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

[Terje Norderhaug]

On Jan 26, 2010, at 11:58 AM, Ron Garret wrote:
> Just out of curiosity, why would anyone ever want to read an  
> uninterned symbol whose name began with a macro character?

A better question is "why not?", particularly when it comes to user- 
defined macro characters rather than dispatch characters in the  
Common Lisp standard syntax.

The case I have encountered involves my long time use of the special  
sequence "<![[" to signify the beginning of markup inlined in lisp  
code, adhering to the syntax of an SGML/XML "marked section". I am  
mostly using it to inline XSL transformations as a little language  
within lisp code.

Loading a third party module surprisingly failed when reading an  
uninterned symbol that started with "#:<" even if the string after  
the sharpsign colon would have been read as a symbol.

> On Jan 26, 2010, at 11:38 AM, Terje Norderhaug wrote:
>
>> Regarding CLZ not allowing a user defined macro character after the
>> #: of an uninterned symbol:
>>
>> On Jan 25, 2010, at 1:56 PM, Gary Byers wrote:
>>> The spec says:
>>>
>>> "A non-terminating macro character is treated as a constituent when
>>> it appears in the middle of an extended token being accumulated."
>>>
>>> (An initial macro character would cause the function associated with
>>> that character to be called in the standard reader algorithm
>>> describted
>>> in section 2.2; a token that begins with an unescaped macro  
>>> character
>>> would not have "the syntax of a symbol.")
>>>
>>> The spec says that the <<symbol-name>> following #:"must have the
>>> syntax of a symbol"; I think that I'd rather have the reader  
>>> complain
>>> when this is violated than quietly accept invalid/undefined syntax.
>>
>> Wouldn't it be reasonable to understand "syntax of a symbol" in the
>> spec to refer to the Standard Syntax rather than the user extended
>> syntax? If so, the reader could complain when the symbol name
>> following #: starts with a character in the Standard Readtable yet
>> allow custom macro characters in the Current Readtable.
>>
>> Not allowing a user defined macro character after the #: of an
>> uninterned symbol discourages even careful use of custom reader
>> macros. An alternative resolution could be to signal a *continuable*
>> error when encountering an uninterned symbol that starts with a macro
>> character, giving the developer the final say.
>>
>>> On Mon, 25 Jan 2010, Terje Norderhaug wrote:
>>>
>>>> CLZ fails to read an uninterned symbol if the sharpsign colon is
>>>> followed by a macro character. Is this a bug or correct behavior? I
>>>> got no wiser by reading the hyperspec, but presume it's in there
>>>> somewhere:
>>>>
>>>>   http://www.lispworks.com/documentation/lw50/CLHS/Body/02_b.htm
>>>>
>>>> Replicate by evaluating the following:
>>>>
>>>> (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.
>>>>
>>>> Backtrace leads to #'read-symbol-token in the l1-reader.
>>>>
>>>> LispWorks (5.0 Personal) reads '#:!abc without reporting an error.
>>>> Which implementation is right?
>>>>
>>>> -- Terje Norderhaug
>>>>  terje at in-progress.com
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> Openmcl-devel mailing list
>>>> Openmcl-devel at clozure.com
>>>> http://clozure.com/mailman/listinfo/openmcl-devel
>>>>
>>>>
>>
>> -- Terje Norderhaug
>>   terje at in-progress.com
>>
>>
>>
>>
>> _______________________________________________
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>

-- Terje Norderhaug
   terje at in-progress.com