Unbundle Cabal from GHC

Recently, Duncan made it so that GHC proper does not depend on Cabal (so it is just ghc-pkg that is the user-facing executable which links against Cabal). We should now seriously consider unbundling Cabal from GHC, so that the default global database we provide does NOT include Cabal.

Pros:

• Distributions will be more likely to take point version releases to Cabal, as they no longer have to finesse updating Cabal without updating GHC as they have now
• Stack is (improperly) coupling the version of Cabal they build with the release of GHC; while they should fix this, unbundling Cabal would also give them more flexibility with picking LTS packages.

Cons:

• Bootstrapping Cabal/cabal-install becomes modestly harder. Fortunately, cabal-install is already pretty obnoxious to bootstrap, so SOP is to just distribute binaries for this, in which case things are as easy as before.
• We wouldn't be strictly adhering to the Cabal spec, which requires that the compiler always be able to build the Setup executable.
• ghc-pkg would have to be statically linked

Trac metadata

Trac field	Value
Version	8.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	dcoutts, hvr
Operating system
Architecture

changed milestone to %8.2.1

changed weight to 5

added Ttask Trac import labels

For reference, a previous discussion which also suggested not installing Cabal into the package db:

• https://mail.haskell.org/pipermail/ghc-devs/2014-December/007825.html

as well as the original CABAL specifcation from 2004 or so:

• https://www.haskell.org/cabal/proposal/pkg-spec.pdf

As the Debian maintainer: Yay!

ghc-pkg would have to be statically linked

That should be a matter of setting utils/ghc-pkg_dist-install_PROGRAM_WAY = v in utils/ghc-pkg/ghc.mk, and probably removing the utils/ghc-pkg_dist-install_SHELL_WRAPPER and utils/ghc-pkg_dist-install_INSTALL_SHELL_WRAPPER_NAME settings.

De-milestoning due to lack of progress.

added Pnormal label