Use Cabal library directly (instead of cabal-install executable) for building

[snoyberg]

Advantages:

• No need to build cabal-install before running the actual build
• Avoid problems with accidentally polluting the user-wide package database
• Less surface area for bugs to creep in

Disadvantages:

• Slightly different build flow than projects normally receive, possibly resulting in bugs
• Concrete example: if a project lists build-type as Simple and then has a different Setup.hs, a naive implementation may not compile properly (I know this affects Nix and affected Stack in the past)

[DanBurton]

How much of cabal-install would we be reimplementing in order to accomplish this? Are the appropriate points of abstraction available in the Cabal library such that this would be trivial to implement (in stackage-curator, I presume)?

[snoyberg]

I think so, I just need to research it a bit. We wouldn't even be using the Cabal library as such, but rather runghc Setup.hs, which has a very similar command line API as cabal-install. The big thing we don't need is cabal-install's ability to solve dependencies and download them for us from Hackage, so we may be golden here.

[DanBurton]

runghc Setup.hs

Oh right! That makes a lot of sense. 👍