Related
After making it through the major parts of an introductory Lisp book, I still couldn't understand what the special operator (quote) (or equivalent ') function does, yet this has been all over Lisp code that I've seen.
What does it do?
Short answer
Bypass the default evaluation rules and do not evaluate the expression (symbol or s-exp), passing it along to the function exactly as typed.
Long Answer: The Default Evaluation Rule
When a regular (I'll come to that later) function is invoked, all arguments passed to it are evaluated. This means you can write this:
(* (+ a 2)
3)
Which in turn evaluates (+ a 2), by evaluating a and 2. The value of the symbol a is looked up in the current variable binding set, and then replaced. Say a is currently bound to the value 3:
(let ((a 3))
(* (+ a 2)
3))
We'd get (+ 3 2), + is then invoked on 3 and 2 yielding 5. Our original form is now (* 5 3) yielding 15.
Explain quote Already!
Alright. As seen above, all arguments to a function are evaluated, so if you would like to pass the symbol a and not its value, you don't want to evaluate it. Lisp symbols can double both as their values, and markers where you in other languages would have used strings, such as keys to hash tables.
This is where quote comes in. Say you want to plot resource allocations from a Python application, but rather do the plotting in Lisp. Have your Python app do something like this:
print("'(")
while allocating:
if random.random() > 0.5:
print(f"(allocate {random.randint(0, 20)})")
else:
print(f"(free {random.randint(0, 20)})")
...
print(")")
Giving you output looking like this (slightly prettyfied):
'((allocate 3)
(allocate 7)
(free 14)
(allocate 19)
...)
Remember what I said about quote ("tick") causing the default rule not to apply? Good. What would otherwise happen is that the values of allocate and free are looked up, and we don't want that. In our Lisp, we wish to do:
(dolist (entry allocation-log)
(case (first entry)
(allocate (plot-allocation (second entry)))
(free (plot-free (second entry)))))
For the data given above, the following sequence of function calls would have been made:
(plot-allocation 3)
(plot-allocation 7)
(plot-free 14)
(plot-allocation 19)
But What About list?
Well, sometimes you do want to evaluate the arguments. Say you have a nifty function manipulating a number and a string and returning a list of the resulting ... things. Let's make a false start:
(defun mess-with (number string)
'(value-of-number (1+ number) something-with-string (length string)))
Lisp> (mess-with 20 "foo")
(VALUE-OF-NUMBER (1+ NUMBER) SOMETHING-WITH-STRING (LENGTH STRING))
Hey! That's not what we wanted. We want to selectively evaluate some arguments, and leave the others as symbols. Try #2!
(defun mess-with (number string)
(list 'value-of-number (1+ number) 'something-with-string (length string)))
Lisp> (mess-with 20 "foo")
(VALUE-OF-NUMBER 21 SOMETHING-WITH-STRING 3)
Not Just quote, But backquote
Much better! Incidently, this pattern is so common in (mostly) macros, that there is special syntax for doing just that. The backquote:
(defun mess-with (number string)
`(value-of-number ,(1+ number) something-with-string ,(length string)))
It's like using quote, but with the option to explicitly evaluate some arguments by prefixing them with comma. The result is equivalent to using list, but if you're generating code from a macro you often only want to evaluate small parts of the code returned, so the backquote is more suited. For shorter lists, list can be more readable.
Hey, You Forgot About quote!
So, where does this leave us? Oh right, what does quote actually do? It simply returns its argument(s) unevaluated! Remember what I said in the beginning about regular functions? Turns out that some operators/functions need to not evaluate their arguments. Such as IF -- you wouldn't want the else branch to be evaluated if it wasn't taken, right? So-called special operators, together with macros, work like that. Special operators are also the "axiom" of the language -- minimal set of rules -- upon which you can implement the rest of Lisp by combining them together in different ways.
Back to quote, though:
Lisp> (quote spiffy-symbol)
SPIFFY-SYMBOL
Lisp> 'spiffy-symbol ; ' is just a shorthand ("reader macro"), as shown above
SPIFFY-SYMBOL
Compare to (on Steel-Bank Common Lisp):
Lisp> spiffy-symbol
debugger invoked on a UNBOUND-VARIABLE in thread #<THREAD "initial thread" RUNNING {A69F6A9}>:
The variable SPIFFY-SYMBOL is unbound.
Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.
restarts (invokable by number or by possibly-abbreviated name):
0: [ABORT] Exit debugger, returning to top level.
(SB-INT:SIMPLE-EVAL-IN-LEXENV SPIFFY-SYMBOL #<NULL-LEXENV>)
0]
Because there is no spiffy-symbol in the current scope!
Summing Up
quote, backquote (with comma), and list are some of the tools you use to create lists, that are not only lists of values, but as you seen can be used as lightweight (no need to define a struct) data structures!
If you wish to learn more, I recommend Peter Seibel's book Practical Common Lisp for a practical approach to learning Lisp, if you're already into programming at large. Eventually on your Lisp journey, you'll start using packages too. Ron Garret's The Idiot's Guide to Common Lisp Packages will give you good explanation of those.
Happy hacking!
It says "don't evaluate me". For example, if you wanted to use a list as data, and not as code, you'd put a quote in front of it. For example,
(print '(+ 3 4)) prints "(+ 3 4)", whereas
(print (+ 3 4)) prints "7"
Other people have answered this question admirably, and Matthias Benkard brings up an excellent warning.
