[Openmcl-devel] Error on macro character after sharpsign colon

Ron Garret 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  
>>>> symbol.
>> 
>>> 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)

'!abc
=> ABC

'#:!abc
=> 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.

rg




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