On Tue, Jun 5, 2012 at 9:51 AM, Warren Harris wrote:

On Jun 5, 2012, at 9:38 AM, Johan Tibell wrote:

...

You want to perform your test as

   d1 - d2 < epsilon

What's the best way to do this though, since aeson's Value type already provides instance Eq? I guess I can write my own suite of equality comparisons, but these aeson values are the leaves of an extensive grammar and it seems a shame to mimic everything that Eq does (without the benefits of deriving) just to compare doubles differently.

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. -- Johan

On Jun 5, 2012, at 10:51 AM, Bryan O'Sullivan wrote:

On Tue, Jun 5, 2012 at 9:12 AM, Warren Harris wrote:

which helps in many cases, but for some the parsing seems bi-stable, alternating between two imprecise double values and causing the test to fail. I was wondering if anyone could suggest a better work-around for this problem, or explain why Attoparsec's double parser can't be isomorphic to haskell's.

If you need the full precision, use rational instead. The double parser is there because parsing floating point numbers is often a bottleneck, and double intentionally trades speed for precision.

I'm actually using Aeson and the parser it provides, so I don't really have the option of using rational.