[Openmcl-devel] Error on macro character after sharpsign colon
ron at flownet.com
Wed Jan 27 07:54:32 UTC 2010
On Jan 26, 2010, at 11:00 PM, Terje Norderhaug wrote:
> On Jan 26, 2010, at 6:28 PM, Ron Garret wrote:
>> On Jan 26, 2010, at 5:55 PM, Terje Norderhaug wrote:
>>>>> Don't let the particular situation lead you astray: This does
>>>>> have to do with reading of uninterned symbols. Rather than
>>>>> loading a third party module, I could just as well have written
>>>>> the uninterned symbol "#:<" in my own code and expected it to
>>>>> be read without error, just as "<" would be read as a symbol.
>>>> If tokens were first-class entities that might be a reasonable
>>>> expectation, but they aren't. If you think through the process
>>>> in detail you will see that having "<" be read as a (possibly
>>>> interned) symbol by a user defined reader macro function is
>>>> fundamentally incompatible with "#:<" being read as an uninterned
>>> Let's not confuse the textual representation with the object
>> That's not the issue. The issue is that in order to read a symbol,
>> the reader first has to assemble a TOKEN. By what method do you
>> propose that it do so in the case that #: is followed by another
>> macro character? Calling the macro function for that character
>> can't work because there is no way that that function can return a
>> token because tokens are not first-class data structures in Common
>> Lisp. So what is it supposed to do?
> It's not a practical problem. Simply commenting out the (logbitp
> $cht_macbit attr) in CLZ's #'read-symbol-token makes it read #:<
> without reporting an error even if the character '<' is a non-
> terminating macro character.
So your answer is: it should read the token as if the current readtable were not in effect. That is not altogether unreasonable, but it is IMHO (and Gary's apparently) in violation of the spec.
I would also like to point out, in the furtherance of pedagogy, that your motivating example:
(defun symbol-reader (stream char)
(declare (ignore char))
(read stream t nil t))
(set-macro-character #\! #'symbol-reader T)
=> Reader error: Illegal symbol syntax.
was very misleading. The fact that !abc returns a symbol turns out to be a red herring. Under the hack you are proposing, #:!abc would read as a symbol no matter what the reader macro for #\! did (including generating an error) and your emphasizing that !abc read as a symbol really lead me down the wrong line of thinking.
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