XEmacs 21.2.27 "Hera".

[chise/xemacs-chise.git.1] / man / lispref / control.texi

@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/control.info
@node Control Structures, Variables, Evaluation, Top
@chapter Control Structures
@cindex special forms for control structures
@cindex control structures

  A Lisp program consists of expressions or @dfn{forms} (@pxref{Forms}).
We control the order of execution of the forms by enclosing them in
@dfn{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
@var{a}, then form @var{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 @dfn{textual order}.  For example, if a function
body consists of two forms @var{a} and @var{b}, evaluation of the
function evaluates first @var{a} and then @var{b}, and the function's
value is the value of @var{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 forms since their subforms are not necessarily evaluated or not
evaluated sequentially.  You can use macros to define your own control
structure constructs (@pxref{Macros}).

@menu
* Sequencing::             Evaluation in textual order.
* Conditionals::           @code{if}, @code{cond}.
* Combining Conditions::   @code{and}, @code{or}, @code{not}.
* Iteration::              @code{while} loops.
* Nonlocal Exits::         Jumping out of a sequence.
@end menu

@node Sequencing
@section 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: @code{progn}, the simplest
control construct of Lisp.

  A @code{progn} special form looks like this:

@example
@group
(progn @var{a} @var{b} @var{c} @dots{})
@end group
@end example

@noindent
and it says to execute the forms @var{a}, @var{b}, @var{c} and so on, in
that order.  These forms are called the body of the @code{progn} form.
The value of the last form in the body becomes the value of the entire
@code{progn}.

@cindex implicit @code{progn}
  In the early days of Lisp, @code{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 @code{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 @code{progn}'':
several forms are allowed just as in the body of an actual @code{progn}.
Many other control structures likewise contain an implicit @code{progn}.
As a result, @code{progn} is not used as often as it used to be.  It is
needed now most often inside an @code{unwind-protect}, @code{and},
@code{or}, or in the @var{then}-part of an @code{if}.

@defspec progn forms@dots{}
This special form evaluates all of the @var{forms}, in textual
order, returning the result of the final form.

@example
@group
(progn (print "The first form")
       (print "The second form")
       (print "The third form"))
     @print{} "The first form"
     @print{} "The second form"
     @print{} "The third form"
@result{} "The third form"
@end group
@end example
@end defspec

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

@defspec prog1 form1 forms@dots{}
This special form evaluates @var{form1} and all of the @var{forms}, in
textual order, returning the result of @var{form1}.

@example
@group
(prog1 (print "The first form")
       (print "The second form")
       (print "The third form"))
     @print{} "The first form"
     @print{} "The second form"
     @print{} "The third form"
@result{} "The first form"
@end group
@end example

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

@example
(prog1 (car x) (setq x (cdr x)))
@end example
@end defspec

@defspec prog2 form1 form2 forms@dots{}
This special form evaluates @var{form1}, @var{form2}, and all of the
following @var{forms}, in textual order, returning the result of
@var{form2}.

@example
@group
(prog2 (print "The first form")
       (print "The second form")
       (print "The third form"))
     @print{} "The first form"
     @print{} "The second form"
     @print{} "The third form"
@result{} "The second form"
@end group
@end example
@end defspec

@node Conditionals
@section Conditionals
@cindex conditional evaluation

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

@defspec if condition then-form else-forms@dots{}
@code{if} chooses between the @var{then-form} and the @var{else-forms}
based on the value of @var{condition}.  If the evaluated @var{condition} is
non-@code{nil}, @var{then-form} is evaluated and the result returned.
Otherwise, the @var{else-forms} are evaluated in textual order, and the
value of the last one is returned.  (The @var{else} part of @code{if} is
an example of an implicit @code{progn}.  @xref{Sequencing}.) 

If @var{condition} has the value @code{nil}, and no @var{else-forms} are
given, @code{if} returns @code{nil}.

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

@example
@group
(if nil 
    (print 'true) 
  'very-false)
@result{} very-false
@end group
@end example
@end defspec

@defspec cond clause@dots{}
@code{cond} chooses among an arbitrary number of alternatives.  Each
@var{clause} in the @code{cond} must be a list.  The @sc{car} of this
list is the @var{condition}; the remaining elements, if any, the
@var{body-forms}.  Thus, a clause looks like this:

@example
(@var{condition} @var{body-forms}@dots{})
@end example

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

If the value of @var{condition} is @code{nil}, the clause ``fails'', so
the @code{cond} moves on to the following clause, trying its
@var{condition}.

If every @var{condition} evaluates to @code{nil}, so that every clause
fails, @code{cond} returns @code{nil}.

