Incompatible API change in Cabal 1.24.1.0 / PVP Violation

[hvr]

It appears as if Cabal 1.24.1.0 introduced a non-backward compatible change;

I noticed this because when cabal-install-1.24.0.0 is compiled with Cabal-1.24.1.0 one ends up with the following compile error:

[106 of 106] Compiling Main             ( Main.hs, /tmp/matrix-worker/1479637119/dist-newstyle/build/x86_64-linux/ghc-7.10.3/cabal-install-1.24.0.0/build/cabal/cabal-tmp/Main.o )

Main.hs:904:35:
    Couldn't match type ‘Cabal.HaddockTarget’ with ‘Bool’
    Expected type: Flag Bool
      Actual type: Flag Cabal.HaddockTarget
    In the second argument of ‘($)’, namely
      ‘haddockForHackage haddockFlags’
    In the first argument of ‘when’, namely
      ‘(fromFlagOrDefault False $ haddockForHackage haddockFlags)’

Main.hs:1106:69:
    Couldn't match expected type ‘Cabal.HaddockTarget’
                with actual type ‘Bool’
    In the first argument of ‘Flag’, namely ‘True’
    In the ‘haddockForHackage’ field of a record

Further investigation reveals that the culprit appears to be a0b4956

which changed the type-signatures in Distribution.Simple.BuildPaths:

-hscolourPref :: FilePath -> PackageDescription -> FilePath
+hscolourPref :: HaddockTarget -> FilePath -> PackageDescription -> FilePath

-haddockPref :: FilePath -> PackageDescription -> FilePath
+haddockPref :: HaddockTarget -> FilePath -> PackageDescription -> FilePath

This is a serious breach of API contract, especially since Setup.hs scripts are still rather fragile to such kinds of API breakages.

IMO, we should undo this API breakage, and release a Cabal 1.24.1.1 (& cabal-install-1.24.0.2) ASAP, and blacklist-deprecate Cabal 1.24.1.0

/cc @bgamari since depending on the resolution of this issue, GHC 8.0.2, may need to update to version different from Cabal 1.24.1.0

[hvr]

I edited in a

build-depends: Cabal < 1.24.1.0 || > 1.24.1.0

to the cabal-install-1.24.0.0 meta-data: https://hackage.haskell.org/package/cabal-install-1.24.0.0/revisions/

[bgamari]

It would be helpful for the GHC 8.0.2 release cycle if we could sort this out soon; I'd like to get another release candidate out later this week.

[23Skidoo]

I'll try to.

[bgamari]

Any movement here?

[23Skidoo]

@bgamari See #4146.

[23Skidoo]

Fixed by #4146, waiting for @rthomas to cut new 1.24 releases...

[23Skidoo]

@bgamari New Cabal/cabal-install point releases from the 1.24 branch are now out.