DO NOT USE QUOTE TO CREATE LISTS THAT YOU WILL LATER MODIFY. The spec allows the compiler to treat quoted lists as constants. Often, a compiler will optimize constants by creating a single value for them in memory and then referencing that single value from all locations where the constant appears. In other words, it may treat the constant like an anonymous global variable.
This can cause obvious problems. If you modify a constant, it may very well modify other uses of the same constant in completely unrelated code. For example, you may compare some variable to '(1 1) in some function, and in a completely different function, start a list with '(1 1) and then add more stuff to it. Upon running these functions, you may find that the first function doesn't match things properly anymore, because it's now trying to compare the variable to '(1 1 2 3 5 8 13), which is what the second function returned. These two functions are completely unrelated, but they have an effect on each other because of the use of constants. Even crazier bad effects can happen, like a perfectly normal list iteration suddenly infinite looping.
Use quote when you need a constant list, such as for comparison. Use list when you will be modifying the result.
One answer to this question says that QUOTE “creates list data structures”. This isn't quite right. QUOTE is more fundamental than this. In fact, QUOTE is a trivial operator: Its purpose is to prevent anything from happening at all. In particular, it doesn't create anything.
What (QUOTE X) says is basically “don't do anything, just give me X.” X needn't be a list as in (QUOTE (A B C)) or a symbol as in (QUOTE FOO). It can be any object whatever. Indeed, the result of evaluating the list that is produced by (LIST 'QUOTE SOME-OBJECT) will always just return SOME-OBJECT, whatever it is.
Now, the reason that (QUOTE (A B C)) seems as if it created a list whose elements are A, B, and C is that such a list really is what it returns; but at the time the QUOTE form is evaluated, the list has generally already been in existence for a while (as a component of the QUOTE form!), created either by the loader or the reader prior to execution of the code.
One implication of this that tends to trip up newbies fairly often is that it's very unwise to modify a list returned by a QUOTE form. Data returned by QUOTE is, for all intents and purposes, to be considered as part of the code being executed and should therefore be treated as read-only!
The quote prevents execution or evaluation of a form, turning it instead into data. In general you can execute the data by then eval'ing it.
quote creates list data structures, for example, the following are equivalent:
(quote a)
'a
It can also be used to create lists (or trees):
(quote (1 2 3))
'(1 2 3)
You're probably best off getting an introductary book on lisp, such as Practical Common Lisp (which is available to read on-line).
In Emacs Lisp:
What can be quoted ?
Lists and symbols.
Quoting a number evaluates to the number itself:
'5 is the same as 5.
What happens when you quote lists ?
For example:
'(one two) evaluates to
(list 'one 'two) which evaluates to
(list (intern "one") (intern ("two"))).
(intern "one") creates a symbol named "one" and stores it in a "central" hash-map, so anytime you say 'one then the symbol named "one" will be looked up in that central hash-map.
But what is a symbol ?
For example, in OO-languages (Java/Javascript/Python) a symbol could be represented as an object that has a name field, which is the symbol's name like "one" above, and data and/or code can be associated with it this object.
So an symbol in Python could be implemented as:
class Symbol:
def __init__(self,name,code,value):
self.name=name
self.code=code
self.value=value
In Emacs Lisp for example a symbol can have 1) data associated with it AND (at the same time - for the same symbol) 2) code associated with it - depending on the context, either the data or the code gets called.
For example, in Elisp:
(progn
(fset 'add '+ )
(set 'add 2)
(add add add)
)
evaluates to 4.
Because (add add add) evaluates as:
(add add add)
(+ add add)
(+ 2 add)
(+ 2 2)
4
So, for example, using the Symbol class we defined in Python above, this add ELisp-Symbol could be written in Python as Symbol("add",(lambda x,y: x+y),2).
Many thanks for folks on IRC #emacs for explaining symbols and quotes to me.
Code is data and data is code. There is no clear distinction between them.
This is a classical statement any lisp programmer knows.
When you quote a code, that code will be data.
1 ]=> '(+ 2 3 4)
;Value: (+ 2 3 4)
1 ]=> (+ 2 3 4)
;Value: 9
When you quote a code, the result will be data that represent that code. So, when you want to work with data that represents a program you quote that program. This is also valid for atomic expressions, not only for lists:
1 ]=> 'code
;Value: code
1 ]=> '10
;Value: 10
1 ]=> '"ok"
;Value: "ok"
1 ]=> code
;Unbound variable: code
Supposing you want to create a programming language embedded in lisp -- you will work with programs that are quoted in scheme (like '(+ 2 3)) and that are interpreted as code in the language you create, by giving programs a semantic interpretation. In this case you need to use quote to keep the data, otherwise it will be evaluated in external language.
When we want to pass an argument itself instead of passing the value of the argument then we use quote. It is mostly related to the procedure passing during using lists, pairs and atoms
which are not available in C programming Language ( most people start programming using C programming, Hence we get confused)
This is code in Scheme programming language which is a dialect of lisp and I guess you can understand this code.
(define atom? ; defining a procedure atom?
