On Jun 5, 2012, at 9:57 AM, Johan Tibell wrote:
I don't think applying == to something that contains floating point values at the leaves makes much sense. You want some approxEq function that uses approximate equality on floating point value or you want to equality function that ignores the floating point values. Probably not the answer you like, but I don't know how to define Eq in a robust way for types that include floating point values.
I buy that in general for comparing floats (those that result from arithmetic operations), but this is a case where attoparsec's parser is munging the value. I would like to have a law that says "parse . print == id" ... which is why this seems more like a bug than the usual floating point concerns. This law seems to hold for haskell's double parser: quickCheck (\d -> read (show d) == d)