Allow cabal to execute user-defined subcommands

[fujimura]

I propose to add a functionality to allow Cabal to execute user-defined subcommands like git.
With this, user or library author can add their custom command like cabal doctest etc. (see #2327).

I tried to implement this roughly and basic part is working.
https://github.com/fujimura/cabal/compare/user-defined-subcommands?expand=1

If you can agree with this, I'm happy to finish implementation and make an pull request. Please let me know how do you think and concerns about this. Thanks!

[tibbe]

How do users create subcommands with your approach?

@dcoutts thoughts?

[luite]

If this is added, it should probably use the existing cabal exec functionality (and perhaps extend it where needed) to work with the correct package environment. Just having cabal userCommand as an alias for cabal-userCommand isn't terribly useful by itself in my opinion (which is what I understand it's doing in the WIP, correct me if i'm wrong).

[tibbe]

We should also look at other systems, e.g. git, and see how they provide this kind of facility. @dcoutts and I have discussed this in the past. For example, it would be useful to allow a subcommand to run in an environment where the preprocessors have already been run.

An example of such a use case is a source code indexer (think hashtags). Here we want to process the source code with our own "compiler" instead of GHC, but we still want the source code to be preprocessed for us.

[fujimura]

@tibbe

How do users create subcommands with your approach?

By creating an executable prefixed with cabal- and put it under PATH. In most case, it will be a Cabal package with executable since it's Haskell.

What I used to test the code above is:

$ cat .cabal-sandbox/bin/cabal-foo
#! /bin/sh
echo "FOO"
$ cabal foo
FOO

other systems

Git seems to be just invoking executable starts with git- in PATH. RubyGems has interface to register custom command(http://ruby-doc.org/stdlib-2.2.0/libdoc/rubygems/rdoc/Gem/CommandManager.html). Why I chose the way Git is using for my implementation is, it's easier than creating interface for subcommands.

@luite

it should probably use the existing cabal exec functionality

Thanks for pointing it out. This is yet another missing implementation in my code.

[23Skidoo]

Is this the same as aliases? I think there's already a ticket for that.

[fujimura]

Not same as aliases. Aliases discussed in #1089 is to extend existing commands, not to add new command.

Just FYI, Links below are articles about how to add custom command to git.
http://stackoverflow.com/questions/10978257/extending-git-functionality
http://thediscoblog.com/blog/2014/03/29/custom-git-commands-in-3-steps/

[BardurArantsson]

EDIT: Nvm. I'm a dork. Sorry about the spam.

[hvr]

Why do you need to restore the CWD during the cabal process' lifetime?

[alok]

I'd just like to second this idea. Rust has this, and it allows things like https://github.com/killercup/cargo-edit, which are very useful for simple project management