(lambda (x) ; which as one argument x
(and (not (null? x)) (not(pair? x) )))) ; checks if the argument is atom or not
(atom? '(a b c)) ; since it is a list it is false #f
The last line (atom? 'abc) is passing abc as it is to the procedure to check if abc is an atom or not, but when you pass(atom? abc) then it checks for the value of abc and passses the value to it. Since, we haven't provided any value to it
Quote returns the internal representation of its arguments. After plowing through way too many explanations of what quote doesn't do, that's when the light-bulb went on. If the REPL didn't convert function names to UPPER-CASE when I quoted them, it might not have dawned on me.
So. Ordinary Lisp functions convert their arguments into an internal representation, evaluate the arguments, and apply the function. Quote converts its arguments to an internal representation, and just returns that. Technically it's correct to say that quote says, "don't evaluate", but when I was trying to understand what it did, telling me what it doesn't do was frustrating. My toaster doesn't evaluate Lisp functions either; but that's not how you explain what a toaster does.
Anoter short answer:
quote means without evaluating it, and backquote is quote but leave back doors.
A good referrence:
Emacs Lisp Reference Manual make it very clear
9.3 Quoting
The special form quote returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.)
Special Form: quote object
This special form returns object, without evaluating it.
Because quote is used so often in programs, Lisp provides a convenient read syntax for it. An apostrophe character (‘'’) followed by a Lisp object (in read syntax) expands to a list whose first element is quote, and whose second element is the object. Thus, the read syntax 'x is an abbreviation for (quote x).
Here are some examples of expressions that use quote:
(quote (+ 1 2))
⇒ (+ 1 2)
(quote foo)
⇒ foo
'foo
⇒ foo
''foo
⇒ (quote foo)
'(quote foo)
⇒ (quote foo)
9.4 Backquote
Backquote constructs allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form quote (described in the previous section; see Quoting). For example, these two forms yield identical results:
`(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
'(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
The special marker ‘,’ inside of the argument to backquote indicates a value that isn’t constant. The Emacs Lisp evaluator evaluates the argument of ‘,’, and puts the value in the list structure:
`(a list of ,(+ 2 3) elements)
⇒ (a list of 5 elements)
Substitution with ‘,’ is allowed at deeper levels of the list structure also. For example:
`(1 2 (3 ,(+ 4 5)))
⇒ (1 2 (3 9))
You can also splice an evaluated value into the resulting list, using the special marker ‘,#’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘`’ is often unreadable. Here are some examples:
(setq some-list '(2 3))
⇒ (2 3)
(cons 1 (append some-list '(4) some-list))
⇒ (1 2 3 4 2 3)
`(1 ,#some-list 4 ,#some-list)
⇒ (1 2 3 4 2 3)
These all work:
(defun testcaseexpr (thecase)
(case thecase
('foo (format t "matched foo"))
(bar (format t "matched bar"))
((funk) (format t "matched funky"))))
Which of these three expressions is considered the idiomatic way? And perhaps as a side point, why are they all working, when clearly they are not the same syntax. In fact in other contexts they have different semantics completely. A list (funk) is certainly not the same as a quoted atom, 'foo. Yet just passing in the words foo bar and funk all work the same.
First, note that you've actually only got two cases here. 'foo is expanded by the reader as (quote foo), so your code is equivalent to
(defun testcaseexpr (thecase)
(case thecase
((quote foo) (format t "matched foo"))
(bar (format t "matched bar"))
((funk) (format t "matched funky"))))
wherein the first and third cases have the same structure; the keys part of the clause is a list of objects.
Perhaps this question is off-topic, since it's asking for the “best”, and that might be primarily opinion based. I agree with the points made in wvxvw's answer, but I tend to use the style you've shown in the third case almost exclusively. I've got a couple reasons for this:
It's the most general form.
It's the most general form. In the documentation for case, we read that in an normal-clause ::= (keys form*) keys is a designator for a list of keys. This means that a clause like (2 (print 'two)) is equivalent to ((2) (print 'two)). You never lose anything by using a list instead of a non-list, but if you have some clauses with multiple objects and some with single objects, you'll have consistent syntax for all of them. E.g., you can have
(case operator
((and or) ...)
((if iff) ...)
((not) ...))
It's harder to mess up.
It makes it harder to mess up the special cases of t and otherwise. The documentation says about keys that (emphasis added):
keys—a designator for a list of objects. In the case of case, the
symbols t and otherwise may not be used as the keys designator. To
refer to these symbols by themselves as keys, the designators (t) and
(otherwise), respectively, must be used instead.
In practice, some implementations will let you use t and otherwise as keys in normal-clauses, even though it seems like this shouldn't be allowed. E.g., in SBCL:
CL-USER> (macroexpand-1 '(case keyform
(otherwise 'a)
(otherwise 'b)))
(LET ((#:G962 KEYFORM))
(DECLARE (IGNORABLE #:G962))
(COND ((EQL #:G962 'OTHERWISE) NIL 'A)
(T NIL 'B)))
Using explicit lists removes any ambiguity about what you're trying to do. Even though t and otherwise are called out specifically, keys is a list designator, which means that nil (an atom and a list) needs some special consideration. Will the following code produce a or b? (Can you tell without testing it or checking the spec? This case is actually highlighted in the examples.)
(case nil
(nil 'a)
(otherwise 'b))
It returns b. To return a, the first normal-clause would have to be ((nil) 'a).
