Drop support for cabal < 2

[angerman]

My experience with cabal sandboxes and hadrian has been that they've been buggy and did not rebuild as needed. As hadrian is a tech preview in 8.4 I would like to put cabal >= 2.0 as a requirement for hadrian up for discussion.

It also kills one script to be maintained. Instead of falling back to sandboxes, tell the user to grab a binary distribution of cabal2 from https://www.haskell.org/cabal/download.html.

[snowleopard]

Yes, I'm in favour of this too.

A few of the recent issues seem to be related to the old Cabal, e.g. #463 and #465.

[bgamari]

Andrey Mokhov notifications@github.com writes:

Yes, I'm in favour of this too.

A few of the recent issues seem to be related to the old Cabal, e.g. #463 and #465.

Yes, this sounds reasonable to me.

[bgamari]

Actually, wait a moment. Unless I'm mistaken we don't support building in anything but sandbox mode with cabal 2. Don't we require cabal-install >= 2.1 to use new-build mode?

[snowleopard]

@bgamari Yes, the current constraint is actually cabal-install >= 2.1, since otherwise we could not pass additional parameters to the executable, see #438. Maybe we can relax this to cabal-install >= 2.0 by finding a workaround for #438.

[duog]

Perhaps we could pass parameters to the executable through an environment variable rather than the command line?

[bgamari]

So apparently the plan is to release cabal-install 2.1 well before the GHC 8.4 release, so it's possible we could do mandate 2.1 and keep the global database codepath around for users who really can't upgrade for some reason.

[snowleopard]

@duog I don't think we want to rework Hadrian just to support pre-2.1 Cabal...

@bgamari Sounds good!

[snowleopard]

We have dropped support for Cabal < 2.2 in #668.