Implement cabal sandbox hc-pkg / cabal sandbox buildopts

[23Skidoo]

cabal-dev has a command ghc-pkg which is handy for manipulating the sandbox package DB using ghc-pkg:

cabal-dev ghc-pkg unregister foo

It'd be nice if cabal sandbox also supported this feature (see discussion in #1195). Additionally, there is cabal-dev buildopts that extracts the options that would be passed to the compiler when building.

[mightybyte]

In the discussion in #1195 it was suggested that I just use ghc-pkg to remove things from my sandbox, so I've been doing that. But I just encountered a situation where that is not sufficient. I have three projects, call them alpha, beta, and gamma. beta depends on alpha, and gamma depends on both alpha and beta. gamma is my top-level project and it is the one where I'm using a cabal sandbox. (It also depends on a number of other local packages that may or may not be on hackage.)

Originally I was working on changes that affected beta and gamma. If I made changes to beta, I would use ghc-pkg to unregister it from gamma's sandbox and then do sandbox-install to build gamma with the new changes. In spite of the advice against this approach, it seemed to be the most efficient way to make improvements to beta and test those changes in the context of gamma. But when I made a change to alpha this pattern failed. I used ghc-pkg to unregister both alpha and beta from the gamma sandbox and then did sandbox-install as before. But this time I got a linker error:

libHSbeta-0.1.a: unknown symbol `alphazm0zi1zi0zi0...

So it appears that unregistering with ghc-pkg is not sufficient. I was able to fix the problem with find .cabal-sandbox "alpha*" | xargs rm -fr and then repeating that command for beta. After that, sandbox-install worked correctly.

[23Skidoo]

@mightybyte Can you open a separate ticket for this issue?

[mightybyte]

Yes, but I'm not sure what the ticket should say. To me this just seems like an implementation detail for this ticket.

[23Skidoo]

@mightybyte I suspect that it's a ghc-pkg issue and the same error happens with cabal-dev.

[hvr]

btw, imo cabal buildopts wouldn't need to be a sandbox-only feature.

[phadej]

sandbox functionality is removed.