Conclusion
If you always make sure that keys is a list, you'll:
end up with more consistent looking code;
avoid edge-case bugs (especially if you're writing macros that expand into case); and
make your intentions clearer.
Second :)
First is never used, unless you expand a macro into something like it by accident, and third is used when you have more then one matching symbol (a fall-through case).
I've written an ad hoc parser generator that creates code to convert an old and little known 7-bit character set into unicode. The call to the parser generator expands into a bunch of defuns enclosed in a progn, which then get compiled. I only want to expose one of the generated defuns--the top-level one--to the rest of the system; all the others are internal to the parser and only get called from within the dynamic scope of the top-level one. Therefore, the other defuns generated have uninterned names (created with gensym). This strategy works fine with SBCL, but I recently tested it for the first time with CLISP, and I get errors like:
*** - FUNCALL: undefined function #:G16985
It seems that CLISP can't handle functions with uninterned names. (Interestingly enough, the system compiled without a problem.) EDIT: It seems that it can handle functions with uninterned names in most cases. See the answer by Rörd below.
My questions is: Is this a problem with CLISP, or is it a limitation of Common Lisp that certain implementations (e.g. SBCL) happen to overcome?
EDIT:
For example, the macro expansion of the top-level generated function (called parse) has an expression like this:
(PRINC (#:G75735 #:G75731 #:G75733 #:G75734) #:G75732)
Evaluating this expression (by calling parse) causes an error like the one above, even though the function is definitely defined within the very same macro expansion:
(DEFUN #:G75735 (#:G75742 #:G75743 #:G75744) (DECLARE (OPTIMIZE (DEBUG 2)))
(DECLARE (LEXER #:G75742) (CONS #:G75743 #:G75744))
(MULTIPLE-VALUE-BIND (#:G75745 #:G75746) (POP-TOKEN #:G75742)
...
The two instances of #:G75735 are definitely the same symbol--not two different symbols with the same name. As I said, this works with SBCL, but not with CLISP.
EDIT:
SO user Joshua Taylor has pointed out that this is due to a long standing CLISP bug.
You don't show one of the lines that give you the error, so I can only guess, but the only thing that could cause this problem as far as I can see is that you are referring to the name of the symbol instead of the symbol itself when trying to call it.
If you were referring to the symbol itself, all your lisp implementation would have to do is lookup that symbol's symbol-function. Whether it's interned or not couldn't possibly matter.
May I ask why you haven't considered another way to hide the functions, i.e. a labels statement or defining the functions within a new package that exports only the one external function?
EDIT: The following example is copied literally from an interaction with the CLISP prompt.
As you can see, calling the function named by a gensym is working as expected.
[1]> (defmacro test ()
(let ((name (gensym)))
`(progn
(defun ,name () (format t "Hello!"))
(,name))))
TEST
[2]> (test)
Hello!
NIL
Maybe your code that's trying to call the function gets evaluated before the defun? If there's any code in the macro expansion besides the various defuns, it may be implementation-dependent what gets evaluated first, and so the behaviour of SBCL and CLISP may differ without any of them violating the standard.
EDIT 2: Some further investigation shows that CLISP's behaviour varies depending upon whether the code is interpreted directly or whether it's first compiled and then interpreted. You can see the difference by either directly loading a Lisp file in CLISP or by first calling compile-file on it and then loading the FASL.
You can see what's going on by looking at the first restart that CLISP offers. It says something like "Input a value to be used instead of (FDEFINITION '#:G3219)." So for compiled code, CLISP quotes the symbol and refers to it by name.
It seems though that this behaviour is standard-conforming. The following definition can be found in the HyperSpec:
function designator n. a designator for a function; that is, an object that denotes a function and that is one of: a symbol (denoting the function named by that symbol in the global environment), or a function (denoting itself). The consequences are undefined if a symbol is used as a function designator but it does not have a global definition as a function, or it has a global definition as a macro or a special form. See also extended function designator.
I think an uninterned symbol matches the "a symbol is used as a function designator but it does not have a global definition as a function" case for unspecified consequences.
EDIT 3: (I can agree that I'm not sure whether CLISP's behaviour is a bug or not. Someone more experienced with details of the standard's terminology should judge this. It comes down to whether the function cell of an uninterned symbol - i.e. a symbol that cannot be referred to by name, only by having a direct hold on the symbol object - would be considered a "global definition" or not)
Anyway, here's an example solution that solves the problem in CLISP by interning the symbols in a throwaway package, avoiding the matter of uninterned symbols:
(defmacro test ()
(let* ((pkg (make-package (gensym)))
(name (intern (symbol-name (gensym)) pkg)))
`(progn
(defun ,name () (format t "Hello!"))
(,name))))
(test)
EDIT 4: As Joshua Taylor notes in a comment to the question, this seems to be a case of the (10 year old) CLISP bug #180.
I've tested both workarounds suggested in that bug report and found that replacing the progn with locally actually doesn't help, but replacing it with let () does.
You can most certainly define functions whose names are uninterned symbols. For instance:
CL-USER> (defun #:foo (x)
(list x))
#:FOO
CL-USER> (defparameter *name-of-function* *)
*NAME-OF-FUNCTION*
CL-USER> *name-of-function*
#:FOO
CL-USER> (funcall *name-of-function* 3)
(3)
However, the sharpsign colon syntax introduces a new symbol each time such a form is read read:
#: introduces an uninterned symbol whose name is symbol-name. Every time this syntax is encountered, a distinct uninterned symbol is created. The symbol-name must have the syntax of a symbol with no package prefix.
