Turns out if you do that you end up with hs-mtl and friends which will conflict with yesod's dependencies. You'll probably have to do an install from https://www.haskell.org/cabal/release/cabal-install-1.22.0.0/cabal-1.22.0.0-x86_64-apple-darwin-mavericks.tar.gz (apparently there are no binary builds for non-Mavericks, so if you need that then you have a problem :/ )

Or you can remove the MacPorts-based ghc stuff and do an install as recommended by https://stackage.org and make certain it can't find any MacPorts-installed library because that will lead to libiconv conflicts.

Apparently the yesod/Stackage folks think this insanity is a good thing.
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