A clause may also look like this:

@example
(@var{condition})
@end example

@noindent
Then, if @var{condition} is non-@code{nil} when tested, the value of
@var{condition} becomes the value of the @code{cond} form.

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

@example
@group
(cond ((numberp x) x)
      ((stringp x) x)
      ((bufferp x)
       (setq temporary-hack x) ; @r{multiple body-forms}
       (buffer-name x))        ; @r{in one clause}
      ((symbolp x) (symbol-value x)))
@end group
@end example

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

For example, 

@example
@group
(cond ((eq a 'hack) 'foo)
      (t "default"))
@result{} "default"
@end group
@end example

@noindent
This expression is a @code{cond} which returns @code{foo} if the value
of @code{a} is 1, and returns the string @code{"default"} otherwise.
@end defspec

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

@example
@group
(if @var{a} @var{b} @var{c})
@equiv{}
(cond (@var{a} @var{b}) (t @var{c}))
@end group
@end example

@node Combining Conditions
@section Constructs for Combining Conditions

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

@defun not condition
This function tests for the falsehood of @var{condition}.  It returns
@code{t} if @var{condition} is @code{nil}, and @code{nil} otherwise.
The function @code{not} is identical to @code{null}, and we recommend
using the name @code{null} if you are testing for an empty list.
@end defun

@defspec and conditions@dots{}
The @code{and} special form tests whether all the @var{conditions} are
true.  It works by evaluating the @var{conditions} one by one in the
order written.

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

If all the @var{conditions} turn out non-@code{nil}, then the value of
the last of them becomes the value of the @code{and} form.

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

@example
@group
(and (print 1) (print 2) nil (print 3))
     @print{} 1
     @print{} 2
@result{} nil
@end group
@end example

Here is a more realistic example of using @code{and}:

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

@noindent
Note that @code{(car foo)} is not executed if @code{(consp foo)} returns
@code{nil}, thus avoiding an error.

@code{and} can be expressed in terms of either @code{if} or @code{cond}.
For example:

@example
@group
(and @var{arg1} @var{arg2} @var{arg3})
@equiv{}
(if @var{arg1} (if @var{arg2} @var{arg3}))
@equiv{}
(cond (@var{arg1} (cond (@var{arg2} @var{arg3}))))
@end group
@end example
@end defspec

@defspec or conditions@dots{}
The @code{or} special form tests whether at least one of the
@var{conditions} is true.  It works by evaluating all the
@var{conditions} one by one in the order written.

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

If all the @var{conditions} turn out @code{nil}, then the @code{or}
expression returns @code{nil}.

For example, this expression tests whether @code{x} is either 0 or
@code{nil}:

@example
(or (eq x nil) (eq x 0))
@end example

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

@example
@group
(or @var{arg1} @var{arg2} @var{arg3})
@equiv{}
(cond (@var{arg1})
      (@var{arg2})
      (@var{arg3}))
@end group
@end example

You could almost write @code{or} in terms of @code{if}, but not quite:

@example
@group
(if @var{arg1} @var{arg1}
  (if @var{arg2} @var{arg2} 
    @var{arg3}))
@end group
@end example

@noindent
This is not completely equivalent because it can evaluate @var{arg1} or
@var{arg2} twice.  By contrast, @code{(or @var{arg1} @var{arg2}
@var{arg3})} never evaluates any argument more than once.
@end defspec

@node Iteration
@section Iteration
@cindex iteration
@cindex recursion

  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 @var{n}.  You can do this
in XEmacs Lisp with the special form @code{while}:

@defspec while condition forms@dots{}
@code{while} first evaluates @var{condition}.  If the result is
non-@code{nil}, it evaluates @var{forms} in textual order.  Then it
reevaluates @var{condition}, and if the result is non-@code{nil}, it
evaluates @var{forms} again.  This process repeats until @var{condition}
evaluates to @code{nil}.

There is no limit on the number of iterations that may occur.  The loop
will continue until either @var{condition} evaluates to @code{nil} or
until an error or @code{throw} jumps out of it (@pxref{Nonlocal Exits}).

The value of a @code{while} form is always @code{nil}.

@example
@group
(setq num 0)
     @result{} 0
@end group
@group
(while (< num 4)
  (princ (format "Iteration %d." num))
  (setq num (1+ num)))
     @print{} Iteration 0.
     @print{} Iteration 1.
     @print{} Iteration 2.
     @print{} Iteration 3.
     @result{} nil
@end group
@end example

If you would like to execute something on each iteration before the
end-test, put it together with the end-test in a @code{progn} as the
first argument of @code{while}, as shown here:

@example
@group
(while (progn
         (forward-line 1)
         (not (looking-at "^$"))))
@end group
@end example

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

@node Nonlocal Exits
@section Nonlocal Exits
@cindex nonlocal exits

  A @dfn{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.