This means that even though something like
CL-USER> (list '#:foo '#:foo)
;=> (#:FOO #:FOO)
shows the same printed representation, you actually have two different symbols, as the following demonstrates:
CL-USER> (eq '#:foo '#:foo)
NIL
This means that if you try to call such a function by typing #: and then the name of the symbol naming the function, you're going to have trouble:
CL-USER> (#:foo 3)
; undefined function #:foo error
So, while you can call the function using something like the first example I gave, you can't do this last one. This can be kind of confusing, because the printed representation makes it look like this is what's happening. For instance, you could write such a factorial function like this:
(defun #1=#:fact (n &optional (acc 1))
(if (zerop n) acc
(#1# (1- n) (* acc n))))
using the special reader notation #1=#:fact and #1# to later refer to the same symbol. However, look what happens when you print that same form:
CL-USER> (pprint '(defun #1=#:fact (n &optional (acc 1))
(if (zerop n) acc
(#1# (1- n) (* acc n)))))
(DEFUN #:FACT (N &OPTIONAL (ACC 1))
(IF (ZEROP N)
ACC
(#:FACT (1- N) (* ACC N))))
If you take that printed output, and try to copy and paste it as a definition, the reader creates two symbols named "FACT" when it comes to the two occurrences of #:FACT, and the function won't work (and you might even get undefined function warnings):
CL-USER> (DEFUN #:FACT (N &OPTIONAL (ACC 1))
(IF (ZEROP N)
ACC
(#:FACT (1- N) (* ACC N))))
; in: DEFUN #:FACT
; (#:FACT (1- N) (* ACC N))
;
; caught STYLE-WARNING:
; undefined function: #:FACT
;
; compilation unit finished
; Undefined function:
; #:FACT
; caught 1 STYLE-WARNING condition
I hope I get the issue right. For me it works in CLISP.
I tried it like this: using a macro for creating a function with a GENSYM-ed name.
(defmacro test ()
(let ((name (gensym)))
`(progn
(defun ,name (x) (* x x))
',name)))
Now I can get the name (setf x (test)) and call it (funcall x 2).
Yes, it is perfectly fine defining functions that have names that are unintenred symbols. The problem is that you cannot then call them "by name", since you can't fetch the uninterned symbol by name (that is what "uninterned" means, essentially).
You would need to store the uninterned symbol in some sort of data structure, to then be able to fetch the symbol. Alternatively, store the defined function in some sort of data structure.
Surprisingly, CLISP bug 180 isn't actually an ANSI CL conformance bug. Not only that, but evidently, ANSI Common Lisp is itself so broken in this regard that even the progn based workaround is a courtesy of the implementation.
Common Lisp is a language intended for compilation, and compilation produces issues regarding the identity of objects which are placed into compiled files and later loaded ("externalized" objects). ANSI Common Lisp requires that literal objects reproduced from compiled files are only similar to the original objects. (CLHS 3.2.4 Literal Objects in Compiled Files).
Firstly, according to the definition similarity (3.2.4.2.2 Definition of Similarity), the rules for uninterned symbols is that similarity is name based. If we compile code with a literal that contains an uninterned symbol, then when we load the compiled file, we get a symbol which is similar and not (necessarily) the same object: a symbol which has the same name.
What if the same uninterned symbol is inserted into two different top-level forms which are then compiled as a file? When the file is loaded, are those two similar to each other at least? No, there is no such requirement.
But it gets worse: there is also no requirement that two occurrences of the same uninterned symbol in the same form will be externalized in such a way that their relative identity is preserved: that the re-loaded version of that object will have the same symbol object in all the places where the original was. In fact, the definition of similarity contains no provision for preserving the circular structure and substructure sharing. If we have a literal like '#1=(a b . #1#), as a literal in a compiled file, there appears to be no requirement that this be reproduced as a circular object with the same graph structure as the original (a graph isomorphism). The similarity rule for conses is given as naive recursion: two conses are similar if their respective cars and cdrs are similar. (The rule can't even be evaluated for circular objects; it doesn't terminate).
That the above works is because of implementations going beyond what is required in the spec; they are providing an extension consistent with (3.2.4.3 Extensions to Similarity Rules).
Thus, purely according to ANSI CL, we cannot expect to use macros with gensyms in compiled files, at least in some ways. The expectation expressed in code like the following runs afoul of the spec:
(defmacro foo (arg)
(let ((g (gensym))
(literal '(blah ,g ,g ,arg)))
...))
(defun bar ()
(foo 42))
The bar function contains a literal with two insertions of a gensym, which according to the similarity rules for conses and symbols need not reproduce as a list containing two occurrences of the same object in the second and third positions.
If the above works as expected, it's due to "extensions to the similarity rules".
So the answer to the "Why can't CLISP ..." question is that although CLISP does provide an extension for similarity which preserves the graph structure of literal forms, it doesn't do it across the entire compiled file, only within individual top level items within that file. (It uses *print-circle* to emit the individual items.) The bug is that CLISP doesn't conform to the best possible behavior users can imagine, or at least to a better behavior exhibited by other implementations.
