Hi Oleg, Thanks for your reply!
> Really? Here are two 'cons' that seem to satisfy all the criteria Thanks - your definitions are similar to Roman's suggestion. Unfortunately my criteria 3) is not quite what I actually wanted - I really wanted something "GHC-optimisable" - (so non-recursive definitions are a necessary but not sufficient condition). The problem is that I'd like to do the static argument transformation on the "Stream" argument to "cons" so that GHC can optimise it properly. This is why I've made my "cons" pattern match on str directly, so the local "run"/"step" loop can refer to the lexically bound step/state fields of the stream being consed on to. As Roman suggests, the best way to get GHC to optimise these compositions would be to do something like extend GHC so it can do the SAT by itself :-). Alternatively, a type-safe eta for data types involving existentials would let me do what I want without GHC changes - but I don't think there is a way to define this. > Finally, if one doesn't like existentials, one can try > universals: > http://okmij.org/ftp/Algorithms.html#zip-folds > http://okmij.org/ftp/Haskell/zip-folds.lhs I hadn't seen this - thanks! It is certainly a neat trick. I can't quite see how to use it to eliminate the existential I'm actually interested eta-expanding without causing work duplication, but I'll keep thinking about it. Cheers, Max _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
