Find out which flags have changed

[dspies-leapyear]

Often ghc-mod seems to rebuild more than necessary because it's passing a different set of flags to ghc then those that were originally used to build the dependencies. How can I find out which flags ghc thinks has changed? (and to what?)

[dspies-leapyear]

When running with --verbose 7 I get

$ ghc-mod --verbose 7 list
info: Found Stack project at: /Users/davidspies/workspace/myproject
info: Using Stack project at: /Users/davidspies/workspace/myproject
DEBUG: setup configuration is out of date
DEBUG: reconfiguring Stack project

and then it proceeds to unregister and rebuild everything

[DanielG]

The list command shouldn't actually start a GHC session in ghc-mod, it does however invoke stack build as necessary to update the Cabal configuration. I think that's the compilation you're seeing. Now the output above indicates that ghc-mod thinks you edited your .cabal file or more technically the .stack-work/**/setup-config file's modtime was older than your-project.cabal's. If this is true then it's perfectly alright for it to invoke stack build to get that updated. It shouldn't actually rebuild your entire project though, ~~only dependencies and even those should already be cached~~ we actually don't even do that by default anymore. The stack command we use is: stack build --only-configure .. The relevant code is here: https://github.com/DanielG/ghc-mod/blob/master/core/GhcMod/CabalHelper.hs#L165 if you wan to have a look.

[dspies-leapyear]

Ok, I think I get it now. Our project is configured like:

project:
  stack.yaml
  .stack-work
  projectsomepackage:
    projectsomepackage.cabal
    src/Project/Some:
        MyModule.hs
        MyModuleDependency.hs
  projectotherpackage:
    projectotherpackage.cabal
    src/Project/Other:
      OtherModule.hs

If I try to run ghc-mod check $HOME/project/projectsomepackage/src/Project/Some/MyModule.hs from inside project/projectsomepackage then it expects the .stack-work directory to be there as well so when it doesn't find anything, it attempts to build. On the other hand if I run from within the root-level project directory, then it treats projectsomepackage as a dependency and complains about Project.Some.MyModuleDependency being a hidden module in the package projectsomepackage. What should I do?

[dspies-leapyear]

Oh, but also, there's still no setup-config file. We're actually using shake which in turn calls stack, but maybe it does something to prevent that file from being generated. What is the setup-config file anyway? Do I need it? Can ghc-mod work without it?

[DanielG]

Can you paste the output of ghc-mod debug run from one of the project directories somewhere please?