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10. Control Structures

A Lisp program consists of expressions or forms (see section 9.2 Kinds of Forms). We control the order of execution of these forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.

The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code--the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b. The result of evaluating b becomes the value of the function.

Explicit control structures make possible an order of execution other than sequential.

Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps--all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see section 13. Macros).

10.1 Sequencing  Evaluation in textual order.
10.2 Conditionals  if, cond, when, unless.
10.3 Constructs for Combining Conditions  and, or, not.
10.4 Iteration  while loops.
10.5 Nonlocal Exits  Jumping out of a sequence.

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10.1 Sequencing

Evaluating forms in the order they appear is the most common way control passes from one form to another. In some contexts, such as in a function body, this happens automatically. Elsewhere you must use a control structure construct to do this: progn, the simplest control construct of Lisp.

A progn special form looks like this:

(progn a b c ...)

and it says to execute the forms a, b, c, and so on, in that order. These forms are called the body of the progn form. The value of the last form in the body becomes the value of the entire progn. (progn) returns nil.

In the early days of Lisp, progn was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a progn in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an "implicit progn": several forms are allowed just as in the body of an actual progn. Many other control structures likewise contain an implicit progn. As a result, progn is not used as much as it was many years ago. It is needed now most often inside an unwind-protect, and, or, or in the then-part of an if.

Special Form: progn forms...
This special form evaluates all of the forms, in textual order, returning the result of the final form.

(progn (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
=> "The third form"

Two other control constructs likewise evaluate a series of forms but return a different value:

Special Form: prog1 form1 forms...
This special form evaluates form1 and all of the forms, in textual order, returning the result of form1.

(prog1 (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
=> "The first form"

Here is a way to remove the first element from a list in the variable x, then return the value of that former element:

(prog1 (car x) (setq x (cdr x)))

Special Form: prog2 form1 form2 forms...
This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.

(prog2 (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
=> "The second form"

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10.2 Conditionals

Conditional control structures choose among alternatives. Emacs Lisp has four conditional forms: if, which is much the same as in other languages; when and unless, which are variants of if; and cond, which is a generalized case statement.

Special Form: if condition then-form else-forms...
if chooses between the then-form and the else-forms based on the value of condition. If the evaluated condition is non-nil, then-form is evaluated and the result returned. Otherwise, the else-forms are evaluated in textual order, and the value of the last one is returned. (The else part of if is an example of an implicit progn. See section 10.1 Sequencing.)

If condition has the value nil, and no else-forms are given, if returns nil.

if is a special form because the branch that is not selected is never evaluated--it is ignored. Thus, in the example below, true is not printed because print is never called.

(if nil 
    (print 'true) 
=> very-false

Macro: when condition then-forms...
This is a variant of if where there are no else-forms, and possibly several then-forms. In particular,

(when condition a b c)

is entirely equivalent to

(if condition (progn a b c) nil)

Macro: unless condition forms...
This is a variant of if where there is no then-form:

(unless condition a b c)

is entirely equivalent to

(if condition nil
   a b c)

Special Form: cond clause...
cond chooses among an arbitrary number of alternatives. Each clause in the cond must be a list. The CAR of this list is the condition; the remaining elements, if any, the body-forms. Thus, a clause looks like this:

(condition body-forms...)

cond tries the clauses in textual order, by evaluating the condition of each clause. If the value of condition is non-nil, the clause "succeeds"; then cond evaluates its body-forms, and the value of the last of body-forms becomes the value of the cond. The remaining clauses are ignored.

If the value of condition is nil, the clause "fails", so the cond moves on to the following clause, trying its condition.

If every condition evaluates to nil, so that every clause fails, cond returns nil.

A clause may also look like this:


Then, if condition is non-nil when tested, the value of condition becomes the value of the cond form.

The following example has four clauses, which test for the cases where the value of x is a number, string, buffer and symbol, respectively:

(cond ((numberp x) x)
      ((stringp x) x)
      ((bufferp x)
       (setq temporary-hack x) ; multiple body-forms
       (buffer-name x))        ; in one clause
      ((symbolp x) (symbol-value x)))

Often we want to execute the last clause whenever none of the previous clauses was successful. To do this, we use t as the condition of the last clause, like this: (t body-forms). The form t evaluates to t, which is never nil, so this clause never fails, provided the cond gets to it at all.

