Offer install flag: do not download dependencies

[snoyberg]

There are times where we would like to prevent cabal-install from downloading dependencies, such as when bandwidth is limited or when using an external tool to download tarballs. It would be good to have a flag to the install command (strawman: --no-download) to prevent it from trying to perform downloads.

[23Skidoo]

I actually implemented this some time ago because I (thought that I) needed it for sandboxes. The sandbox issue was solved in a different way, so the offline mode patch wasn't merged. Should be easy to resurrect it.

[23Skidoo]

My patch is a bit simplistic, though: it works by rejecting install plans that contain non-local packages. This could be also implemented by modifying the solver to not consider such packages.

[snoyberg]

For my specific use case, failing would be acceptable, though I can certainly imagine other use cases that would have different requirements.

[snoyberg]

(And thanks for the quick reply!)

[kosmikus]

I would like this too.

[hvr]

Btw, I've been wanting a related feature that allows to limit the solver to only consider an explicitly specified set of additional dependencies for computing the install-plan. Right now I have to repeatedly call cabal install --dry and find out if the solver picked up any additional build-dep I didn't allow/specify, and add a respective --constraint flag, and repeat this process until a fixpoint is reached.

[kosmikus]

@hvr Could you open a separate ticket for that request? As I see it, @snoyberg's request doesn't affect the solver at all. Your request would, but the feature should be extremely easy to add, modulo bikeshedding about the name and syntax of the flag.

[kosmikus]

Ah, sorry, I've not read the discussion in detail. Ok, a solution affecting the solver for the original change has also been discussed, even though that's not how I originally understood the request. I'm open to add solver-changing flags for this (in particular as this is only about adding additional preferences, which is very easy to do), if that's what people would prefer.