I think there should be a post/page/whatever somewhere explaining how to go back to ghc 6.10.4.

Here is how I did :

pacman -Qu http://arm.konnichi.com/extra/os/i686/ghc-6.10.4-1-i686.pkg.tar.gz

-- the 64 bits version is : http://arm.konnichi.com/extra/os/x86_64/ghc-6.10.4-1-x86_64.pkg.tar.gz

after having removed ghc 6.12 properly, and all the packages built for/by 6.12.

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