For example,

(setq a 5)
(cond ((eq a 'hack) 'foo)
      (t "default"))
=> "default"

This cond expression returns foo if the value of a is hack, and returns the string "default" otherwise.

Any conditional construct can be expressed with cond or with if. Therefore, the choice between them is a matter of style. For example:

(if a b c)
(cond (a b) (t c))

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10.3 Constructs for Combining Conditions

This section describes three constructs that are often used together with if and cond to express complicated conditions. The constructs and and or can also be used individually as kinds of multiple conditional constructs.

Function: not condition
This function tests for the falsehood of condition. It returns t if condition is nil, and nil otherwise. The function not is identical to null, and we recommend using the name null if you are testing for an empty list.

Special Form: and conditions...
The and special form tests whether all the conditions are true. It works by evaluating the conditions one by one in the order written.

If any of the conditions evaluates to nil, then the result of the and must be nil regardless of the remaining conditions; so and returns nil right away, ignoring the remaining conditions.

If all the conditions turn out non-nil, then the value of the last of them becomes the value of the and form. Just (and), with no conditions, returns t, appropriate because all the conditions turned out non-nil. (Think about it; which one did not?)

Here is an example. The first condition returns the integer 1, which is not nil. Similarly, the second condition returns the integer 2, which is not nil. The third condition is nil, so the remaining condition is never evaluated.

(and (print 1) (print 2) nil (print 3))
     -| 1
     -| 2
=> nil

Here is a more realistic example of using and:

(if (and (consp foo) (eq (car foo) 'x))
    (message "foo is a list starting with x"))

Note that (car foo) is not executed if (consp foo) returns nil, thus avoiding an error.

and can be expressed in terms of either if or cond. For example:

(and arg1 arg2 arg3)
(if arg1 (if arg2 arg3))
(cond (arg1 (cond (arg2 arg3))))

Special Form: or conditions...
The or special form tests whether at least one of the conditions is true. It works by evaluating all the conditions one by one in the order written.

If any of the conditions evaluates to a non-nil value, then the result of the or must be non-nil; so or returns right away, ignoring the remaining conditions. The value it returns is the non-nil value of the condition just evaluated.

If all the conditions turn out nil, then the or expression returns nil. Just (or), with no conditions, returns nil, appropriate because all the conditions turned out nil. (Think about it; which one did not?)

For example, this expression tests whether x is either nil or the integer zero:

(or (eq x nil) (eq x 0))

Like the and construct, or can be written in terms of cond. For example:

(or arg1 arg2 arg3)
(cond (arg1)

You could almost write or in terms of if, but not quite:

(if arg1 arg1
  (if arg2 arg2 

This is not completely equivalent because it can evaluate arg1 or arg2 twice. By contrast, (or arg1 arg2 arg3) never evaluates any argument more than once.

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10.4 Iteration

Iteration means executing part of a program repetitively. For example, you might want to repeat some computation once for each element of a list, or once for each integer from 0 to n. You can do this in Emacs Lisp with the special form while:

Special Form: while condition forms...
while first evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates to nil.

There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to nil or until an error or throw jumps out of it (see section 10.5 Nonlocal Exits).

The value of a while form is always nil.

(setq num 0)
     => 0
(while (< num 4)
  (princ (format "Iteration %d." num))
  (setq num (1+ num)))
     -| Iteration 0.
     -| Iteration 1.
     -| Iteration 2.
     -| Iteration 3.
     => nil

To write a "repeat...until" loop, which will execute something on each iteration and then do the end-test, put the body followed by the end-test in a progn as the first argument of while, as shown here:

(while (progn
         (forward-line 1)
         (not (looking-at "^$"))))

This moves forward one line and continues moving by lines until it reaches an empty line. It is peculiar in that the while has no body, just the end test (which also does the real work of moving point).

The dolist and dotimes macros provide convenient ways to write two common kinds of loops.

