I am currently reading the chapter on read-time macros from Paul Graham's "On Lisp" book.
The problem I am encountering is the following. When I run one of his examples:
(set-dispatch-macro-character #\# #\?
#’(lambda (stream char1 char2)
‘#’(lambda (&rest ,(gensym))
,(read stream t nil t))))
I get the following error:
No dispatch function defined for #\’
Why is it happening? Could it be because I am running it at the REPL? What can one do to fix it?
The PDF from which you are copying the code uses punctuation marks outside the range of basic ASCII characters you are supposed to use here:
CL-USER> (char-name #\’)
"RIGHT_SINGLE_QUOTATION_MARK"
The usual quote symbol should instead use the apostrophe character:
CL-USER> (char-name #\')
"APOSTROPHE"
The same goes for backquote:
CL-USER> (char-name #\‘)
"LEFT_SINGLE_QUOTATION_MARK"
You should be writing instead:
(set-dispatch-macro-character #\# #\?
#'(lambda (stream char1 char2)
`#'(lambda (&rest ,(gensym))
,(read stream t nil t))))
The #' is not necessary before lambda, since Common Lisp also defines a macro named lambda which expands into (function (lambda ...)).
You can test your new read macro as follows:
CL-USER> #?10
#<FUNCTION (LAMBDA (&REST #:G617)) {1001C541FB}>
CL-USER> (funcall *)
10
When using SBCL, I obtain warnings about unused variables. This happens because the code declares variables in anonymous functions but never uses them. This is not a serious problem, but generally speaking, it is better to declare which variables are ignored:
(set-dispatch-macro-character
#\# #\?
(lambda (stream &rest chars)
(declare (ignore chars))
(let ((rest (gensym)))
`(lambda (&rest ,rest)
(declare (ignore ,rest))
,(read stream t nil t)))))
