Hi all, I have these functions: toDouble :: (Rounding r, Precision p) => Rounded r p -> Double fromDouble :: (Rounding r, Precision p) => Double -> Rounded r p and I want to use RULES for optimization: {-# RULES "realToFrac/toDouble" realToFrac = toDouble #-} {-# RULES "realToFrac/fromDouble" realToFrac = fromDouble #-} This doesn't work. I found out (thanks to Stack Overflow via #haskell) that I can modify the first rule to the following, which does compile and also seems to fire in a simple test: {-# RULES "realToFrac/toDouble" forall (x :: (Rounding r, Precision p) => Rounded r p) . realToFrac x = toDouble x #-} But I'm at a loss with the fromDouble rule, the error message is: • Could not deduce (Rounding r) arising from a use of ‘fromDouble’ from the context: (Real Double, Fractional (Rounded r p)) bound by the RULE "realToFrac/fromDouble" at src/Numeric/Rounded.hs:359:11-57 Possible fix: add (Rounding r) to the context of the RULE "realToFrac/fromDouble" • In the expression: fromDouble When checking the transformation rule "realToFrac/fromDouble" How do I apply the "possible fix" to the fromDouble rule? Is it even possible with the RULES syntax? Note, there are these instances (among others): instance (Rounding r, Precision p) => Real (Rounded r p) instance (Rounding r, Precision p) => Fractional (Rounded r p) Full code at https://code.mathr.co.uk/rounded (claude5 branch). Thanks for any insight, Claude -- https://mathr.co.uk