Macro: dolist (var list [result]) body...
This construct executes body once for each element of list, using the variable var to hold the current element. Then it returns the value of evaluating result, or nil if result is omitted. For example, here is how you could use dolist to define the reverse function:

(defun reverse (list)
  (let (value)
    (dolist (elt list value)
      (setq value (cons elt value)))))

Macro: dotimes (var count [result]) body...
This construct executes body once for each integer from 0 (inclusive) to count (exclusive), using the variable var to hold the integer for the current iteration. Then it returns the value of evaluating result, or nil if result is omitted. Here is an example of using dotimes do something 100 times:

(dotimes (i 100)
  (insert "I will not obey absurd orders\n"))

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10.5 Nonlocal Exits

A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.

10.5.1 Explicit Nonlocal Exits: catch and throw  Nonlocal exits for the program's own purposes.
10.5.2 Examples of catch and throw  Showing how such nonlocal exits can be written.
10.5.3 Errors  How errors are signaled and handled.
10.5.4 Cleaning Up from Nonlocal Exits  Arranging to run a cleanup form if an error happens.

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10.5.1 Explicit Nonlocal Exits: catch and throw

Most control constructs affect only the flow of control within the construct itself. The function throw is the exception to this rule of normal program execution: it performs a nonlocal exit on request. (There are other exceptions, but they are for error handling only.) throw is used inside a catch, and jumps back to that catch. For example:

(defun foo-outer ()
  (catch 'foo

(defun foo-inner ()
  (if x
      (throw 'foo t))

The throw form, if executed, transfers control straight back to the corresponding catch, which returns immediately. The code following the throw is not executed. The second argument of throw is used as the return value of the catch.

The function throw finds the matching catch based on the first argument: it searches for a catch whose first argument is eq to the one specified in the throw. If there is more than one applicable catch, the innermost one takes precedence. Thus, in the above example, the throw specifies foo, and the catch in foo-outer specifies the same symbol, so that catch is the applicable one (assuming there is no other matching catch in between).

Executing throw exits all Lisp constructs up to the matching catch, including function calls. When binding constructs such as let or function calls are exited in this way, the bindings are unbound, just as they are when these constructs exit normally (see section 11.3 Local Variables). Likewise, throw restores the buffer and position saved by save-excursion (see section 30.3 Excursions), and the narrowing status saved by save-restriction and the window selection saved by save-window-excursion (see section 28.17 Window Configurations). It also runs any cleanups established with the unwind-protect special form when it exits that form (see section 10.5.4 Cleaning Up from Nonlocal Exits).

The throw need not appear lexically within the catch that it jumps to. It can equally well be called from another function called within the catch. As long as the throw takes place chronologically after entry to the catch, and chronologically before exit from it, it has access to that catch. This is why throw can be used in commands such as exit-recursive-edit that throw back to the editor command loop (see section 21.12 Recursive Editing).

Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially: return, return-from, and go, for example. Emacs Lisp has only throw.

Special Form: catch tag body...
catch establishes a return point for the throw function. The return point is distinguished from other such return points by tag, which may be any Lisp object except nil. The argument tag is evaluated normally before the return point is established.

With the return point in effect, catch evaluates the forms of the body in textual order. If the forms execute normally (without error or nonlocal exit) the value of the last body form is returned from the catch.

If a throw is executed during the execution of body, specifying the same value tag, the catch form exits immediately; the value it returns is whatever was specified as the second argument of throw.

Function: throw tag value
The purpose of throw is to return from a return point previously established with catch. The argument tag is used to choose among the various existing return points; it must be eq to the value specified in the catch. If multiple return points match tag, the innermost one is used.

The argument value is used as the value to return from that catch.

If no return point is in effect with tag tag, then a no-catch error is signaled with data (tag value).

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10.5.2 Examples of catch and throw

One way to use catch and throw is to exit from a doubly nested loop. (In most languages, this would be done with a "go to".) Here we compute (foo i j) for i and j varying from 0 to 9:

(defun search-foo ()
  (catch 'loop
    (let ((i 0))
      (while (< i 10)
        (let ((j 0))
          (while (< j 10)
            (if (foo i j)
                (throw 'loop (list i j)))
            (setq j (1+ j))))
        (setq i (1+ i))))))

If foo ever returns non-nil, we stop immediately and return a list of i and j. If foo always returns nil, the catch returns normally, and the value is nil, since that is the result of the while.

Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, hack:

(defun catch2 (tag)
  (catch tag
    (throw 'hack 'yes)))
=> catch2

(catch 'hack 
  (print (catch2 'hack))
-| yes
=> no

Since both return points have tags that match the throw, it goes to the inner one, the one established in catch2. Therefore, catch2 returns normally with value yes, and this value is printed. Finally the second body form in the outer catch, which is 'no, is evaluated and returned from the outer catch.

Now let's change the argument given to catch2:

(catch 'hack
  (print (catch2 'quux))
=> yes

We still have two return points, but this time only the outer one has the tag hack; the inner one has the tag quux instead. Therefore, throw makes the outer catch return the value yes. The function print is never called, and the body-form 'no is never evaluated.

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10.5.3 Errors

When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.

When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.

In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers that should be deleted before the program is finished. In such cases, you would use unwind-protect to establish cleanup expressions to be evaluated in case of error. (See section 10.5.4 Cleaning Up from Nonlocal Exits.) Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use condition-case to establish error handlers to recover control in case of error.

Resist the temptation to use error handling to transfer control from one part of the program to another; use catch and throw instead. See section 10.5.1 Explicit Nonlocal Exits: catch and throw. How to Signal an Error  How to report an error. How Emacs Processes Errors  What Emacs does when you report an error. Writing Code to Handle Errors  How you can trap errors and continue execution. Error Symbols and Condition Names  How errors are classified for trapping them.

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Most errors are signaled "automatically" within Lisp primitives which you call for other purposes, such as if you try to take the CAR of an integer or move forward a character at the end of the buffer. You can also signal errors explicitly with the functions error and signal.

Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See section 21.10 Quitting.

The error message should state what is wrong ("File does not exist"), not how things ought to be ("File must exist"). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation.

Function: error format-string &rest args
This function signals an error with an error message constructed by applying format (see section 4.6 Conversion of Characters and Strings) to format-string and args.

These examples show typical uses of error:

(error "That is an error -- try something else")
     error--> That is an error -- try something else

(error "You have committed %d errors" 10)
     error--> You have committed 10 errors

error works by calling signal with two arguments: the error symbol error, and a list containing the string returned by format.

Warning: If you want to use your own string as an error message verbatim, don't just write (error string). If string contains `%', it will be interpreted as a format specifier, with undesirable results. Instead, use (error "%s" string).

Function: signal error-symbol data
This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error.

The argument error-symbol must be an error symbol---a symbol bearing a property error-conditions whose value is a list of condition names. This is how Emacs Lisp classifies different sorts of errors.

The number and significance of the objects in data depends on error-symbol. For example, with a wrong-type-arg error, there should be two objects in the list: a predicate that describes the type that was expected, and the object that failed to fit that type. See section Error Symbols and Condition Names, for a description of error symbols.

Both error-symbol and data are available to any error handlers that handle the error: condition-case binds a local variable to a list of the form (error-symbol . data) (see section Writing Code to Handle Errors). If the error is not handled, these two values are used in printing the error message.

The function signal never returns (though in older Emacs versions it could sometimes return).

(signal 'wrong-number-of-arguments '(x y))
     error--> Wrong number of arguments: x, y

(signal 'no-such-error '("My unknown error condition"))
     error--> peculiar error: "My unknown error condition"

Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.

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When an error is signaled, signal searches for an active handler for the error. A handler is a sequence of Lisp expressions designated to be executed if an error happens in part of the Lisp program. If the error has an applicable handler, the handler is executed, and control resumes following the handler. The handler executes in the environment of the condition-case that established it; all functions called within that condition-case have already been exited, and the handler cannot return to them.

If there is no applicable handler for the error, the current command is terminated and control returns to the editor command loop, because the command loop has an implicit handler for all kinds of errors. The command loop's handler uses the error symbol and associated data to print an error message.

An error that has no explicit handler may call the Lisp debugger. The debugger is enabled if the variable debug-on-error (see section 18.1.1 Entering the Debugger on an Error) is non-nil. Unlike error handlers, the debugger runs in the environment of the error, so that you can examine values of variables precisely as they were at the time of the error.

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The usual effect of signaling an error is to terminate the command that is running and return immediately to the Emacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler, with the special form condition-case. A simple example looks like this:

(condition-case nil
    (delete-file filename)
  (error nil))

This deletes the file named filename, catching any error and returning nil if an error occurs.