Consider this piece of code:
(defvar lst '(1 1))
(defmacro get-x (x lst)
`(nth ,x ,lst))
(defun get-y (y lst)
(nth y lst))
Now let us assume that I want to change the value of the elements of the list called lst, the car with get-x and the cdr with get-y.
As I try to change the value with get-x (with setf) everything goes fine but if I try it with get-y it signals an error (shortened):
; caught STYLE-WARNING:
; undefined function: (SETF GET-STUFF)
Why does this happen?
I myself suspect that this happens because the macro simply expands and the function nth simply returns a reference to the value of an element in the list and the function on the other hand evaluates the function-call to nth and returns the value of the referenced value (sounds confusing).
Am I correct in my suspicions?
If I am correct then how will one know what is simply a reference to a value and an actual value?
The error does not happen with the macro version, because, as you assumed, the expression (setf (get-x some-x some-list) some-value) will be expanded (at compile-time) into something like (setf (nth some-x some-list) some-value) (not really, but the details of setf-expansion are complex), and the compiler knows, how to deal with that (i.e., there is a suitable setf expander defined for function nth).
However, in the case of get-y, the compiler has no setf expander, unless you provide one. The easiest way to do so would be
(defun (setf get-y) (new-value x ls) ; Note the function's name: setf get-y
(setf (nth x ls) new-value))
Note, that there are a few conventions regarding setf-expanders:
The new value is always provided as the first argument to the setf function
All setf functions are supposed to return the new value as their result (as this is, what the entire setf form is supposed to return)
There is, BTW, no such concept as a "reference" in Common Lisp (at least not in the C++ sense), though there once were Lisp dialects which had locatives. Generalized place forms (ie., setf and its machinery) work very differently from plain C++ style references. See the CLHS, if you are curious about the details.
SETF is a macro.
The idea is that to set and read elements from data structures are two operations, but usually require two different names (or maybe even something more complex). SETF now enables you to use just one name for both:
(get-something x)
Above reads a datastructure. The inverse then simply is:
(setf (get-something x) :foobar)
Above sets the datastructure at X with :FOOBAR.
SETF does not treat (get-something x) as a reference or something like that. It just has a database of inverse operations for each operation. If you use GET-SOMETHING, it knows what the inverse operation is.
How does SETF know it? Simple: you have to tell it.
For The NTH operation, SETF knows how to set the nth element. That's builtin into Common Lisp.
For your own GET-Y operation SETF does not have that information. You have to tell it. See the Common Lisp HyperSpec for examples. One example is to use DEFUN and (SETF GET-Y) as a function name.
Also note following style problems with your example:
lst is not a good name for a DEFVAR variable. Use *list* as a name to make clear that it is a special variable declared by DEFVAR (or similar).
'(1 2) is a literal constant. If you write a Common Lisp program, the effects of changing it are undefined. If you want to change a list later, you should cons it with LIST or something like COPY-LIST.
After making it through the major parts of an introductory Lisp book, I still couldn't understand what the special operator (quote) (or equivalent ') function does, yet this has been all over Lisp code that I've seen.
What does it do?
Short answer
Bypass the default evaluation rules and do not evaluate the expression (symbol or s-exp), passing it along to the function exactly as typed.
Long Answer: The Default Evaluation Rule
When a regular (I'll come to that later) function is invoked, all arguments passed to it are evaluated. This means you can write this:
(* (+ a 2)
3)
Which in turn evaluates (+ a 2), by evaluating a and 2. The value of the symbol a is looked up in the current variable binding set, and then replaced. Say a is currently bound to the value 3:
(let ((a 3))
(* (+ a 2)
3))
We'd get (+ 3 2), + is then invoked on 3 and 2 yielding 5. Our original form is now (* 5 3) yielding 15.
Explain quote Already!
Alright. As seen above, all arguments to a function are evaluated, so if you would like to pass the symbol a and not its value, you don't want to evaluate it. Lisp symbols can double both as their values, and markers where you in other languages would have used strings, such as keys to hash tables.
This is where quote comes in. Say you want to plot resource allocations from a Python application, but rather do the plotting in Lisp. Have your Python app do something like this:
print("'(")
while allocating:
if random.random() > 0.5:
print(f"(allocate {random.randint(0, 20)})")
else:
print(f"(free {random.randint(0, 20)})")
...
print(")")
Giving you output looking like this (slightly prettyfied):
'((allocate 3)
(allocate 7)
(free 14)
(allocate 19)
...)
Remember what I said about quote ("tick") causing the default rule not to apply? Good. What would otherwise happen is that the values of allocate and free are looked up, and we don't want that. In our Lisp, we wish to do:
(dolist (entry allocation-log)
(case (first entry)
(allocate (plot-allocation (second entry)))
(free (plot-free (second entry)))))
For the data given above, the following sequence of function calls would have been made:
(plot-allocation 3)
(plot-allocation 7)
(plot-free 14)
(plot-allocation 19)
But What About list?
Well, sometimes you do want to evaluate the arguments. Say you have a nifty function manipulating a number and a string and returning a list of the resulting ... things. Let's make a false start:
(defun mess-with (number string)
'(value-of-number (1+ number) something-with-string (length string)))
Lisp> (mess-with 20 "foo")
(VALUE-OF-NUMBER (1+ NUMBER) SOMETHING-WITH-STRING (LENGTH STRING))
Hey! That's not what we wanted. We want to selectively evaluate some arguments, and leave the others as symbols. Try #2!