@menu
* Catch and Throw::     Nonlocal exits for the program's own purposes.
* Examples of Catch::   Showing how such nonlocal exits can be written.
* Errors::              How errors are signaled and handled.
* Cleanups::            Arranging to run a cleanup form if an error happens.
@end menu

@node Catch and Throw
@subsection Explicit Nonlocal Exits: @code{catch} and @code{throw}

  Most control constructs affect only the flow of control within the
construct itself.  The function @code{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.)  @code{throw} is used inside a @code{catch}, and jumps back to
that @code{catch}.  For example:

@example
@group
(catch 'foo
  (progn
    @dots{}
    (throw 'foo t)
    @dots{}))
@end group
@end example

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

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

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

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

@cindex CL note---only @code{throw} in Emacs
@quotation
@b{Common Lisp note:} Most other versions of Lisp, including Common Lisp,
have several ways of transferring control nonsequentially: @code{return},
@code{return-from}, and @code{go}, for example.  XEmacs Lisp has only
@code{throw}.
@end quotation

@defspec catch tag body@dots{}
@cindex tag on run time stack
@code{catch} establishes a return point for the @code{throw} function.  The
return point is distinguished from other such return points by @var{tag},
which may be any Lisp object.  The argument @var{tag} is evaluated normally
before the return point is established.

With the return point in effect, @code{catch} evaluates the forms of the
@var{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 @code{catch}.

If a @code{throw} is done within @var{body} specifying the same value
@var{tag}, the @code{catch} exits immediately; the value it returns is
whatever was specified as the second argument of @code{throw}.
@end defspec

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

The argument @var{value} is used as the value to return from that
@code{catch}.

@kindex no-catch
If no return point is in effect with tag @var{tag}, then a @code{no-catch}
error is signaled with data @code{(@var{tag} @var{value})}.
@end defun

@node Examples of Catch
@subsection Examples of @code{catch} and @code{throw}

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

@example
@group
(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))))))
@end group
@end example

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

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

@example
@group
(defun catch2 (tag)
  (catch tag
    (throw 'hack 'yes)))
@result{} catch2
@end group

@group
(catch 'hack 
  (print (catch2 'hack))
  'no)
@print{} yes
@result{} no
@end group
@end example

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

  Now let's change the argument given to @code{catch2}:

@example
@group
(defun catch2 (tag)
  (catch tag
    (throw 'hack 'yes)))
@result{} catch2
@end group

@group
(catch 'hack
  (print (catch2 'quux))
  'no)
@result{} yes
@end group
@end example

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

@node Errors
@subsection Errors
@cindex errors

  When XEmacs Lisp attempts to evaluate a form that, for some reason,
cannot be evaluated, it @dfn{signals} an @dfn{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 @kbd{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
@code{unwind-protect} to establish @dfn{cleanup expressions} to be
evaluated in case of error.  (@xref{Cleanups}.)  Occasionally, you may
wish the program to continue execution despite an error in a subroutine.
In these cases, you would use @code{condition-case} to establish
@dfn{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 @code{catch} and @code{throw}
instead.  @xref{Catch and Throw}.

@menu
* Signaling Errors::      How to report an error.
* Processing of Errors::  What XEmacs does when you report an error.
* Handling Errors::       How you can trap errors and continue execution.
* Error Symbols::         How errors are classified for trapping them.
@end menu

@node Signaling Errors
@subsubsection How to Signal an Error
@cindex signaling errors

  Most errors are signaled ``automatically'' within Lisp primitives
which you call for other purposes, such as if you try to take the
@sc{car} of an integer or move forward a character at the end of the
buffer; you can also signal errors explicitly with the functions
@code{error}, @code{signal}, and others.

  Quitting, which happens when the user types @kbd{C-g}, is not 
considered an error, but it is handled almost like an error.
@xref{Quitting}.

@defun error format-string &rest args
This function signals an error with an error message constructed by
applying @code{format} (@pxref{String Conversion}) to
@var{format-string} and @var{args}.

This error is not continuable: you cannot continue execution after the
error using the debugger @kbd{r} or @kbd{c} commands.  If you wish the
user to be able to continue execution, use @code{cerror} or
@code{signal} instead.

