RULES and type classes

Mike Hamburg haskell at
Thu Mar 29 05:38:20 EDT 2007

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 

    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.

Mike Hamburg

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