Use write-ghc-environment-files to run doctest with cabal

Imagine you have a test suite in your cabal file that uses doctest, and you’re trying to run it with cabal test.

This might fail because doctest cannot find some dependencies. This happens because doctest doesn’t look into the cabal file for dependencies when it runs tests.

You can put write-ghc-environment-files in your cabal.project file to fix this.

write-ghc-environment-files: always

This makes ghc to write the environment to .ghc.environment.aarch64-darwin-9.2.5 (the last part depends on your environment), and doctest loads it to find dependencies.

• Cabal-3: doctest-discover fails for non-boot libraries #6087
• 8. cabal.project Reference