Adopt cabal-add?

[ulysses4ever]

Describe the feature request
People often ask for a programmatic way to manipulate parts of the cabal file, especially w.r.t dependencies. A recent tool, cabal-add, delivers on the particular task. Should it be integrated into cabal proper?

Additional context
cabal-add (a tool adding entries in build-depends without ruining formatting in a cabal file) is being integrated into HLS: Bodigrim/cabal-add#7 This confirms my feeling that there's a user base for this feature, however partial this feature is.

I tried to read the implementation (it's reasonably small and clean). It juggles between [Field Position] and GenericPackageDescription like no tomorrow, starting from:
https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L202-L210

... as is expected in the absence of a holistic solution for exact parsing (cf. #7544).

The core of it is three "algorithms" to solve the task (add a dependency) that spread across the spectrum: accuracy --- robustness (e.g. the first algorithm preserves formatting the best but is more fragile than the others)

• https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L403-L407
• https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L450-L453
• https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L472-L475

Given that all of them are Maybe, I assume, the tool doesn't guarantee success...

Maybe we should wait and see how the HLS feature fares and what its users think?

[geekosaur]

Sounds like, at least at first, it's a candidate for an external command.

I had been thinking about this myself, but without much consideration for formatting. The basic idea was to add a dependency with multiple ways to specify versioning:

• add [section:]dep, use current Hackage version in a ^>=
• add [section:]dep ver, use specified version with ^>=
• add [section:]dep ver1 ver2, specify a version range

[geekosaur]

Also, seems to me HLS has more of a specific need for dependency adding, just as it automatically adds imports.

[Bodigrim]

Given that all of them are Maybe, I assume, the tool doesn't guarantee success...

It pretty much always succeeds, unless Cabal file has an ancient, unsectioned format. Which is when there are no sections library / executable / test-suite, but everything (including build-depends) somehow happens on the top level.

[Mikolaj]

For context, this has been discussed before: https://discourse.haskell.org/t/cabal-add-extend-cabal-build-depends-from-the-command-line/7911/6