haskell/cabal (original) (raw)

Summary by @ezyang. An invocation like

$ cabal new-configure --with-package=/home/user/sample/sample.cabal

should write out a cabal.project.local that includes

packages: /home/user/sample/sample.cabal

The new flag would probably live in ConfigExFlags (Distribution.Client.Setup). The top-level function which converts command line flags to project config is commandLineFlagsToProjectConfig in Distribution.Client.ProjectConfig.Legacy. The packages field in cabal.project is the projectPackages field; you would add it as another field setter to the mempty in commandLineFlagsToProjectConfig.

Don't forget to add a test to integration-tests or integration-tests2.

What you will learn. How the configuration logic for Nix-style local builds (new-build) works, and in particular how the new-configure command is implemented.

We can already include a packages field in both cabal.project and in cabal.project.local, and cabal new-build will consult and use both. What I'd like—and AFAICT doesn't currently exist—is the ability to pass a flag to cabal new-configure (maybe something like cabal new-configure --extra-project=path-or-url-of-cabal-project) that will add the specified project locations (either local paths or, as per #2189, VCS urls) to the generated cabal.project.local file.

This is useful for a few reasons. One use-case is when you depend on a library that's not part of the same project, is available on a local filesystem, and won't necessarily be in the same absolute or relative location on every developer's machine. Another use-case is when you want to locally override an existing dependency to specify a WIP version of that dependency either on the local filesystem or in a VCS. In both these cases, the location of the dependency—being a local detail—would belong in cabal.project.local instead of in cabal.project.

The functionality to make this work is mostly already in place, except that cabal.project.local is usually generated by cabal new-configure and there's no way to tell it to add a project except by hand-editing the generated file.