[Python-Dev] Why is nan != nan?

[Nick Coghlan]

Mark Dickinson wrote:
> Here's an interesting recent blog post on this subject, from the
> creator of Eiffel:
> 
> http://bertrandmeyer.com/2010/02/06/reflexivity-and-other-pillars-of-civilization/

Interesting. So the natural tweak that would arise from that perspective
is for us to restore reflexivity by declaring that any given NaN is
equal to *itself* but not to any other NaN (even one with the same payload).

With NaN (in general) not being interned, that would actually fit the
idea of a NaN implicitly carrying the operation that created the NaN as
part of its definition of equivalence.

So, I'm specifically putting that proposal on the table for both float
and Decimal NaNs in Python:

  "Not a Number" is not a single floating point value. Instead each
  instance is a distinct value representing the precise conditions that
  created it. Thus, two "NaN" values x and y will compare equal iff they
  are the exact same NaN object (i.e. "if isnan(x) then x == y iff
  x is y".

As stated above, such a change would allow us to restore reflexivity
(eliminating a bunch of weirdness) while still honouring the idea of NaN
being a set of values rather than a single value.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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