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

Gary Byers gb at clozure.com
Tue Jan 26 21:13:26 UTC 2010



On Tue, 26 Jan 2010, Gary Byers wrote:

> What the hell is "CLZ" ?
>
>

And, to try to be more responsive to Terje's question:

Suppose that someone decided (for barely plausible reasons) that #\;
shouldn't be a macro character and should just be a ordinary constituent
character.

  (set-macro-character #\; nil nil)
  (set-syntax-from-char #\; #\a) ; just in case.

Now, ';abc is just a symbol with a "non-standard" name.

Should '#:;abc be rejected, because the first character of its name
is a macro character in a (standard) readtable that isn't otherwise
in effect ?

Signaling an error on invalid syntax discourages the use of invalid
syntax; I don't understand how it discourages careful use of custom
reader macros (or even fairly casual use of them.)

Use of custom reader macros -does- change the syntax of the language.
In a fresh lisp:

(defun factorial (n) (if (zerop n) 1 (* n (factorial (1- n)))))

(defun ! (x) (factorial x))

(defun !bar (x) (bar (factorial x)))


#\! is a constituent character, no different from (for instance) a
standard alphabetic character.

If we decide that it'd be really neat if #\! was a macro character:

(set-macro-character #\!
   (lambda (stream subchar)
     (declare (ignore subchar))
     (factorial (read stream)))
    t)

? !10
3628800


then we've decided that it's better (more convenient, whatever) to
have #\! be a macro-character than to have it continue to be a
constituent; we've (slightly) changed the syntax of the language
that the reader accepts: we've added the ability to use ! at read
time to introduce "factorial constants", and we've lost the ability
to use #\! as the first character of a symbol name:

? !10
3628800  ; Neat!
? (defun !foo (x) x)
> Error: value FOO is not of the expected type NUMBER. ; Less neat!

This tradeoff is something that has to be kept in mind when defining
reader macros; it isn't some obscure thing that only affects #:.







> On Tue, 26 Jan 2010, 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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>> Openmcl-devel at clozure.com
>> http://clozure.com/mailman/listinfo/openmcl-devel
>>
>>
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