In Lisp (Clojure, Emacs Lisp), what is the difference between list and quote?

Question

From reading introductory material on Lisp, I now consider the following to be identical:

(list 1 2 3)

'(1 2 3)

However, judging from problems I face when using the quoted form in both Clojure and Emacs Lisp, they are not the same. Can you tell me what the difference is?

Answer 1

The primary difference is that quote prevents evaluation of the elements, whereas list does not:

user=> '(1 2 (+ 1 2))
(1 2 (+ 1 2))
user=> (list 1 2 (+ 1 2))
(1 2 3)

For this reason (among others), it is idiomatic clojure to use a vector when describing a literal collection:

user=> [1 2 (+ 1 2)]
[1 2 3]

Answer 2

In Common Lisp, quoted objects are constant literal data. The data is not evaluated. You should not modify this data, as the consequences are undefined. Possible consequences are: modification of shared data, attempt to modify read-only data, an error might be signalled, it might just work, shared data, ...

Literal lists:

'(1 2 3)

Above is a constant list, which will be constructed by the reader and evaluating to itself, because it is quoted. If it appears in Lisp code, a compiler will embed this data somehow in the FASL code.

(quote (1 2 3)) is another way to write it.

(list 1 2 3)

this is a call of the Common Lisp function LIST with three arguments 1, 2 and 3. Each of the arguments will be evaluated. Since they are numbers, they evaluate to themselves. When evaluated the result is a fresh new list (1 2 3).

Data sharing in compiled code

Imagine in a Lisp file the following four definitions:

(defparameter *list1* (list 1 2 3))
(defparameter *list2* (list 1 2 3))
(defparameter *list3* '(1 2 3))
(defparameter *list4* '(1 2 3))

Then we compile and load the file.

! (eq *list3* *list4*) now may evaluate to either T or NIL depending on the implementation and compiler settings !

Reason: in Common Lisp the Lisp compiler may share structure of literal lists (!) if they are similar. The compiler detects that here the lists are similar and will allocate only one list. Both variables *list1* and *list2* the will point to this one list.

All other EQ (object equality) comparisons of two of the above lists will return NIL.

Notations for some other data structures:

'(1 . 2)            and  (cons 1 2)            ; cons cell

'#(1 2 3)           and  (vector 1 2 3)        ; vector

'#S(FOO :a 1 :b 2)  and (make-foo :a 1 :b 2)   ; structure

One is the literal data and the other is a function call that constructs such a data structure.

Answer 3

Quoted lists (e.g. '(1 2 3)) should be treated carefully (generally as read-only). (see SO answers When to use 'quote in Lisp and When to use 'quote in Lisp).

(list 1 2 3) will "cons" up a fresh list, independent of all others.

You can see an example of a pitfall of using quoted lists in the manual for nconc.

And, as you probably know, when you call 'list - the arguments will obviously be evaluated versus the contents of a quoted list. And 'quote takes a single argument, versus 'lists variable number of arguments.

(list (+ 1 2) 3)     -->  (3 3)
(quote ((+ 1 2) 3))  -->  ((+ 1 2) 3)

Answer 4

Their relation can be analogous to function invocation with 'function name' and funcall.

When you have no idea what function you'll get at runtime, you use funcall.

When you have no idea what element you may get at runtime, your use list.

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For people like me who get confused because of existence of backquote and count it as quote tacitly.

backquote is not quote

It's a reader-macro that expands into quote, list or others:

(macroexpand ''(1 2));=> '(1 2)
(macroexpand '`(1 2));=> '(1 2)
(macroexpand '`(1 ,2));=> (list 1 2)
(macroexpand '`(1 ,@foo));=> (cons 1 foo)
(macroexpand '`(1 ,@foo 2));=> (cons 1 (append foo '(2)))