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15. Control Structures

A Lisp program consists of expressions or forms (see section 14.3 Kinds of Forms). We control the order of execution of the forms by enclosing them in control structures. Control structures are special operators which control when, whether, or how many times to execute the subforms of their containing forms.

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, and the function's value is the value of b.

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

XEmacs 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 operators since their enclosing forms' subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see section 18. Macros).

15.1 Sequencing  Evaluation in textual order.
15.2 Conditionals  if, cond.
15.3 Constructs for Combining Conditions  and, or, not.
15.4 Iteration  while loops.
15.5 Nonlocal Exits  Jumping out of a sequence.


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15.1 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.

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 often as it used to be. It is needed now most often inside an unwind-protect, and, or, or in the then-part of an if.

Special Operator: progn forms...
This special operator 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 Operator: prog1 form1 forms...
This special operator 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 Operator: prog2 form1 form2 forms...
This special operator 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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15.2 Conditionals

Conditional control structures choose among alternatives. XEmacs Lisp has two conditional forms: if, which is much the same as in other languages, and cond, which is a generalized case statement.

Special Operator: 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 15.1 Sequencing.)

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

if is a special operator 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)
=> very-false

Special Operator: 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:

 
(condition)

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,

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

This expression is a cond which returns foo if the value of a is 1, 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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15.3 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 Operator: and conditions...
The and special operator 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 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.

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 Operator: or conditions...
The or special operator 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.

For example, this expression tests whether x is either 0 or nil:

 
(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)
      (arg2)
      (arg3))

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

 
(if arg1 arg1
  (if arg2 arg2
    arg3))

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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15.4 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 XEmacs Lisp with the special operator while:

Special Operator: 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 15.5 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

If you would like to execute something on each iteration before the end-test, put it together with 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. It is unusual in that the while has no body, just the end test (which also does the real work of moving point).


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15.5 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 XEmacs 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.

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


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15.5.1 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:

 
(catch 'foo
  (progn
    ...
    (throw 'foo t)
    ...))

The throw 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 throw and the catch are matched through the first argument: throw searches for a catch whose first argument is eq to the one specified. Thus, in the above example, the throw specifies foo, and the catch specifies the same symbol, so that catch is applicable. If there is more than one applicable catch, the innermost one takes precedence.

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 16.3 Local Variables). Likewise, throw restores the buffer and position saved by save-excursion (see section 41.3 Excursions), and the narrowing status saved by save-restriction and the window selection saved by save-window-excursion (see section 38.16 Window Configurations). It also runs any cleanups established with the unwind-protect special operator when it exits that form (see section 15.5.4 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 25.10 Recursive Editing).

Special Operator: 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. 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 done within body specifying the same value tag, the catch 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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15.5.2 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))
  'no)
-| 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:

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

(catch 'hack
  (print (catch2 'quux))
  'no)
=> 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.

In most cases the formal tag for a catch is a quoted symbol or a variable whose value is a symbol. Both styles are demonstrated above. In definitions of derived control structures, an anonymous tag may be desired. A gensym could be used, but since catch tags are compared using eq, any Lisp object can be used. An occasionally encountered idiom is to bind a local variable to (cons nil nil), and use the variable as the formal tag.


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15.5.3 Errors

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

When an error is signaled, XEmacs'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 15.5.4 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 15.5.1 Explicit Nonlocal Exits: catch and throw.

15.5.3.1 How to Signal an Error  How to report an error.
15.5.3.2 How XEmacs Processes Errors  What XEmacs does when you report an error.
15.5.3.3 Writing Code to Handle Errors  How you can trap errors and continue execution.
15.5.3.4 Error Symbols and Condition Names  How errors are classified for trapping them.


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15.5.3.1 How to Signal an Error

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, signal, and others.

Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See section 25.8 Quitting.