These examples show typical uses of @code{error}:

@example
@group
(error "You have committed an error.  
        Try something else.")
     @error{} You have committed an error.  
        Try something else.
@end group

@group
(error "You have committed %d errors." 10)
     @error{} You have committed 10 errors.  
@end group
@end example

@code{error} works by calling @code{signal} with two arguments: the
error symbol @code{error}, and a list containing the string returned by
@code{format}.  This is repeated in an endless loop, to ensure that
@code{error} never returns.

If you want to use your own string as an error message verbatim, don't
just write @code{(error @var{string})}.  If @var{string} contains
@samp{%}, it will be interpreted as a format specifier, with undesirable
results.  Instead, use @code{(error "%s" @var{string})}.
@end defun

@defun cerror format-string &rest args
This function behaves like @code{error}, except that the error it
signals is continuable.  That means that debugger commands @kbd{c} and
@kbd{r} can resume execution.
@end defun

@defun signal error-symbol data
This function signals a continuable error named by @var{error-symbol}.
The argument @var{data} is a list of additional Lisp objects relevant to
the circumstances of the error.

The argument @var{error-symbol} must be an @dfn{error symbol}---a symbol
bearing a property @code{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 @var{data} depends on
@var{error-symbol}.  For example, with a @code{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.
@xref{Error Symbols}, for a description of error symbols.

Both @var{error-symbol} and @var{data} are available to any error
handlers that handle the error: @code{condition-case} binds a local
variable to a list of the form @code{(@var{error-symbol} .@:
@var{data})} (@pxref{Handling Errors}).  If the error is not handled,
these two values are used in printing the error message.

The function @code{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 @code{signal-error} instead.  Note that
in FSF Emacs @code{signal} never returns.

@smallexample
@group
(signal 'wrong-number-of-arguments '(x y))
     @error{} Wrong number of arguments: x, y
@end group

@group
(signal 'no-such-error '("My unknown error condition"))
     @error{} Peculiar error (no-such-error "My unknown error condition")
@end group
@end smallexample
@end defun

@defun signal-error error-symbol data
This function behaves like @code{signal}, except that the error it
signals is not continuable.
@end defun

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

@var{argument} is not evaluated, and should be a symbol.
@var{predicate} is evaluated, and should name a function.

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

@example
@group
(defun cache-object-internal (object wlist)
  ;; @r{Before doing anything, make sure that @var{wlist} is indeed}
  ;; @r{a weak list, which is what we expect.}
  (check-argument-type 'weak-list-p wlist)
  @dots{})
@end group
@end example
@end defmac

@node Processing of Errors
@subsubsection How XEmacs Processes Errors

When an error is signaled, @code{signal} searches for an active
@dfn{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 @code{condition-case} that
established it; all functions called within that @code{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 @code{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.

@defun display-error error-object stream
This function displays @var{error-object} on @var{stream}.
@var{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 an @code{display-error} property, that
function is invoked for printing the actual error message.  Otherwise,
the error is printed as @samp{Error: arg1, arg2, ...}.
@end defun

@defun error-message-string error-object
This function converts @var{error-object} to an error message string,
and returns it.  The message is equivalent to the one that would be
printed by @code{display-error}, except that it is conveniently returned
in string form.
@end defun

@cindex @code{debug-on-error} use
An error that has no explicit handler may call the Lisp debugger.  The
debugger is enabled if the variable @code{debug-on-error} (@pxref{Error
Debugging}) is non-@code{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.

@node Handling Errors
@subsubsection Writing Code to Handle Errors
@cindex error handler
@cindex handling 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 form
@code{condition-case}.  A simple example looks like this:

@example
@group
(condition-case nil
    (delete-file filename)
  (error nil))
@end group
@end example

@noindent
This deletes the file named @var{filename}, catching any error and
returning @code{nil} if an error occurs.

  The second argument of @code{condition-case} is called the
@dfn{protected form}.  (In the example above, the protected form is a
call to @code{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
@code{signal} and @code{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 @dfn{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,
@code{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
@code{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
@code{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 @code{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.

  @code{condition-case} is often used to trap errors that are
predictable, such as failure to open a file in a call to
@code{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.

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

  Error signaling and handling have some resemblance to @code{throw} and
@code{catch}, but they are entirely separate facilities.  An error
cannot be caught by a @code{catch}, and a @code{throw} cannot be handled
by an error handler (though using @code{throw} when there is no suitable
@code{catch} signals an error that can be handled).

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

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

@smallexample
@group
(error nil)

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

((arith-error file-error)
 (message
  "Either division by zero or failure to open a file"))
@end group
@end smallexample

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

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

The argument @var{var} is a variable.  @code{condition-case} does not
bind this variable when executing the @var{protected-form}, only when it
handles an error.  At that time, it binds @var{var} locally to a list of
the form @code{(@var{error-symbol} . @var{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 @var{data}---the third element of
@var{var}.

