yaml-0.8.31 causes issues on GHC 7.10 and below

[ndmitchell]

I'm seeing this on Travis for HLint: https://travis-ci.org/ndmitchell/hlint/builds/397826785

And the Hackage matrix is showing black squares: https://matrix.hackage.haskell.org/package/yaml

CC @snoyberg. Looking at the code, it has a lower bound on base of GHC 8, so not sure what the problem is...

[jgm]

I'm having the same issue with my pandoc CI builds on ghc 7.10.3 + cabal 2.0.

[jgm]

--- yaml-0.8.30/yaml.cabal	2018-06-29 19:58:56.000000000 +0200
+++ yaml-0.8.31/yaml.cabal	2018-06-29 19:59:56.000000000 +0200
@@ -1,5 +1,5 @@
 name:            yaml
-version:         0.8.30
+version:         0.8.31
 license:         BSD3
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>, Anton Ageev <antage@gmail.com>,Kirill Simonov
@@ -46,7 +46,9 @@
 
 library
     other-extensions: LambdaCase
-    build-depends:   base >= 4 && < 5
+    if impl(ghc < 8.0.2)
+      buildable: False
+    build-depends:   base >= 4.9.1 && < 5
                    , transformers >= 0.1
                    , bytestring >= 0.9.1.4
                    , conduit >= 1.2.8 && < 1.4

Looks like the main thing that changed was raising the minimum bound for base to 4.9.1 and the restriction to ghc >= 8.0.2. But as @ndmitchell says, it's hard to see why cabal's solver didn't just reject the package.

@snoyberg is it impossible to continue to support ghc 7.10.3? We'll have to drop support for it in pandoc solely because of this package. [EDIT: Well, I suppose I can just put an upper bound on the package for older ghcs.]

[ndmitchell]

@jgm as long as Cabal solver is smart enough to pick an older version (which it isn't for no obvious reason), you can continue supporting 7.10.3 even if it results in using an older yaml.

[hvr]

@ndmitchell the reason matrix.hho has black squares is that the construct if impl(ghc < 8.0.2) buildable: False on a library component is something the cabal solver doesn't like much and so doesn't backtrack;

The issues with buildable: False have known for some time (and how buildable: False and build-depends were supposed to interact with each other semantically wasn't too obvious initially either), and until recently there was no urgency to act on it; but when http-client started using this construct, a preliminary fix was created (see haskell/cabal#5337) and it's going to be available in the next cabal-2.2 point release; (but the fix is already available in my PPA's cabal-install-2.2 packages);

@jgm btw, if you just need a standards compliant YAML 1.2 parser that avoids FFI and supports GHCs back till GHC 7.4, HsYAML is currently officially the most compliant YAML 1.2 parser available :-)

[ndmitchell]

@hvr perfectly happy with the yaml package, I just want it to install :)

Is the advice that @snoyberg should avoid the impl(ghc) constraint because that breaks older Cabal users? Saying that people on 5 year old compilers will be fine if only they upgraded their Cabal is really just ditching support for them :(.

[phadej]

@ndmitchell Old Cabal (the lib) is fine. People only need to update cabal-install (the tool), which is not a bad idea.

[jgm]

@hvr thanks for the pointer about HsYaml; I'd like to use a pure Haskell library if possible. I'll try it out.

[ndmitchell]

@phadej saying you can use GHC out the box if you upgrade all the things around it to me doesn't work - the people who are on old GHC are also on old Cabal and old cabal-install.

[gbaz]

@ndmitchell true enough, but upgrading from an older ghc is hard -- you may have a codebase which is not compatible library changes, other couplings, etc. On the other hand, updating cabal-install is just replacing a binary, and should otherwise be just fine.

(also: another, certainly nonideal, workaround, would be for the cabal files of their projects, which are likely proprietary, to simply just exclude the offending version of yaml)

[snoyberg]

Before I make a release to Hackage, can someone confirm that this release will properly work around the bug in cabal-install? snoyberg/yaml@2735b09

[snoyberg]

And here's a workaround for http-client: snoyberg/http-client@1390d6e.

[jgm]

Michael Snoyman <notifications@github.com> writes:

Before I make a release to Hackage, can someone confirm that this release will properly work around the bug in cabal-install? snoyberg/yaml@2735b09

If you add a cabal 2.0 + ghc 7.10.3 combination to your travis CI testing, that should give you the answer.

[ndmitchell]

@snoyberg unfortunately my CI is set up thus that it's almost impossible to test before release is made... If you do make it thought I can give quick feedback after that.