On Sat, Sep 21, 2013 at 2:21 AM, Bardur Arantsson <spam@scientician.net> wrote:
On 2013-09-21 06:16, Mike Meyer wrote:
The single biggest gotcha is that two calculations we expect to be equal often aren't. As a result of this, we warn people not to do equality comparison on floats. The Eq instance for Float violates at least one expected law of Eq:
Prelude> let nan = 0/0 Prelude> nan == nan False
Yeah, Nan's are a whole 'nother bucket of strange. But if violating an expected law of a class is a reason to drop it as an instance, consider: Prelude> e > 0 True Prelude> 1 + e > 1 False Of course, values "not equal when you expect them to be" breaking equality means that they also don't order the way you expect: Prelude> e + e + 1 > 1 + e + e True So, should Float's also not be an instance of Ord? I don't think you can turn IEEE 754 floats into a well-behaved numeric type. A wrapper around a hardware type for people who want that performance and can deal with its quirks should provide access to as much of the types behavior as possible, and equality comparison is part of IEEE 754 floats. <mike