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

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