[Scheme-reports] Definition of expt (was #391 - a selection of error predicates)

[Mark H Weaver <mhw@x>]

On 08/08/2012 02:49 AM, Alex Shinn wrote (on the WG1 list):
> These predicates seem random, reflecting only the
> historical precedent of what was stated as "signals
> an error" vs "is an error".  I'm not even convinced that
> distinction was made by design rather than the
> stylistic convention of the editors at the time.
>
> For expt, in particular, we added this note ourselves,
> and the actual text is:
>
>    0.0^z is 1.0 if z = 0.0, and 0.0 if (real-part z) is positive.
>    For other cases in which the first argument is zero, either
>    an error is signalled or an unspecified number is returned.
>
> Apart from the wording being a clumsy, saying
> an "error is signalled _or_ an unspecified result
> is returned" is the same as saying "the result is
> an error."

The text does not say "or an unspecified result is returned".  It says 
"or an unspecified _number_ is returned".  Therefore, it is not the same.

> I propose changing the text to:
>
>    0^z is 1 if z = 0, and 0 if (real-part z) is positive.
>    It is an error if (real-part z) is negative.

You've made two questionable changes to the meaning of this definition, 
beyond the description of what happens in the error/unspecified case.

1. The current text uses "0.0" and "1.0" which suggests inexact numbers.
You've changed these to exact numbers.   I interpret the current text to 
apply only when the base is an _inexact_ zero, and to specify inexact 
results.  I interpret your proposed text as mandating exact return 
values, and possibly also applying when the base is an exact zero.

2. In the current text, the error/unspecified case consists of the 
following possibilities: (real-part z) is negative, or (real-part z) is 
zero and (imag-part z) is non-zero.  Your change in wording now fails to 
specify the behavior in the latter case.

> [although it should probably also be an error if
> (imag-part z) is non-zero.]

Why do you say that?

      Mark

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