(defun mess-with (number string)
(list 'value-of-number (1+ number) 'something-with-string (length string)))
Lisp> (mess-with 20 "foo")
(VALUE-OF-NUMBER 21 SOMETHING-WITH-STRING 3)
Not Just quote, But backquote
Much better! Incidently, this pattern is so common in (mostly) macros, that there is special syntax for doing just that. The backquote:
(defun mess-with (number string)
`(value-of-number ,(1+ number) something-with-string ,(length string)))
It's like using quote, but with the option to explicitly evaluate some arguments by prefixing them with comma. The result is equivalent to using list, but if you're generating code from a macro you often only want to evaluate small parts of the code returned, so the backquote is more suited. For shorter lists, list can be more readable.
Hey, You Forgot About quote!
So, where does this leave us? Oh right, what does quote actually do? It simply returns its argument(s) unevaluated! Remember what I said in the beginning about regular functions? Turns out that some operators/functions need to not evaluate their arguments. Such as IF -- you wouldn't want the else branch to be evaluated if it wasn't taken, right? So-called special operators, together with macros, work like that. Special operators are also the "axiom" of the language -- minimal set of rules -- upon which you can implement the rest of Lisp by combining them together in different ways.
Back to quote, though:
Lisp> (quote spiffy-symbol)
SPIFFY-SYMBOL
Lisp> 'spiffy-symbol ; ' is just a shorthand ("reader macro"), as shown above
SPIFFY-SYMBOL
Compare to (on Steel-Bank Common Lisp):
Lisp> spiffy-symbol
debugger invoked on a UNBOUND-VARIABLE in thread #<THREAD "initial thread" RUNNING {A69F6A9}>:
The variable SPIFFY-SYMBOL is unbound.
Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.
restarts (invokable by number or by possibly-abbreviated name):
0: [ABORT] Exit debugger, returning to top level.
(SB-INT:SIMPLE-EVAL-IN-LEXENV SPIFFY-SYMBOL #<NULL-LEXENV>)
0]
Because there is no spiffy-symbol in the current scope!
Summing Up
quote, backquote (with comma), and list are some of the tools you use to create lists, that are not only lists of values, but as you seen can be used as lightweight (no need to define a struct) data structures!
If you wish to learn more, I recommend Peter Seibel's book Practical Common Lisp for a practical approach to learning Lisp, if you're already into programming at large. Eventually on your Lisp journey, you'll start using packages too. Ron Garret's The Idiot's Guide to Common Lisp Packages will give you good explanation of those.
Happy hacking!
It says "don't evaluate me". For example, if you wanted to use a list as data, and not as code, you'd put a quote in front of it. For example,
(print '(+ 3 4)) prints "(+ 3 4)", whereas
(print (+ 3 4)) prints "7"
Other people have answered this question admirably, and Matthias Benkard brings up an excellent warning.
DO NOT USE QUOTE TO CREATE LISTS THAT YOU WILL LATER MODIFY. The spec allows the compiler to treat quoted lists as constants. Often, a compiler will optimize constants by creating a single value for them in memory and then referencing that single value from all locations where the constant appears. In other words, it may treat the constant like an anonymous global variable.
This can cause obvious problems. If you modify a constant, it may very well modify other uses of the same constant in completely unrelated code. For example, you may compare some variable to '(1 1) in some function, and in a completely different function, start a list with '(1 1) and then add more stuff to it. Upon running these functions, you may find that the first function doesn't match things properly anymore, because it's now trying to compare the variable to '(1 1 2 3 5 8 13), which is what the second function returned. These two functions are completely unrelated, but they have an effect on each other because of the use of constants. Even crazier bad effects can happen, like a perfectly normal list iteration suddenly infinite looping.
Use quote when you need a constant list, such as for comparison. Use list when you will be modifying the result.
One answer to this question says that QUOTE “creates list data structures”. This isn't quite right. QUOTE is more fundamental than this. In fact, QUOTE is a trivial operator: Its purpose is to prevent anything from happening at all. In particular, it doesn't create anything.
What (QUOTE X) says is basically “don't do anything, just give me X.” X needn't be a list as in (QUOTE (A B C)) or a symbol as in (QUOTE FOO). It can be any object whatever. Indeed, the result of evaluating the list that is produced by (LIST 'QUOTE SOME-OBJECT) will always just return SOME-OBJECT, whatever it is.
Now, the reason that (QUOTE (A B C)) seems as if it created a list whose elements are A, B, and C is that such a list really is what it returns; but at the time the QUOTE form is evaluated, the list has generally already been in existence for a while (as a component of the QUOTE form!), created either by the loader or the reader prior to execution of the code.
One implication of this that tends to trip up newbies fairly often is that it's very unwise to modify a list returned by a QUOTE form. Data returned by QUOTE is, for all intents and purposes, to be considered as part of the code being executed and should therefore be treated as read-only!
The quote prevents execution or evaluation of a form, turning it instead into data. In general you can execute the data by then eval'ing it.
quote creates list data structures, for example, the following are equivalent:
(quote a)
'a
It can also be used to create lists (or trees):
(quote (1 2 3))
'(1 2 3)
You're probably best off getting an introductary book on lisp, such as Practical Common Lisp (which is available to read on-line).
In Emacs Lisp:
What can be quoted ?