XEmacs has a rich hierarchy of error symbols predefined via deferror.

 
error
  syntax-error
    invalid-read-syntax
    list-formation-error
      malformed-list
        malformed-property-list
      circular-list
        circular-property-list

  invalid-argument
    wrong-type-argument
    args-out-of-range
    wrong-number-of-arguments
    invalid-function
    no-catch

  invalid-state
    void-function
    cyclic-function-indirection
    void-variable
    cyclic-variable-indirection

  invalid-operation
    invalid-change
      setting-constant
    editing-error
      beginning-of-buffer
      end-of-buffer
      buffer-read-only
    io-error
      end-of-file
    arith-error
      range-error
      domain-error
      singularity-error
      overflow-error
      underflow-error

The five most common errors you will probably use or base your new errors off of are syntax-error, invalid-argument, invalid-state, invalid-operation, and invalid-change. Note the semantic differences:

Function: error datum &rest args
This function signals a non-continuable error.

datum should normally be an error symbol, i.e. a symbol defined using define-error. args will be made into a list, and datum and args passed as the two arguments to signal, the most basic error handling function.

This error is not continuable: you cannot continue execution after the error using the debugger r command. See also cerror.

The correct semantics of args varies from error to error, but for most errors that need to be generated in Lisp code, the first argument should be a string describing the *context* of the error (i.e. the exact operation being performed and what went wrong), and the remaining arguments or \"frobs\" (most often, there is one) specify the offending object(s) and/or provide additional details such as the exact error when a file error occurred, e.g.:

For historical compatibility, DATUM can also be a string. In this case, datum and args are passed together as the arguments to format, and then an error is signalled using the error symbol error and formatted string. Although this usage of error is very common, it is deprecated because it totally defeats the purpose of having structured errors. There is now a rich set of defined errors to use.

See also cerror, signal, and signal-error."

These examples show typical uses of error:

 
(error 'syntax-error
       "Dialog descriptor must supply at least one button"
	descriptor)

(error "You have committed an error.
        Try something else.")
     error--> You have committed an error.
        Try something else.

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

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: cerror datum &rest args
This function behaves like error, except that the error it signals is continuable. That means that debugger commands c and r can resume execution.

Function: signal error-symbol data
This function signals a continuable 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 XEmacs 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-argument error, there are 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 15.5.3.4 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 15.5.3.3 Writing Code to Handle Errors). If the error is not handled, these two values are used in printing the error message.

The function signal can return, if the debugger is invoked and the user invokes the "return from signal" option. If you want the error not to be continuable, use signal-error instead. Note that in FSF Emacs signal never returns.

 
(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 (no-such-error "My unknown error condition")

Function: signal-error error-symbol data
This function behaves like signal, except that the error it signals is not continuable.

Macro: check-argument-type predicate argument
This macro checks that argument satisfies predicate. If that is not the case, it signals a continuable wrong-type-argument error until the returned value satisfies predicate, and assigns the returned value to argument. In other words, execution of the program will not continue until predicate is met.

argument is not evaluated, and should be a symbol. predicate is evaluated, and should name a function.

As shown in the following example, check-argument-type is useful in low-level code that attempts to ensure the sanity of its data before proceeding.

 
(defun cache-object-internal (object wlist)
  ;; Before doing anything, make sure that wlist is indeed
  ;; a weak list, which is what we expect.
  (check-argument-type 'weak-list-p wlist)
  ...)


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15.5.3.2 How XEmacs Processes Errors

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.

Errors in command loop are processed using the command-error function, which takes care of some necessary cleanup, and prints a formatted error message to the echo area. The functions that do the formatting are explained below.

Function: display-error error-object stream
This function displays error-object on stream. error-object is a cons of error type, a symbol, and error arguments, a list. If the error type symbol of one of its error condition superclasses has a display-error property, that function is invoked for printing the actual error message. Otherwise, the error is printed as `Error: arg1, arg2, ...'.

Function: error-message-string error-object
This function converts error-object to an error message string, and returns it. The message is equivalent to the one that would be printed by display-error, except that it is conveniently returned in string form.

