Use .cabal or cabal.project to discover which files to format

[turion]

It would be great if, instead of having to specify files to format via a directory or a complicated call to git, fourmolu would simply detect the files to format in the same way cabal does. Basically, if the command cabal build all --enable-tests --enable-benchmarks would read a particular Haskell source file, then fourmolu -i should format that file.

Why the current situation is not good enough

For any particular project, I can somehow adapt the fourmolu call. But when I want to automatize (e.g. to write a good git alias, or a github action), I want to be able to deal with any sort of Haskell repo.

Specifying a directory

The docs recommend e.g. fourmolu -i src. This doesn't work for projects that have several folders (e.g. test, benchmark, app) generically because a project might have any sort of folders. Also, it doesn't work when you have a cabal.project structure.

git ls-files

The docs recommend fourmolu --mode inplace $(git ls-files '*.hs'), but this doesn't work in general because there might be other files like *.lhs, *.md. Also, it doesn't work if there is no git repository (e.g. downloading a tarball).

What would be the most straightforward way

Like cabal, fourmolu should automatically discover whether there is a *.cabal file, and then format all of the Haskell source files specified in it, or if there is a cabal.project file, format all the source files found from all the cabal projects in there. Advantages:

• Single point of definition what is part of the project and what is not
• No additional assumptions about directory structure or available tooling

[brandonchinn178]

This would be great, perhaps behind an --all-files flag! Can you file an issue with ormolu? It would be great if this could be implemented in ormolu and we'd get it for free after. If ormolu rejects it, we can implement it ourselves

[turion]

Can you file an issue with ormolu?

Good point :) See tweag/ormolu#1054