Lists and symbols.
Quoting a number evaluates to the number itself:
'5 is the same as 5.
What happens when you quote lists ?
For example:
'(one two) evaluates to
(list 'one 'two) which evaluates to
(list (intern "one") (intern ("two"))).
(intern "one") creates a symbol named "one" and stores it in a "central" hash-map, so anytime you say 'one then the symbol named "one" will be looked up in that central hash-map.
But what is a symbol ?
For example, in OO-languages (Java/Javascript/Python) a symbol could be represented as an object that has a name field, which is the symbol's name like "one" above, and data and/or code can be associated with it this object.
So an symbol in Python could be implemented as:
class Symbol:
def __init__(self,name,code,value):
self.name=name
self.code=code
self.value=value
In Emacs Lisp for example a symbol can have 1) data associated with it AND (at the same time - for the same symbol) 2) code associated with it - depending on the context, either the data or the code gets called.
For example, in Elisp:
(progn
(fset 'add '+ )
(set 'add 2)
(add add add)
)
evaluates to 4.
Because (add add add) evaluates as:
(add add add)
(+ add add)
(+ 2 add)
(+ 2 2)
4
So, for example, using the Symbol class we defined in Python above, this add ELisp-Symbol could be written in Python as Symbol("add",(lambda x,y: x+y),2).
Many thanks for folks on IRC #emacs for explaining symbols and quotes to me.
Code is data and data is code. There is no clear distinction between them.
This is a classical statement any lisp programmer knows.
When you quote a code, that code will be data.
1 ]=> '(+ 2 3 4)
;Value: (+ 2 3 4)
1 ]=> (+ 2 3 4)
;Value: 9
When you quote a code, the result will be data that represent that code. So, when you want to work with data that represents a program you quote that program. This is also valid for atomic expressions, not only for lists:
1 ]=> 'code
;Value: code
1 ]=> '10
;Value: 10
1 ]=> '"ok"
;Value: "ok"
1 ]=> code
;Unbound variable: code
Supposing you want to create a programming language embedded in lisp -- you will work with programs that are quoted in scheme (like '(+ 2 3)) and that are interpreted as code in the language you create, by giving programs a semantic interpretation. In this case you need to use quote to keep the data, otherwise it will be evaluated in external language.
When we want to pass an argument itself instead of passing the value of the argument then we use quote. It is mostly related to the procedure passing during using lists, pairs and atoms
which are not available in C programming Language ( most people start programming using C programming, Hence we get confused)
This is code in Scheme programming language which is a dialect of lisp and I guess you can understand this code.
(define atom? ; defining a procedure atom?
(lambda (x) ; which as one argument x
(and (not (null? x)) (not(pair? x) )))) ; checks if the argument is atom or not
(atom? '(a b c)) ; since it is a list it is false #f
The last line (atom? 'abc) is passing abc as it is to the procedure to check if abc is an atom or not, but when you pass(atom? abc) then it checks for the value of abc and passses the value to it. Since, we haven't provided any value to it
Quote returns the internal representation of its arguments. After plowing through way too many explanations of what quote doesn't do, that's when the light-bulb went on. If the REPL didn't convert function names to UPPER-CASE when I quoted them, it might not have dawned on me.
So. Ordinary Lisp functions convert their arguments into an internal representation, evaluate the arguments, and apply the function. Quote converts its arguments to an internal representation, and just returns that. Technically it's correct to say that quote says, "don't evaluate", but when I was trying to understand what it did, telling me what it doesn't do was frustrating. My toaster doesn't evaluate Lisp functions either; but that's not how you explain what a toaster does.
Anoter short answer:
quote means without evaluating it, and backquote is quote but leave back doors.
A good referrence:
Emacs Lisp Reference Manual make it very clear
9.3 Quoting
The special form quote returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.)
Special Form: quote object
This special form returns object, without evaluating it.
Because quote is used so often in programs, Lisp provides a convenient read syntax for it. An apostrophe character (‘'’) followed by a Lisp object (in read syntax) expands to a list whose first element is quote, and whose second element is the object. Thus, the read syntax 'x is an abbreviation for (quote x).
Here are some examples of expressions that use quote:
(quote (+ 1 2))
⇒ (+ 1 2)
(quote foo)
⇒ foo
'foo
⇒ foo
''foo
⇒ (quote foo)
'(quote foo)
⇒ (quote foo)
9.4 Backquote
Backquote constructs allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form quote (described in the previous section; see Quoting). For example, these two forms yield identical results:
`(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
'(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
The special marker ‘,’ inside of the argument to backquote indicates a value that isn’t constant. The Emacs Lisp evaluator evaluates the argument of ‘,’, and puts the value in the list structure:
`(a list of ,(+ 2 3) elements)
⇒ (a list of 5 elements)
Substitution with ‘,’ is allowed at deeper levels of the list structure also. For example:
`(1 2 (3 ,(+ 4 5)))
⇒ (1 2 (3 9))
You can also splice an evaluated value into the resulting list, using the special marker ‘,#’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘`’ is often unreadable. Here are some examples:
(setq some-list '(2 3))
⇒ (2 3)
(cons 1 (append some-list '(4) some-list))
⇒ (1 2 3 4 2 3)
`(1 ,#some-list 4 ,#some-list)
⇒ (1 2 3 4 2 3)