Add http support to `cabal sandbox add-source`

[hvr]

This would be particularly convenient in combination with Hackage's candidate feature, as then you could e.g.

cabal sandbox add-source http://hackage.haskell.org/package/lzma-0.0.0.0/candidate/lzma-0.0.0.0.tar.gz

and then cabal could keep track of the etag to detect if the package needs to be redownloaded and rebuilt...

[23Skidoo]

So you suggest that it should do a HTTP request for each remote add-source package on each cabal build?

[hvr]

Yes. This may sound weird at first, but otoh if one doesn't want that behaviour, there's the add-source --snapshot flag already available to decouple from filesystem based "upstream" locations. So it'd be only consistent with the existing UI to have http-based tested source-deps tested for freshness as well, IMO.

[23Skidoo]

OK, understood. I considered implementing this for git:// URLs, but then decided that standard add-source and submodules/manual git pull work better.

[hvr]

In fact, this reminds me about adding some kind of syntax-sugar to cabal for Hackage candidates. Specifically something like a candidate:-prefix that would expand to a http URL, then one could conveniently say:

cabal install candidate:lzma-0.0.0.0

or

cabal sandbox add-source candidate:lzma-0.0.0.0

etc... I think this would increase the value of the package candidate feature. As right I don't think many users are aware of the package candidate feature, nor that it can be used as a staging area for packages quite easily.

/cc @dcoutts

[phadej]

sandbox functionality is removed.