[Haskell-cafe] attoparsec double precision, quickCheck and aeson

[Warren Harris]

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)