Is there any way to use RULES substitutions with type classes? I'm writing a reactive programming arrow (same idea as Yampa, different design goals), and it would help performance (and not just in the speed sense) to be able to tell when a value derived with arr hasn't changed. So I'd like to be able to specialize arr to functions whose result is an instance of Eq. I tried {-# RULES "reactiveArr/Eq" reactiveArr = reactiveArrEq #-} but got the message Control/Arrow/Reactive/Reactive.hs:89:41: No instance for (Eq b) arising from instantiating a type signature at Control/Arrow/Reactive/Reactive.hs:89:41-89 Possible fix: add (Eq b) to the tcRule When checking the transformation rule "reactiveArr/Eq" I tried adding various sorts of type signatures, but I couldn't find any way around this... is it a restriction in the RULES rewrite engine? Is there a workaround, or some mechanism other than RULES that I should be using? I could write a special "arrEq" function, but I'd like to minimize the number of extraneous operations outside the Arrow class. Thanks, Mike Hamburg