Drop support for GHC 7.4

[ttuegel]

GHC 7.4 is more than four years old now, well outside our three-year
support window. (Actually, GHC 7.6 is outside our support window too,
but I don't have any pending patches which are broken with that
version.)

Attn. @23Skidoo @rthomas

[mention-bot]

@ttuegel, thanks for your PR! By analyzing the annotation information on this pull request, we identified @phadej, @ezyang and @23Skidoo to be potential reviewers

[23Skidoo]

Sure, if it's blocking your work, then it's okay. But please add a changelog note.

[ttuegel]

Actually, I just thought of a way around this. False alarm!

[ttuegel]

It turns out there is a separate issue with GHC < 7.6 blocking my patch, so I'm reopening this. 😄

[23Skidoo]

#3111 was a bit different from this PR, I'd add a separate line saying "Cabal/cabal-install can no longer be compiled with GHC < 7.6" instead. Otherwise it should be possible to use 2.0 with 7.4.

[23Skidoo]

Fixed.

[hvr]

wait a minute... why exactly are we dropping GHC 7.4 support?

We need cabal new-build to work back till GHC 7.0 (and at the very least till GHC 7.4), but if the Cabal lib can't be built even with GHC 7.4 then we've got a big problem :-(

@ttuegel Can we revert this please?

[ttuegel]

We need cabal new-build to work back till GHC 7.0 (and at the very least till GHC 7.4), but if the Cabal lib can't be built even with GHC 7.4 then we've got a big problem :-

Cabal still works with GHC >= 7, you'll just need a newer GHC to build it. This is fine; Cabal can be built with a different version of GHC than it is building your package with. This is also in line with our GHC support window policy.

[hvr]

I'm not talking about cabal the executable, but rather Cabal the lib, which needs a larger support window than cabal-the-exe, especially with new-build + setup-depends giving the solver more incentive to recompile a recent Cabal with GHC 7.4 (or when some cabal mode of operations result in an implicit lower bound on Cabal being imposed); Cabal has had a tradition of supporting up to 6 major GHC versions in the past, we shouldn't sacrifice that merely for convenience.

So is there are real blocker that you need to drop support for GHC 7.4?

[ezyang]

To be fair, our GHC window policy (which I wrote) specifies three versions for GHC, and more on a best effort basis. So we should change our window policy.

[ttuegel]

I'm not talking about cabal the executable, but rather Cabal the lib

Yes, I was being specific about punctuation.

which needs a larger support window than cabal-the-exe, especially with new-build + setup-depends giving the solver more incentive to recompile a recent Cabal with GHC 7.4 (or when some cabal mode of operations result in an implicit lower bound on Cabal being imposed); Cabal has had a tradition of supporting up to 6 major GHC versions in the past, we shouldn't sacrifice that merely for convenience.

The stated policy has always been:

• Cabal must build out-of-the-box with any GHC release in the last 3 years
• cabal-install must build with any GHC release in the last 3 years.

If we need to reconsider this policy, please open a new issue. This discussion is not relevant to this PR, which complies with the stated policy. (Actually, this PR is more generous, as GHC 7.6 is also more than three years old.)

[dcoutts]

There may be a good argument now for us reconsidering and expanding our support window. With the advent of setup-deps we can have a packages that need older versions of Cabal, and conversely using a new cabal-install with an older ghc may sill involve using packages that require a more recent Cabal.

So we may have to expand our support window in both directions: point releases of older Cabal to work with newer ghc, and keeping latest Cabal building with older ghc.

[ezyang]

New ticket #3838.