The second argument of condition-case is called the protected form. (In the example above, the protected form is a call to delete-file.) The error handlers go into effect when this form begins execution and are deactivated when this form returns. They remain in effect for all the intervening time. In particular, they are in effect during the execution of functions called by this form, in their subroutines, and so on. This is a good thing, since, strictly speaking, errors can be signaled only by Lisp primitives (including signal and error) called by the protected form, not by the protected form itself.

The arguments after the protected form are handlers. Each handler lists one or more condition names (which are symbols) to specify which errors it will handle. The error symbol specified when an error is signaled also defines a list of condition names. A handler applies to an error if they have any condition names in common. In the example above, there is one handler, and it specifies one condition name, error, which covers all errors.

The search for an applicable handler checks all the established handlers starting with the most recently established one. Thus, if two nested condition-case forms offer to handle the same error, the inner of the two gets to handle it.

If an error is handled by some condition-case form, this ordinarily prevents the debugger from being run, even if debug-on-error says this error should invoke the debugger. See section 18.1.1 Entering the Debugger on an Error. If you want to be able to debug errors that are caught by a condition-case, set the variable debug-on-signal to a non-nil value.

When an error is handled, control returns to the handler. Before this happens, Emacs unbinds all variable bindings made by binding constructs that are being exited and executes the cleanups of all unwind-protect forms that are exited. Once control arrives at the handler, the body of the handler is executed.

After execution of the handler body, execution returns from the condition-case form. Because the protected form is exited completely before execution of the handler, the handler cannot resume execution at the point of the error, nor can it examine variable bindings that were made within the protected form. All it can do is clean up and proceed.

The condition-case construct is often used to trap errors that are predictable, such as failure to open a file in a call to insert-file-contents. It is also used to trap errors that are totally unpredictable, such as when the program evaluates an expression read from the user.

Error signaling and handling have some resemblance to throw and catch (see section 10.5.1 Explicit Nonlocal Exits: catch and throw), but they are entirely separate facilities. An error cannot be caught by a catch, and a throw cannot be handled by an error handler (though using throw when there is no suitable catch signals an error that can be handled).

Special Form: condition-case var protected-form handlers...
This special form establishes the error handlers handlers around the execution of protected-form. If protected-form executes without error, the value it returns becomes the value of the condition-case form; in this case, the condition-case has no effect. The condition-case form makes a difference when an error occurs during protected-form.

Each of the handlers is a list of the form (conditions body...). Here conditions is an error condition name to be handled, or a list of condition names; body is one or more Lisp expressions to be executed when this handler handles an error. Here are examples of handlers:

(error nil)

(arith-error (message "Division by zero"))

((arith-error file-error)
  "Either division by zero or failure to open a file"))

Each error that occurs has an error symbol that describes what kind of error it is. The error-conditions property of this symbol is a list of condition names (see section Error Symbols and Condition Names). Emacs searches all the active condition-case forms for a handler that specifies one or more of these condition names; the innermost matching condition-case handles the error. Within this condition-case, the first applicable handler handles the error.

After executing the body of the handler, the condition-case returns normally, using the value of the last form in the handler body as the overall value.

The argument var is a variable. condition-case does not bind this variable when executing the protected-form, only when it handles an error. At that time, it binds var locally to an error description, which is a list giving the particulars of the error. The error description has the form (error-symbol . data). The handler can refer to this list to decide what to do. For example, if the error is for failure opening a file, the file name is the second element of data---the third element of the error description.

If var is nil, that means no variable is bound. Then the error symbol and associated data are not available to the handler.

Function: error-message-string error-description
This function returns the error message string for a given error descriptor. It is useful if you want to handle an error by printing the usual error message for that error.

Here is an example of using condition-case to handle the error that results from dividing by zero. The handler displays the error message (but without a beep), then returns a very large number.

(defun safe-divide (dividend divisor)
  (condition-case err                
      ;; Protected form.
      (/ dividend divisor)              
    ;; The handler.
    (arith-error                        ; Condition.
     ;; Display the usual message for this error.
     (message "%s" (error-message-string err))
=> safe-divide

(safe-divide 5 0)
     -| Arithmetic error: (arith-error)
=> 1000000

The handler specifies condition name arith-error so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this condition-case. Thus,

(safe-divide nil 3)
     error--> Wrong type argument: number-or-marker-p, nil

Here is a condition-case that catches all kinds of errors, including those signaled with error:

(setq baz 34)
     => 34

(condition-case err
    (if (eq baz 35)
      ;; This is a call to the function error.
      (error "Rats!  The variable %s was %s, not 35" 'baz baz))
  ;; This is the handler; it is not a form.
  (error (princ (format "The error was: %s" err)) 
-| The error was: (error "Rats!  The variable baz was 34, not 35")
=> 2

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When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.

