Feature Request: Command to add a dependency/package

[andys8]

Most build tools have a command to add a dependency to your project (e. g. npm install). If I'm not mistaken there currently is no such command in cabal and install and get might be misleading.

Is it possible to add such command? Or is there a reason against or this issue being a duplicate?

Thanks :)

[ffaf1]

Does cabal build --only-dependencies fit the bill?

 --only-dependencies            Install only the dependencies necessary to
                                build the given packages

[ulysses4ever]

@andys8 indeed, that would be nice to have. Unfortunately, this is predicated on an ability to read/write .cabal without loss of any information (e.g. user comments, layout, etc.), which is a long-standing task that we have no volunteers for currently. See #7544.

[ulysses4ever]

@ffaf1 I think you didn't get what they ask for. They want to introduce new dependencies (as in build-depends) from command line rather than editing the .cabal file.

[ulysses4ever]

Related work is: https://github.com/sdiehl/cabal-edit

[andys8]

Unfortunately, this is predicated on an ability to read/write .cabal without loss of any information

@ulysses4ever And is cabal-edit able to do it or is it potentially loosing information?

[ulysses4ever]

The latter. It also doesn't support all of the modern features of Cabal, like common stanzas (see its Readme), and it's kinda dormant (doesn't compile with current versions of GHC and Cabal).

[Bodigrim]

hackage-cli add-bound is an abuse of the tool, but works well.

[gbaz]

effectively the same issue #7337

[andreasabel]

hackage-cli add-bound is an abuse of the tool, but works well.

Yeah, a command to add a dependency pays off when you want to bulk-add or script stuff. hackage-cli works in many cases, but does not win a beauty contest. It is good enough for bulk-revising package versions.

In advanced .cabal files, where common stanzas and imports are used, an "add dependency" command might need extra input as to where to place the new dependency, so this is far from trivial to implement. It might only be user friendly with extra intel, like info which .hs modules import from the new dependency. Maybe if the HLS folks have figured out how to do it, it could be backported into cabal...

[Bodigrim]

I've written https://github.com/Bodigrim/cabal-add. It handles common stanzas reasonably well and allows to specify a target component.

[geekosaur]

This is the kind of use I'd been thinking of for #9063, fwiw. cabal-plan might be another.