"complex Cabal issue" error

[begriffs]

When trying to deploy https://github.com/knowledge-map/graph-rtce the build freezes with:

-----> Restoring sandbox ghc-7.8.2
       Downloading http://s3.halcyon.sh/halcyon-sandbox-ghc-7.8.2-1862ba43c1a8c96b8efbe0198750cc3a.tar.gz... done
       Extracting halcyon-sandbox-ghc-7.8.2-1862ba43c1a8c96b8efbe0198750cc3a.tar.gz... done
-----> Extending matched sandbox ghc-7.8.2 to sandbox ghc-7.8.2-graph-rtce-0.0.0.0

       In case of timeout, disable sandbox extending and push again:
       $ heroku config:set NO_EXTEND_SANDBOX=1
       $ git push heroku master

-----> Building sandbox ghc-7.8.2-graph-rtce-0.0.0.0...
   *** WARNING: Installing implicit versions of alex and happy

       Congratulations!  You have reproduced a complex Cabal issue!
       Please let the Cabal developers know:
       https://github.com/haskell/cabal/issues/220
       https://github.com/haskell/cabal/issues/779

[mietek]

Thanks. I see this warning message is misleading, and I will improve it. The build is not frozen — it's just proceeding from an empty sandbox, so it's going to take a while.

The real issue here is that the sandbox prepared for scotty-0.7.3 should not have been rejected. Investigating.

-----> Inferring sandbox digest... done, f5bd2cbdffb7b810726caefc648c4484
-----> Restoring sandbox ghc-7.8.2-graph-rtce-0.0.0.0
       Downloading http://s3.halcyon.sh/halcyon-sandbox-ghc-7.8.2-graph-rtce-0.0.0.0-f5bd2cbdffb7b810726caefc648c4484.tar.gz... 404
   *** WARNING: Sandbox ghc-7.8.2-graph-rtce-0.0.0.0 is not prepared
-----> Locating matched sandboxes
       ...
-----> Scoring matched sandboxes
       ...
   *** DEBUG: Rejecting sandbox ghc-7.8.2-scotty-0.7.3-85f4b20dc9ea76039a0b5582035082c8 because mtl-2.1.3.1 is not mtl-2.2.0.1
       ...
       4    ghc-7.8.2-1862ba43c1a8c96b8efbe0198750cc3a
-----> Restoring sandbox ghc-7.8.2

[mietek]

The misleading messages are gone since 2e2c0e0. Logging has been improved in general. Since 7c2e7e6, build progress is no longer silent by default.

After updating prepared sandboxes, it appears there are two issues affecting graph-rtce:

1. Its type-eq == 0.4 constraint should be relaxed, as type-eq-0.4 does not build.
2. type-eq-0.4.2 needs cpphs, but executable-only dependencies are not supported yet.

This is now #4.

[begriffs]

I relaxed those graph-rtce constraints in my own fork. See these commits

begriffs/graph-rtce@2923544
begriffs/graph-rtce@7c5a981

I've found that graph-rtce is a good project in general to benchmark the buildpack because its requirements are fairly demanding.