begriffs/heroku-buildpack-ghc — pull request #24 by samstokes (2014-02-18 02:38:04+00:00)
Title: Pass --reorder-goals to "cabal install"

I'm submitting this pull request mostly for discussion. The change is required to make my app (a moderate-complexity Yesod app) build, but I'm not entirely sure it would be consequence-free for other apps.

Maybe it would make sense to allow user apps to specify custom options to cabal? Though I don't know what mechanism to use for that now that `user-env-compile` is being removed (so a config variable will no longer work).

More detail
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Tweaks the cabal dependency resolution heuristics in some undocumented way that makes cabal more likely to solve Yesod dependencies. (Without this you see odd errors like yesod-platform getting rejected because it depends on a conflicting version of, say, aeson, than your app does, even though your .cabal file doesn't actually specify which version of aeson, and the whole point of yesod-platform was to lock it to a version that would work.)

This feels like black magic, but it's [recommended by the Yesod documentation](http://www.yesodweb.com/page/quickstart) :(