Hello all, I am writing a tool that uses the ghc api. It works great when I invoke it on user library code but not for test (and executable) components. Consider a very simple user project with this layout lib/Foo.hs lib/Bar.hs test/FooTest.hs test/TestUtils.hs Let's say that Foo imports from Bar and FooTest imports from TestUtils. To start my tool, which for all intents and purposes is just loading a file into ghc, I obtain the ghc flags from cabal's v2-repl (this is the hie-bios hack). I've been sure to make sure that I grab the flags associated to the relevant component. I feel that I need to justify why I'm doing it this way instead of cabal envfiles: they do not include language extensions and other things of that nature, and don't separate library/test/etc. Let's say the user has compiled the project, with tests enabled. The flags for the library might look something like ... -this-unit-id example-1.0-inplace -hide-all-packages -Wmissing-home-modules -no-user-package-db -package-db -/home/me/.cabal/store/ghc-8.6.5/package.db -package-db -/home/me/example/dist-newstyle/packagedb/ghc-8.6.5 --package-db /home/me/example/dist-newstyle/build/x86_64-linux/ghc-8.6.5/example-1.0/noopt/package.conf.inplace -package-id transformers-0.5.6.2 ... which all makes sense. If I use these flags then loading Foo.hs will fail because it cannot find Bar. These modules are needed for compilation but not listed in your .cabal file's other-modules: Bar Of course I could suppress this by ignoring warnings, but I want to see those modules.... so I have a workaround! I hacked it by changing the -this-unit-id to a -package-id and it all works great because the compiler already has the interface files for Bar in the packagedb. My tool doesn't write any object files, so I think this is safe. However, when I come to running my tool on test directories, there is no -this-unit-id from v2-repl. I know what the package-id is (from Cabal's plan.json) but it doesn't exist in my packagedb, so I can't re-use my hack. These modules are needed for compilation but not listed in your .cabal file's other-modules: TestUtils And I'm back where I started. So my question is: which flags do I need to provide to the ghc api so that a module can see the interfaces of other modules in the same component? I'm guessing my -this-unit-id hack is horrible and I shouldn't be doing that. What is the correct thing to do? -- Best regards, Sam