Re: [Haskell-cafe] Mystery of an Eq instance

On Sat, Sep 21, 2013 at 10:26 AM, Mike Meyer wrote:

On Sat, Sep 21, 2013 at 2:21 AM, Bardur Arantsson wrote:

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On 2013-09-21 06:16, Mike Meyer wrote:

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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.

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I do have to agree with Damodar Kulkarni that different laws imply different classes. However, this will break **a lot** of existing software. If we would do this, only Eq and Ord need to be duplicated, as they cause most of the problems. Qualified imports should suffice to differentiate between the two. import qualified Data.Eq.Approximate as A import qualified Data.Ord.Approximate as A main = print $ 3.16227766016837956 A.== 3.16227766016837955