On Sun, Jul 1, 2012 at 9:54 PM, Marc Feeley <feeley@iro.umontreal.ca> wrote:
>
> On 2012-07-01, at 3:15 AM, John Cowan wrote:
>
>> Marc Feeley scripsit:
>>
>>> What is the rationale for digit-value ?  Its usefullness seems limited.
>>> It could be used to parse decimal numbers and convert them to their
>>> numerical value.  However, a more general procedure would be useful
>>> to parse numbers in any base:
>>
>> Its purpose is to determine for any digit character (not merely the
>> European digits 0-9) what its numeric value is.  For example,
>> (digit-value #\x0663) => 3, because ٣ U+0663 is the Arabic-Indic digit 3.
>> Of course, if the implementation does not support this character, that
>> won't work.
>
> I understand, but why is this useful?  It seems rather special purpose (for parsing decimal numbers and converting them to their numerical value), but this is the purpose of the string->number procedure.  Digit-value doesn't help me much in parsing hexadecimal.  Perhaps I have missed some other use case.  Moreover, as Peter Bex has pointed out, there is an inconsistency with read and string->number.

The consistency we were making was between digit-value
and char-numeric?, the latter of which was extended to
Unicode semantics.  It seemed strange to be able to tell
that a character had a numeric interpretation yet not know
what that numeric value was.

Neither of these necessarily apply to read or string->number,
since the read syntax is described only for ASCII numbers.
However, since symbols may not begin with any character
with the Unicode numeric property, an implementation would
be free to return (string->number (string #\x0663)) => 3.

So I think it's more consistent to have digit-value but
consider it a fairly minor issue.

--
Alex

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