Implement --cabal-file, allows multiple Cabal files in directory

[ezyang]

This is primarily intended for use with the Cabal test suite (allowing
us to easily specify multiple Cabal packages for the same Haskell source
files), but maybe some end-users will find it useful as well. If there
are multiple Cabal files in the current working directory, --cabal-file
(for configure) allows you to disambiguate which one to build with.

There's a big hack to handle the BOM check, as it is inconvenient to
plumb the flag value all the way to the check code. Some bigger
refactoring needed, see #3552.

Signed-off-by: Edward Z. Yang ezyang@cs.stanford.edu

[phadej]

So cabal check doesn't support --cabal-file with this?

[ezyang]

Yes, and supporting it would require quite a bit of faffing about.

[23Skidoo]

Bikeshedding: we might want to call it --package-description in case we later add support for other package description formats (e.g. hpack).

/cc @dcoutts

[ezyang]

OK, sure (although it's hard to imagine the library getting support for hpack; it's more of a cabal-install thing.)

[phadej]

I'd keep it as --cabal-file, and if we later would like to add --package-description we can, yet --cabal-file would be always parsed as cabal file (e.g. we could add --hpack-file too)

--package-description needs to guess, and sometimes you really want the tool you use not to guess at all, just do what it's told.

note to myself: verify what automatic reconfiguration can be disabled with some flag

[ezyang]

Well, I have the patch to put it either way.

[hvr]

Please take into account we'll also need a way to specify the cabal.project file location when designing a name for this

[ezyang]

Well... --cabal-project for that one?

[hvr]

Alright, we'd have

• --config-file to specify an alternative ~/.cabal/config
• (suggested) --cabal-file to speficy an alternative *.cabal file

but then, for consistency, I'd suggest

• --project-file to specify a non-default cabal.project file

[23Skidoo]

@ezyang BTW, don't we need a note in the manual about --cabal-file?

[ezyang]

Well, I wasn't intending for it to be a user-visible feature. If we put it in the manual, we need to teach cabal-install how to handle it too, because cabal-install also reads in cabal files itself.

[23Skidoo]

There should be a place for documenting non-user-visible features like --exact-configuration and this one, but I guess this will have to wait until someone rewrites the user guide.

[23Skidoo]

Added a line to the changelog (858981e).