Here is simplified example from book On Lisp by Paul Graham (scheme like syntax).
(define-macro (bar)
(let ((x 10) (y '(1 2 3)) (z 'foo))
`(list ,x `(,',z ,,#y))))
I know how ,,#y should work but not sure exactly how ,',z should work what should be evaluated first and in what order. (I know it should evaluate to symbol foo because it return (10 (foo 1 2 3)) in guile, but I'm not sure what are exact steps).
I need this for my lisp in JavaScript where I have result:
(10 ((unquote z) 1 2 3))
because it just evaluate it form left to right (I'm only handling specially ,, and more commas). How should you evaluate this expression.
There is also this example in the book:
(defmacro propmacro (propname)
`(defmacro ,propname (obj)
`(get ,obj ',',propname)))
how ',', should be evaluated? What are the steps in this case?
Are there any other weird edge cases with backquote/quasiquote? Can you show examples of those and how they should be evaluated and in what order?
How ,',z works is that:
`(list ,x `(,',z ,,#y))))
^ ^
| `- this comma
`- belongs to this backquote
The above comma interpolates, into the inner backquote, the expression ',z or (quote ,z). And that ,z, in turn, belongs to the outer backquote.
Thus the value of z is inserted into (quote ,z) to make (quote <value-of-z>).
Then, effectively, the inner backquote then behaves like `(,'<value-of-z>).
Concretely, suppose z contains the list (+ 2 2). Then we can understand it in terms of the outer backquote inserting (+ 2 2) into the inner one to produce `(,'(+ 2 2) ...). This is now straightforward to understand: when the inner backquote is evaluated, the (+ 2 2) is protected from evaluation, resulting in the object ((+ 2 2) ...).
The pattern ,',',', ... ,',expr is used to obtain a single evaluation of expr during the evaluation of the outermost backquote, such that this value is then propagated through any number of evaluation rounds of the remaining backquote nestings without undergoing further evaluation. There is a kind of "backquote algebra" at play here in which the "commas and quotes cancel out".
You can also visualize the ,',','... as a kind of drill bit that digs through the layers of nesting to allow you plant a literal value anywhere in the structure. E.g.
(defmacro super-nested-macro (arg)
`(... `(.... `(.....`(we simply want arg down here ,',',',arg)))))
The author of super-nested-macro just wants to stick the value of arg into the template, in a position that is buried in three other backquotes. Thus the usual ,arg cannot be used: that comma would be misinterpreted as belonging to the inner-most backquote.
Are there any other weird edge cases with backquote/quasiquote?
One weird edge case in backquote is trying to splice into a dot position:
`(a b c . ,#foo) ;; not allowed
`(a b c . ,foo) ;; OK: equivalent to `(a b c ,#foo)
Not sure how various implementations deal with a backquote in a dot position:
`(a b c . `(d e f))
It doesn't really make sense, and I suspect that the actual result obtained will depend on the backquote implementation internals.
Not all objects are traversed for unquoting:
`#c(,(sin theta) ,(cos theta)) ;; Not required by ANSI CL, oops!
This could work via an implementation's extension.
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)
I'm analyzing LISP, I'm no expert, but something is bothering me:
Some primitives as list accepts more than one parameter. e.g.:
(list 1 2 3)
=> (1 2 3)
On the other hand quote seems to accept just one parameter. e.g:
(quote (1 2 3))
=> (1 2 3)
(quote x)
=> 'x
(quote 1 2 3)
=> 1 ???
Is there a reason why (quote 1 2 3) i.e. quote with multiple params, just ignores the other arguments?
what will happen if (quote 1 2 3) evaluates to (1 2 3), i.e. an special case when more than one argument is supplied?
I do understand that this special case is superfluous, but my question to LISP hackers is:
adding such special case to quote will break everything? will it break the REPL? will it break macros?
Note: tested on http://repl.it/ and http://clojurescript.net/
Note that Lisp is not a single language, but a large family of somewhat similar languages. You seem to have tried out Scheme (repl.it runs BiwaScheme) and ClojureScript.
The Scheme spec only defines one argument for quote, so BiwaScheme seems to be wrong in that respect. (quote 1 2 3) should be an error in Scheme. For example, Racket, another dialect of Scheme, does not accept them:
$ racket
Welcome to Racket v5.3.6.
