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RE: Q on NaN

From: Drew Adams
Subject: RE: Q on NaN
Date: Fri, 24 Jun 2005 15:54:38 -0700

        (equal 0.0e+NaN) <=?=> (and (numberp x) (/= x x))
        That is, are there any objects equal to 0.0e+NaN that are not NaN?

    They are equivalent. internal_equal contains:
          /* If d is a NaN, then d != d. Two NaNs should be `equal' even
             though they are not =. */

Thanks for confirming. I guess I could have searched the code myself - mea

Given the equivalence, I suggest that the clearest idiom is (equal foo

We should mention this use of `equal' with NaN in the doc: anything `equal'
to a NaN is NaN (all NaN are `equal').

We should also mention the trick (and (numberp foo) (/= foo foo)) ==> foo is

We might still want to add a standard predicate for testing NaN-ness:
`nanp'. (`not-a-number-p' would be confusing.)

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