An error that has no explicit handler may call the Lisp debugger. The debugger is enabled if the variable debug-on-error (see section 22.1.1 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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15.5.3.3 Writing Code to Handle Errors

The usual effect of signaling an error is to terminate the command that is running and return immediately to the XEmacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler, with the special operator 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 will actually handle it.

When an error is handled, control returns to the handler. Before this happens, XEmacs 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 continues by returning 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.

condition-case 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.

Even when an error is handled, the debugger may still be called if the variable debug-on-signal (see section 22.1.1 Entering the Debugger on an Error) is non-nil. Note that this may yield unpredictable results with code that traps expected errors as normal part of its operation. Do not set debug-on-signal unless you know what you are doing.

Error signaling and handling have some resemblance to throw and catch, 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 Operator: condition-case var protected-form handlers...
This special operator 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)
 (message
  "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 15.5.3.4 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 a list of the form (error-symbol . data), giving the particulars of the error. 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 var.

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

Here is an example of using condition-case to handle the error that results from dividing by zero. The handler prints out a warning message and returns a very large number.

 
(defun safe-divide (dividend divisor)
  (condition-case err
      ;; Protected form.
      (/ dividend divisor)
    ;; The handler.
    (arith-error                        ; Condition.
     (princ (format "Arithmetic error: %s" err))
     1000000)))
=> 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: integer-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)
        t
      ;; 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))
         2))
-| The error was: (error "Rats!  The variable baz was 34, not 35")
=> 2


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15.5.3.4 Error Symbols and Condition Names

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 XEmacs 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 other words, each error condition inherits from another error condition, with error sitting at the top of the inheritance hierarchy.

Function: define-error error-symbol error-message &optional inherits-from
This function defines a new error, denoted by error-symbol. error-message is an informative message explaining the error, and will be printed out when an unhandled error occurs. error-symbol is a sub-error of inherits-from (which defaults to error).

define-error internally works by putting on error-symbol an error-message property whose value is error-message, and an error-conditions property that is a list of error-symbol followed by each of its super-errors, up to and including error. You will sometimes see code that sets this up directly rather than calling define-error, but you should not do this yourself, unless you wish to maintain compatibility with FSF Emacs, which does not provide define-error.

Here is how we define a new error symbol, new-error, that belongs to a range of errors called my-own-errors:

 
(define-error 'my-own-errors "A whole range of errors" 'error)
(define-error 'new-error "A new error" 'my-own-errors)

new-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.

Note that it is not legal to try to define an error unless its super-error is also defined. For instance, attempting to define new-error before my-own-errors are defined will signal an error.

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, XEmacs will never signal new-error on its own; only an explicit call to signal (see section 15.5.3.1 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 C. Standard Errors, for a list of all the standard error symbols and their conditions.


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15.5.4 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 ensure the data are consistent in the event of an error or throw.

Special Operator: 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 actually protected by the unwind-protect. If any of the cleanup-forms themselves exits nonlocally (e.g., 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 16.3 Local Variables).

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

 
(save-excursion
  (let ((buffer (get-buffer-create " *temp*")))
    (set-buffer buffer)
    (unwind-protect
        body
      (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 in time to kill it.)

Here is an actual example taken from the file `ftp.el'. It creates a process (see section 56. 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, XEmacs might fill up with useless subprocesses.

 
(let ((win nil))
  (unwind-protect
      (progn
        (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 actually 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.

Here is another example which uses unwind-protect to make sure to kill a temporary buffer. In this example, the value returned by unwind-protect is used.

 
(defun shell-command-string (cmd)
  "Return the output of the shell command CMD, as a string."
  (save-excursion
    (set-buffer (generate-new-buffer " OS*cmd"))
    (shell-command cmd t)
    (unwind-protect
        (buffer-string)
      (kill-buffer (current-buffer)))))

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