> (quote 1)
1
> (quote 1 2 3)
stdin::10: quote: wrong number of parts
in: (quote 1 2 3)
context...:
/usr/share/racket/collects/racket/private/misc.rkt:87:7
BiwaScheme is written in JavaScript, and JavaScript simply ignores extra arguments to any function, so the behavior probably comes from there.
ClojureScript might inherit its manners from JavaScript or from Clojure. Clojure's documentation explicitly states that quote with multiple arguments evaluates to the first of them only.
Common Lisp, another popular Lisp language, also only accepts a single argument to quote:
$ sbcl
* (quote 1 2 3)
debugger invoked on a SIMPLE-ERROR in thread
#<THREAD "main thread" RUNNING {1002B2AE83}>:
wrong number of args to QUOTE:
(QUOTE 1 2 3)
Note that in general, for any Lisp, quote is seldom spelled out. It is just a special form that is an expansion of '. In the ' form, it is not even possible to give quote extra arguments:
'(1 2 3) ≡ (quote (1 2 3))
'x ≡ (quote x)
'??? ≡ (quote 1 2 3)
I don't immediately see a problem with expanding quote's definition in any given language to, in case of multiple arguments, evaluate them as a list, but I certainly do not see a use for that feature, either.
The original idea of QUOTE is to denote a constant, especially for symbols and lists:
(quote sin)
(quote (sin 10))
To get the unquoted data we call SECOND or CADR.
(defun unquote (expression)
(second expression))
For example we could call:
(unquote '(quote (sin 10)))
If know allow the idea that (quote sin 10) is the same as (quote (sin 10)), then we would need to rewrite our unquote function for the two cases:
(defun unquote (expression)
(if (consp (cddr expression))
(cdr expression)
(cadr expression)))
By adding that special case we would not get any new capabilities, but it would complicate code which has to deal with such expressions...
In most lisps quote will error given more than one argument. This behaviour seems to be a peculiarity of Clojure (or ClojureScript?).
Allowing multiple arguments to quote to become a list isn't a very nice design. If you have an operation to make a list you should clearly be able to use it to construct a single element list, but modified quote does not allow that.
(I tested SBCL, Emacs Lisp and scheme48, all of which complain about quote with multiple arguments.)
Imagine the following code to dynamically create a macro:
(def a (list '+ 1 2))
(def b (list '- 10 5))
(def c (list '/ 22 2))
(defmacro gg [h]
(let [k# `~h]
k#))
The intent is to pass a vector of symbols to a macro, do some evaluation on each element of the vector such that it returns a nice macro-esque form, then have the macro combine them into a nice macro and evaluate it. The above example works all except the actual evaluation.
When I run it I get:
(gg [a b c])
=> [(+ 1 2) (- 10 5) (/ 22 2)]
What is the secret to passing a symbol that is a list of symbols and getting a macro to evaluate them? I have tried lots of combinations of quoting and have yet to hit the right one.
The real purpose of this question is to build an Archimedes Ogre query based on a definition of a path through the graph. If someone has an example of that, I would be grateful.
EDIT:
(defmacro gg2 [h]
`(do ~#(map identity h)))
(macroexpand '(gg2 [a b c]))
=> (do a b c)
(gg2 [a b c])
=> (/ 22 2)
I was hoping to get 11 rather than the form.
You don't need a macro. Macros don't do what you're looking for here. What you are looking for is eval.
(def a '/)
(def b 22)
(def c 2)
(eval (list* [a b c]))
=> 11
Of course, you can write a macro which expands into (eval (list* ...)) if you want. It could just as well be a function though.
This is a very common mistake when starting out with macros; trying to write a macro which depends on the run-time value of its arguments. Macros run at compile-time, and generally the values of the symbols which you pass to a macro are not yet available when the macro is expanded.
About the use of eval, some cautions are in order. No one said it better than Paul Graham:
Generally it is not a good idea to call eval at runtime, for two reasons:
It’s inefficient: eval is handed a raw list, and either has to compile it on the spot, or evaluate it in an interpreter. Either way is slower than compiling the code beforehand, and just calling it.
