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Hmm, now that I think about it, I should have used "Not so straight forward" instead of "very awkward". In reality, almost everything composes really well!
I've been keeping this a secret, but for the past 18 months, I've been collaborating with Simon Peyton Jones and Richard Eisenberg on formalizing GHC's typechecker! It turns out that there are some very awkward interactions between the extensions.