If @var{var} is @code{nil}, that means no variable is bound.  Then the
error symbol and associated data are not available to the handler.
@end defspec

@cindex @code{arith-error} example
Here is an example of using @code{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.

@smallexample
@group
(defun safe-divide (dividend divisor)
  (condition-case err                
      ;; @r{Protected form.}
      (/ dividend divisor)              
    ;; @r{The handler.}
    (arith-error                        ; @r{Condition.}
     (princ (format "Arithmetic error: %s" err))
     1000000)))
@result{} safe-divide
@end group

@group
(safe-divide 5 0)
     @print{} Arithmetic error: (arith-error)
@result{} 1000000
@end group
@end smallexample

@noindent
The handler specifies condition name @code{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 @code{condition-case}.  Thus,

@smallexample
@group
(safe-divide nil 3)
     @error{} Wrong type argument: integer-or-marker-p, nil
@end group
@end smallexample

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

@smallexample
@group
(setq baz 34)
     @result{} 34
@end group

@group
(condition-case err
    (if (eq baz 35)
        t
      ;; @r{This is a call to the function @code{error}.}
      (error "Rats!  The variable %s was %s, not 35" 'baz baz))
  ;; @r{This is the handler; it is not a form.}
  (error (princ (format "The error was: %s" err)) 
         2))
@print{} The error was: (error "Rats!  The variable baz was 34, not 35")
@result{} 2
@end group
@end smallexample

@node Error Symbols
@subsubsection Error Symbols and Condition Names
@cindex error symbol
@cindex error name
@cindex condition name
@cindex user-defined error
@kindex error-conditions

  When you signal an error, you specify an @dfn{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 @dfn{error conditions}, identified by @dfn{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
@code{error} which takes in all kinds of errors.  Thus, each error has
one or more condition names: @code{error}, the error symbol if that
is distinct from @code{error}, and perhaps some intermediate
classifications.

  In other words, each error condition @dfn{inherits} from another error
condition, with @code{error} sitting at the top of the inheritance
hierarchy.

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

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

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

@example
@group
(define-error 'my-own-errors "A whole range of errors" 'error)
(define-error 'new-error "A new error" 'my-own-errors)
@end group
@end example

@noindent
@code{new-error} has three condition names: @code{new-error}, the
narrowest classification; @code{my-own-errors}, which we imagine is a
wider classification; and @code{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
@code{new-error} before @code{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 @code{new-error} on its own; only
an explicit call to @code{signal} (@pxref{Signaling Errors}) in your
code can do this:

@example
@group
(signal 'new-error '(x y))
     @error{} A new error: x, y
@end group
@end example

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

@example
@group
(condition-case foo
    (bar nil t)
  (my-own-errors nil))
@end group
@end example

  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
@code{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 @code{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.

  

  @xref{Standard Errors}, for a list of all the standard error symbols
and their conditions.

@node Cleanups
@subsection Cleaning Up from Nonlocal Exits

  The @code{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.

@defspec unwind-protect body cleanup-forms@dots{}
@cindex cleanup forms
@cindex protected forms
@cindex error cleanup
@cindex unwinding
@code{unwind-protect} executes the @var{body} with a guarantee that the
@var{cleanup-forms} will be evaluated if control leaves @var{body}, no
matter how that happens.  The @var{body} may complete normally, or
execute a @code{throw} out of the @code{unwind-protect}, or cause an
error; in all cases, the @var{cleanup-forms} will be evaluated.

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

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

The number of currently active @code{unwind-protect} forms counts,
together with the number of local variable bindings, against the limit
@code{max-specpdl-size} (@pxref{Local Variables}).
@end defspec

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

@smallexample
@group
(save-excursion
  (let ((buffer (get-buffer-create " *temp*")))
    (set-buffer buffer)
    (unwind-protect
        @var{body}
      (kill-buffer buffer))))
@end group
@end smallexample

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

@findex ftp-login
  Here is an actual example taken from the file @file{ftp.el}.  It
creates a process (@pxref{Processes}) to try to establish a connection
to a remote machine.  As the function @code{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.

@smallexample
@group
(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)))))
@end group
@end smallexample

  This example actually has a small bug: if the user types @kbd{C-g} to
quit, and the quit happens immediately after the function
@code{ftp-setup-buffer} returns but before the variable @code{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 @code{unwind-protect} to make sure
to kill a temporary buffer.  In this example, the value returned by
@code{unwind-protect} is used.

@smallexample
(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)))))
@end smallexample