SavedConfig Monoid instance: make list fields work like Flags.

[23Skidoo]

Fixes #1884. See code comments for more details.

[23Skidoo]

See #2221 for an alternative approach.

[23Skidoo]

After a discussion with @dcoutts we decided to merge this for the time being, but migrate to an include/include with filtering mechanism in the future.

[snoyberg]

W00t! Do you want me to send PRs for 1.18 and 1.20?

[23Skidoo]

@snoyberg Sure, go ahead. I don't know if @tibbe wants to do another 1.18 release, though.

[snoyberg]

I would really appreciate if we could. I still recommend 1.18 to most people so as to avoid having two different versions of the Cabal library on their systems. (I still regularly run into error messages about conflicting Cabal library versions if I use parallel builds.)

[23Skidoo]

@snoyberg

(I still regularly run into error messages about conflicting Cabal library versions if I use parallel builds.)

Do we have a ticket for this?

[snoyberg]

It was issue #1244. I hadn't noticed that the issue had been closed; I'm still able to reproduce it in some cases. It may be that this issue was resolved with cabal-install-1.20; with 1.18 I can reproduce (which seems contrary to what #1244 says):

$ /opt/cabal/1.18/bin/cabal --version
cabal-install version 1.18.0.4
using version 1.18.1.3 of the Cabal library 
$ /opt/cabal/1.20/bin/cabal --version
cabal-install version 1.20.0.3
using version 1.20.0.0 of the Cabal library 
$ /opt/cabal/1.20/bin/cabal install -j
$ /opt/cabal/1.18/bin/cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal being
used has changed (was Cabal-1.20.0.0, now Cabal-1.18.1.3). Additionally the
compiler is different (was ghc-7.8, now ghc-7.4) which is probably the cause
of the problem.

I believe I've also reproduced this when just using a single version of cabal-install, but I was not able to reproduce that right now.

[23Skidoo]

Additionally the compiler is different (was ghc-7.8, now ghc-7.4) which is probably the cause
of the problem.

Hmm, can it be the case that cabal-1.20 and cabal-1.18 are set to use different versions GHC? Then the error is actually correct. We could improve the UX by calling configure automatically in this case; in fact there's already a ticket for this (#2214).

[snoyberg]

No, they're both using the same version of GHC (ghc-7.8). I believe that error message is simply referring to the version of the compiler which was used to compile cabal-install itself.

[23Skidoo]

I believe that error message is simply referring to the version of the compiler which was used to compile cabal-install itself.

Hmm, yes looks like you're right.

$ /opt/cabal/1.20/bin/cabal install -j
$ /opt/cabal/1.18/bin/cabal build

I thought a bit more about this: it looks like the error is caused by cabal build using the internal setup method, which uses the Cabal version cabal-install was built with. So if you'd stick to just cabal-1.20 or cabal-1.18, you'd be OK.

One approach to making these errors happen less often is to make cabal always use the external/cached setup exe, which would always use the version of Cabal installed on the system.