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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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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
.
(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:
(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))) |
(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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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.
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) => very-false |
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) |
if
where there is no then-form:
(unless condition a b c) |
is entirely equivalent to
(if condition nil a b c) |
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,
(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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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.
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.
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)))) |
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) (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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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
:
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.
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))))) |
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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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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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 (foo-inner))) (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
, andgo
, for example. Emacs Lisp has onlythrow
.
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
.
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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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
:
(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.
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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
.
10.5.3.1 How to Signal an Error | How to report an error. | |
10.5.3.2 How Emacs Processes Errors | What Emacs does when you report an error. | |
10.5.3.3 Writing Code to Handle Errors | How you can trap errors and continue execution. | |
10.5.3.4 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.
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)
.
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 10.5.3.4 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 10.5.3.3 Writing Code to Handle Errors). If the error is not handled,
these two values are used in printing the error message.
The function signal
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).
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 10.5.3.4 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 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.
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)) 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: 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)
t
;; This is a call to the function |
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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-conditions '(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 10.5.3.1 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 F. Standard Errors, for a list of all the standard error symbols and their conditions.
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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.
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:
(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 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)) (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 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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