Hello, Has anyone thought about adding hereditary Harrop formulas, in other words hypothetical reasoning and universal quantification, to the instance contexts in the Hsakell type class system? Just today (and not only today) I wanted to write instance definitions like instance (forall a. C a => D a) => E [a] where ... This is analogous to wanting to write a rank-2 dictionary constructor (forall a. C a -> D a) -> E [a] at the term level, but with type classes, this dictionary constructor should be applied automatically, in a type-directed fashion, at compile time. The theory behind such instance contexts doesn't seem so intractable; indeed it looks decidable to my cursory examination. The opreational intuition is that we would like the type checker to generate an eigenvariable "a" and perform hypothetical reasoning. I would also like to quantify universally over type classes; in other words, if "?" is the kind of a type class constraint (aka a dictionary type; perhaps "o" would be a better choice of notation), then I would like to define not just types of kind "*->*->?" (aka type classes) or kind "(*->*)->?" (aka constructor classes), but also types of kind "(*->?)->(*->?)". But that's for another day... Ken -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig "Hi, my name is Kent, and I let people change my .sig on the internet." "Hi, Ken!" "Put midgets back in midget porn!" -- Not authorized by Ken Shan. Supported by the association of midget porn workers, Local 9823.