It’s less powerful, because the expression is evaluated with no lexical context. Among other things, this means that you can’t refer to ordinary variables visible outside the expression being evaluated.
Usually, calling eval explicitly is like buying something in an airport gift-shop. Having waited till the last moment, you have to pay high prices for a limited selection of second-rate goods.
I had never really thought about whether a symbol could be a number in Lisp, so I played around with it today:
> '1
1
> (+ '1 '1)
2
> (+ '1 1)
2
> (define a '1)
> (+ a 1)
2
The above code is scheme, but it seems to be roughly the same in Common Lisp and Clojure as well. Is there any difference between 1 and quoted 1?
In Common Lisp, '1 is shorthand for (QUOTE 1). When evaluated, (QUOTE something) returns the something part, unevaluated. However, there is no difference between 1 evaluated and 1 unevaluated.
So there is a difference to the reader: '1 reads as (QUOTE 1) and 1 reads as 1. But there is no difference when evaluted.
Numbers are self-evaluating objects. That's why you don't have to worry about quoting them, as you do with, say, lists.
A symbol can be made from any string. If you want the symbol whose name is the single character 1, you can say:
(intern "1")
which prints |1|, suggesting an alternate way to enter it:
'|1|
Quoting prevents expressions from being evaluated until later. For example, the following is not a proper list:
(1 2 3)
This is because Lisp interprets 1 as a function, which it is not. So the list must be quoted:
'(1 2 3)
When you quote a very simple expression such as a number, Lisp effectively does not alter its behavior.
See Wikipedia: Lisp.
Well, they are in fact very different. '1 is however precisely the same as (quote 1). (car ''x) evaluates to the symbol 'quote'.
1 is an S-expression, it's the external representation of a datum, a number 1. To say that 1 is a 'number-object' or an S-expression to enter that object would both be acceptable. Often it is said that 1 is the external representation for the actual number object.
(quote 1) is another S-expression, it's an S-expression for a list whose first element is the symbol 'quote' and whose second element is the number 1. This is where it's already different, syntactic keywords, unlike functions, are not considered objects in the language and they do not evaluate to them.
However, both are external representations of objects (data) which evaluate to the same datum. The number whose external representation is 1, they are however most certainly not the same objects, the same, code, the same datum the same whatever, they just evaluate to the very same thing. Numbers evaluate to themselves. To say that they are the same is to say that:
(+ 1 (* 3 3))
And
(if "Strings are true" (* 5 (- 5 3)) "Strings are not true? This must be a bug!")
Are 'the same', they aren't, they are both different programs which just happen to terminate to the same value, a lisp form is also a program, a form is a datum which is also a program, remember.
Also, I was taught a handy trick once that shows that self-evaluating data are truly not symbols when entered:
(let ((num 4))
(symbol? num) ; ====> evaluates to #f
(symbol? 'num) ; ====> evaluates to #t
(symbol? '4) ; ====> evaluates to #f
(symbol? '#\c) ; #f again, et cetera
(symbol? (car ''x)) ; #t
(symbol? quote) ; error, in most implementations
)
Self evaluating data truly evaluate to themselves, they are not 'predefined symbols' of some sorts.
In Lisp, the apostrophe prevents symbols to be evaluated. Using an apostrophe before a number is not forbidden, it is not necessary as the numbers represent themselves. However, like any other list, it automatically gets transformed to an appropriate function call. The interpreter considers these numbers coincide with their value.
As has been pointed out, there is no difference, as numbers evaluate to themselves. You can confirm this by using eval:
(eval 1) ;=> 1
This is not limited to numbers, by the way. In fact, in Common Lisp, most things evaluate to themselves. It's just that it's very rare for something other than numbers, strings, symbols, and lists to be evaluated. For instance, the following works:
(eval (make-hash-table)) ;equivalent to just (make-hash-table)
In Lisp, quote prevent the following expression to be evaluated. ' is a shorthand for quote. As a result, '1 is same as (quote 1).
However, in Lisp, symbols can never be a number. I mean, 'abc is a symbol, but '123 is not (evaluated into) a symbol. I think this is wrong of the design of Lisp. Another case is not only #t or #f can be used as a Boolean expression.
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)