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lang-hs
IO StringvsStringis one of the classic "common issues" in Haskell. You are trying to do pretty advanced stuff (Happstack) which involves monads (and possibly monad transformers). Grasping monads will not provide the immediate solution to your specific problems, but will tell you that certain approaches like trying to convertIO StringtoStringcan not work (in a sense, a main design feature of theIOmonad is precisely to prevent such conversion).