Meaning of 'quote in Lisp

问题:

This question arose when reading SICP. Why (list 'quote '(a b c)) evaluated by the interpreter (R5RS in Dr.Racket) as '(a b c). For me it should be (quote (a b c)). For instance (list 'quot '(a b c)) is evaluated as (quot (a b c)). What is so special in the 'quote?

回答1:

It also took me a while to understand this problem. But it's just your good-hearted lisp interpreter showing (quote (a b c)) in it's equivalent form '(a b c). Since there is no such equivalence/syntactic sugar for (quott (a b c)), it's shown as is.

回答2:

You'll get different behaviors depending on exactly what Lisp you're using (Scheme, Racket, Common Lisp, etc.) but in general, the system will accept 'x as a shorthand or syntactic sugar for (quote x). The two forms are exactly equivalent and their values are the same: the unevaluated x. When a result is coming out of the system, it might choose to print in the first way to make the result more intuitive to the user. A similar thing happens with cons, too. For instance,

(cons 1 2)
;=> (1 . 2)

because that's the general way that cons cells (pairs) are printed. However, there's a special case defined for when the second part of the pair is another list (either the empty list () or another pair, and that's why we have the following. I've also written a bit more about how lists and cons cells are printed in an answer to Recursive range in Lisp adds a period?.

(cons 1 '())
;=> (1)

(cons 1 '(2 3))
;=> (1 2 3)

Now, I've written the values of the expression above. E.g., the value of the form (cons 1 '(2 3)) is the list (1 2 3). As an additional complication, some systems (I'm thinking of some languages is Dr. Racket, in particular) don't print the value of a form in the interactive prompt, but rather print a form that would produce the same (for certain interpretations of “the same”) values. For instance, you might evaluate '(1 . 2) and see the output (cons 1 2) because that's another form that would produce the same value. This can be helpful if you're doing pure functional programming that has referential transparency, but if you're not expecting it, it can lead to some confusion.

A good way to see that we're getting the results that we should, regardless of how the system prints them, is to inspect them. We expect that (list 'quote '(a b c)) should return a list whose car is the symbol quote and whose cadr is the list (a b c). This is what we get (in Dr. Racket 5.3 with language R⁵RS):

> (display (list 'quote '(a b c)))
'(a b c)
> (display (car (list 'quote '(a b c))))
quote
> (display (cadr (list 'quote '(a b c))))
(a b c)

We get similar results if we use 'qmmmt instead of 'quote:

> (display (list 'qmmmt '(a b c)))
(qmmmt (a b c))
> (display (car (list 'qmmmt '(a b c))))
qmmmt
> (display (cadr (list 'qmmmt '(a b c))))
(a b c)

The only difference is that in the first case, display displays the list whose car is the symbol quote using the shorthand that is available for such lists. That is, instead of displaying (quote (a b c)) it displayed '(a b c).

回答3:

'(a b c) and (quote (a b c)) are actually different notations for the same. So don't be surprised if your Lisp prints the shorter version.

In general '<something> is the same as (quote <something>).

QUOTE is used in Lisp to mark expressions which should evaluate to themselves. Usually a list would be a function or macro call and a symbol would be a variable. If you want to treat those as data, you need to quote them.

Since (quote <something>) is used so often in Lisp, the abbreviated version '<something> has been introduced to save a bit of typing or reading...

回答4:

display emits some behaviors. Eg. '(a . (b . (c . ()))) is displayed (a b c). 'quote is displayed quote and perhaps '(quote x y) is displayed (quote x y) while '(quote x) is displayed 'x or (quote x). Which one is implementation dependent but both mean the same.

As data (ie. quoted, like (quote quote) and it's abbrivation 'quote) the result of the evaluation, the symbol quote is nothing special, for any LISP, just like '+ and 'potato happen to be the symbols + and potato. Any symbol that mean something when not quoted is no special when quoted.