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Tamas K Papp | 17 Apr 2012 16:51
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type of SB-EXT infinities

Sorry if this is a stupid question, but why is

(typep sb-ext:double-float-positive-infinity 'double-float)

true?

DOUBLE-FLOAT is a subtype of FLOAT, and the spec says that

  "A float is a mathematical rational (but not a Common Lisp rational)
  of the form s*f*b^e-p, where s is +1 or -1, the sign"

Clearly infinity isn't.  This makes harmless-looking code like

(typecase f
  (float (decode-float f)))

when f is an IEEE infinity.  Don't get me wrong, I am happy that SBCL
has IEEE infinities, but if I read the standard correctly they can't be
a subtype of float.

(The issue came up because I am working on a compatibility library that
would allow portable treatment if infinities.)

Best,

Tamas

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Paul Khuong | 17 Apr 2012 18:26
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Re: type of SB-EXT infinities

In article <87vckybed4.fsf <at> gmail.com>, Tamas K Papp <tkpapp <at> gmail.com> 
wrote:

> Sorry if this is a stupid question, but why is
> 
> (typep sb-ext:double-float-positive-infinity 'double-float)
> 
> true?
> 
> DOUBLE-FLOAT is a subtype of FLOAT, and the spec says that
> 
>   "A float is a mathematical rational (but not a Common Lisp rational)
>   of the form s*f*b^e-p, where s is +1 or -1, the sign"
> 
> Clearly infinity isn't.
[...] 
> when f is an IEEE infinity.  Don't get me wrong, I am happy that SBCL
> has IEEE infinities, but if I read the standard correctly they can't be
> a subtype of float.

    12.1.4.4 Rule of Float Precision Contagion
    The result of a numerical function is a float of the largest format 
    among all the floating-point arguments to the function.

What's the type of the value of (+ inf 1.0), if infinities aren't floats?

I don't think CL was specified with NaNs or infinities in mind:
    12.1.4.3 Rule of Float Underflow and Overflow
    An error of type floating-point-overflow or floating-point-underflow 
    should be signaled if a floating-point computation causes exponent
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