dependency] a
fixCondBranchConstraints ( CondBranch cond true falseMay ) =
  CondBranch cond
    ( fixCondTreeConstraints true )
    ( fixCondTreeConstraints <$> falseMay )

[Javier Neira]

Wow, thanks for that elegant code. However, do you think that inform that field shouldn't be in cabal-to-dhall? could be done in parsing, at same time that other condTree* fields?

[quasicomputational]

Yeah, makes sense to do it there. I had it in my head that we were doing something with the conditionals that meant that it'd have to be a separate step, but I think I was getting that mixed up with the cabal-to-dhall code or something.