Pfthb

Can you help me understand why there are 7 symbols (including duplicates) in the value of the following expression ?

​'('a ',(string->symbol "b") 'c))

You can write a function that checks whether elements are symbols.

Pair the element with its "symbolness" for clarity:

(define (sym-check ls)
 (cond ((null? ls) '())
 ((not (pair? ls)) (cons ls (symbol? ls)))
 ((pair? (car ls)) (cons (map sym-check (car ls)) (sym-check (cdr ls))))
 (else (cons (sym-check (car ls)) (sym-check (cdr ls))))))

> (sym-check '('a ',(string->symbol "b") 'c))
'(((quote . #t) (a . #t)) ((quote . #t) 
 ((unquote . #t) 
 ((string->symbol . #t) ("b" . #f)))) 
 ((quote . #t) (c . #t)))

and you get seven #ts.

Note that' and , are "shorthand" for the symbols quote and unquote,

> (quote (unquote (string->symbol "b")))
',(string->symbol "b")

and that using list instead of quoting gives a very different result:

> (sym-check (list 'a ',(string->symbol "b") 'c))
'((a . #t) ((unquote . #t) ((string->symbol . #t) ("b" . #f))) (c . #t))

which is the same as leaving quoting out of the quoted list:

> (sym-check '(a ,(string->symbol "b") c))
'((a . #t) ((unquote . #t) ((string->symbol . #t) ("b" . #f))) (c . #t))

​'('a ',(string->symbol "b") 'c)) evaluates to the following structure:

((quote a) 
 (quote (unquote (string->symbol "b"))) 
 (quote c))

I can count 7 so yes. It is 7 symbols in the result of evaluating the quoted expression.

Scheme system has a reader that translates 'x to (quote x) and ,, `, and ,@ corresponds to forms with quasiquote, unquote, and unquote-splicing. That means that the code `(a b c ,d e f) becomes (quasiquote (a b c (unquote d) e f)). Then the macos in the implementation translates it into (list* 'a 'b 'c d '(e f)) or a similar expression that does the same. However with '`(a b c ,d e f) just becomes the value (quasiquote (a b c (unquote d) e f)) since that is the expression that was quoted. Scheme doesn't expand into normal quote expression.

Usually quote characters inside quoted datum is a bug. Beginners doesn't understand the purpose and thin it needs to be everywhere. But really you only need the outer quote. (+ 4 5) is 9 and '(+ 4 5) is (+ 4 5). See the difference?