Related
I'm a Lisp beginner and I'm struggling to understand why the following code gives me an error.
(dolist (elem '(mapcar
mapcon))
(when (fboundp `',elem) (print "hello")))
Thanks.
Edit:
A bit more context. I wrote the following in Elisp and I don't know how to fix it.
(dolist (ui-elem '(menu-bar-mode
tool-bar-mode
tooltip-mode
scroll-bar-mode
horizontal-scroll-bar-mode))
(when (fboundp `',ui-elem) (ui-elem -1)))
Note
In your question you mix common-lisp and elisp, but they are two different languages. The question however touches on concepts that are identical in both languages.
The need to quote symbols
The code you want to write checks if a symbol is bound to a function.
What you already know probably is that you can call fboundp on a symbol to determines this:
(fboundp 'menu-bar-mode)
=> t
When you evalute the above form, 'menu-bar-mode is the same as (quote menu-bar-mode), and is evaluated as the symbol object menu-bar-mode. This is the value that is given as an argument to fboundp.
In you example you want to iterate over a list of symbols, call fboundp on it and call the function if the symbol denotes a function. You can do this as follows:
(dolist (s '(menu-bar-mode and other symbols))
(when (fboundp s)
(funcall s -1)))
The list of symbols '(menu-bar-mode and other symbols) is quoted, which means that when dolist evaluates it, it sees a list of symbols. The value to which s is bound at each iteration of the loop is a symbol object, there is no need to quote them.
Quoting a symbol is something you have to do when writing them in your code so that they are not interpreted as variables. When you iterate over a list of symbols, you already manipulate symbols.
Note also that both Common Lisp and Emacs Lisp are "Lisp-2", meanings that you have to use (funcall ui-elem -1) instead of writing (ui-elem -1). When you write the latter form, that means calling the function literally named ui-elem because for function application, the first symbol in the list is not evaluated, it is taken literally.
Too many levels of quoting
The actual error I have when I execute your code is:
(wrong-type-argument symbolp 'mapcar)
It may look like 'mapcar denotes a symbol, because when you want the interpreter to evaluate some code as a symbol, you need to quote it. However, Lisp printers write objects in a way that they can be read back to "similar" objects. The error message that is printed if I expect a symbol to be a number is the following, where symbol foo is printed unquoted:
(+ 'foo 3)
;; error: (wrong-type-argument number-or-marker-p foo)
In your error message, the form that you are trying to use as a symbol is (quote mapcar). Recall that when you directly call fboundp:
(fboundp 'mapcar)
It is the same as-if you wrote:
(fboundp (quote mapcar))
First, (quote mapcar) is evaluated, as the symbol mapcar. Then, fboundp is applied to that value.
But when you write the following, while ui-elem is bound to symbol mapcar:
(fboundp `',ui-elem)
This is equivalent to:
(fboundp `(quote ,ui-elem))
The argument to fboundp is evaluated as (quote mapcar). You have one extra level of quoting. You could write instead:
(fboundp `,ui-elem)
But then, you don't need to use backquote/comma, you can directly write:
(fboundp ui-elem)
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 am studying the Emacs-Lisp by following the introduction.
I can understand the below defun print out a list in left-to-right order because the print command comes before recursion (as I understood):
(defun print-elements-recursively (list)
"Print each element of LIST on a line of its own.
Uses recursion."
(when list ; do-again-test
(print (car list)) ; body
(print-elements-recursively ; recursive call
(cdr list)))) ; next-step-expression
E.g. for a list of '(gazelle giraffe lion tiger). The print order is gazelle, giraffe, lion, tiger.
However, I could not understand why the same order still holds when I switch the position of the two expression within the when body:
(defun print-elements-recursively (list)
"Print each element of LIST on a line of its own.
Uses recursion."
(when list ; do-again-test
; body
(print-elements-recursively ; recursive call
(cdr list))
(print (car list)))) ; next-step-expression
Per my expectation, the recursion happens before the print function, therefore, the order should be reversed. May I know why?
You probably did not evaluate the second defun after defining it, and that is why the items of the input list are still being printed in the original order. Adding a second function with the same name to the global namespace does not mean that the definition of the first function is automatically overwritten.
I suggest you
rename one of the defuns
evaluate them both
and then call each one of them separately.
The behavior should not persist when you do that.
Aside from printing the elements of the list in a different order, note also that the original function returns nil and the second function returns a printed representation of the last (non-nil) item of the input list. This is because (when list) returns nil and is the last expression that gets evaluated when the base case is reached in the first function. In the second function all invocations of print are evaluated after the base case is reached.
I have the following function (simplified to that version):
(defun append-test (xs)
(let ((ys `(foo ,(nconc `(bar) xs))))
(nconc ys `((baz)))))
If I evaluate the function multiple times (append-test '((foo))) the list keeps increasing in size, but I can't understand why. The nconc modifies the original list, but because the original list is either a function argument or let-variable, they should be created anew per every invocation, right? What is happening here?
If you change `(bar) to (list 'bar), then the code returns the same result every time.
nconc modifies all arguments but the last one. Obviously, if the first argument were '(bar), then the quoted list contained within the function definition would be modified, and we would expect to see the result you see. Apparently using a backquote expression that doesn't contain any commas is equivalent to using a quoted list.
Actually, the following piece of code keeps the backquote, but adds a pointless expression to the cdr of the list. Apparently, this makes it allocate a new list every time the function is called, and thus it returns the same result every time:
(defun append-test (xs)
(let ((ys `(foo ,(nconc `(bar . ,(ignore)) xs))))
(nconc ys `((baz)))))
I was bitten by this. Here are some bugs that resulted.
http://comments.gmane.org/gmane.emacs.bugs/50783
http://lists.gnu.org/archive/html/emacs-bug-tracker/2011-09/msg00220.html
This issue seems to be a rite of passage for lisp developers. :-)
From what I understand, lisp evaluation is split in two phases:
Read : A reader that statically allocates storage and reads forms into memory.
Evaluate : The read form is evaluated.
What about quote?
quote simply returns the read form without evaluation.
Implicitly,
When a quote form is evaluated, it just returns the reader allocated
object directly.
When a quote form is re-evaluated (i.e. function call, etc), it returns
the same reader allocated object.
quote does not allocate new memory
Ta-da!
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)