| But this gives a "less polymorphic" error. I think the problem is that | ghc will not instantiate the "a'" in "IO a'" as a higher-rank type | because it occurs within a type contructor. I believe this is the | restriction referred to at the end of 7.4.9[1]. If I instead write I think that is indeed the heart of your problem. Actually I'm writing a paper right now about how to lift this restriction. An out of date version is here, http://research.microsoft.com/%7Esimonpj/papers/boxy/ that gives the general idea, but a much better version will be done in a week's time. It's not fully implemented yet, but I'll keep your message as an example to try when it is. Simon -cafe