These narrow classifications are grouped into a hierarchy of wider classes called error conditions, identified by condition names. The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name error which takes in all kinds of errors. Thus, each error has one or more condition names: error, the error symbol if that is distinct from error, and perhaps some intermediate classifications.

In order for a symbol to be an error symbol, it must have an error-conditions property which gives a list of condition names. This list defines the conditions that this kind of error belongs to. (The error symbol itself, and the symbol error, should always be members of this list.) Thus, the hierarchy of condition names is defined by the error-conditions properties of the error symbols.

In addition to the error-conditions list, the error symbol should have an error-message property whose value is a string to be printed when that error is signaled but not handled. If the error-message property exists, but is not a string, the error message `peculiar error' is used.

Here is how we define a new error symbol, new-error:

(put 'new-error
     '(error my-own-errors new-error))       
=> (error my-own-errors new-error)
(put 'new-error 'error-message "A new error")
=> "A new error"

This error has three condition names: new-error, the narrowest classification; my-own-errors, which we imagine is a wider classification; and error, which is the widest of all.

The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs. Naturally, Emacs will never signal new-error on its own; only an explicit call to signal (see section How to Signal an Error) in your code can do this:

(signal 'new-error '(x y))
     error--> A new error: x, y

This error can be handled through any of the three condition names. This example handles new-error and any other errors in the class my-own-errors:

(condition-case foo
    (bar nil t)
  (my-own-errors nil))

The significant way that errors are classified is by their condition names--the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. It would be cumbersome to give signal a list of condition names rather than one error symbol.

By contrast, using only error symbols without condition names would seriously decrease the power of condition-case. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification.

See section F. Standard Errors, for a list of all the standard error symbols and their conditions.

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10.5.4 Cleaning Up from Nonlocal Exits

The unwind-protect construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to make the data consistent again in the event of an error or throw.

Special Form: unwind-protect body cleanup-forms...
unwind-protect executes the body with a guarantee that the cleanup-forms will be evaluated if control leaves body, no matter how that happens. The body may complete normally, or execute a throw out of the unwind-protect, or cause an error; in all cases, the cleanup-forms will be evaluated.

If the body forms finish normally, unwind-protect returns the value of the last body form, after it evaluates the cleanup-forms. If the body forms do not finish, unwind-protect does not return any value in the normal sense.

Only the body is protected by the unwind-protect. If any of the cleanup-forms themselves exits nonlocally (via a throw or an error), unwind-protect is not guaranteed to evaluate the rest of them. If the failure of one of the cleanup-forms has the potential to cause trouble, then protect it with another unwind-protect around that form.

The number of currently active unwind-protect forms counts, together with the number of local variable bindings, against the limit max-specpdl-size (see section 11.3 Local Variables).

For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:

  (let ((buffer (get-buffer-create " *temp*")))
    (set-buffer buffer)
      (kill-buffer buffer))))

You might think that we could just as well write (kill-buffer (current-buffer)) and dispense with the variable buffer. However, the way shown above is safer, if body happens to get an error after switching to a different buffer! (Alternatively, you could write another save-excursion around the body, to ensure that the temporary buffer becomes current again in time to kill it.)

Emacs includes a standard macro called with-temp-buffer which expands into more or less the code shown above (see section 27.2 The Current Buffer). Several of the macros defined in this manual use unwind-protect in this way.

Here is an actual example derived from an FTP package. It creates a process (see section 37. Processes) to try to establish a connection to a remote machine. As the function ftp-login is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses.

(let ((win nil))
        (setq process (ftp-setup-buffer host file))
        (if (setq win (ftp-login process host user password))
            (message "Logged in")
          (error "Ftp login failed")))
    (or win (and process (delete-process process)))))

This example has a small bug: if the user types C-g to quit, and the quit happens immediately after the function ftp-setup-buffer returns but before the variable process is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely.

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