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

Terje Norderhaug terje at in-progress.com
Tue Jan 26 19:38:02 UTC 2010

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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