(command-line): If no error data, print "", not nil.

[gnu-emacs] / lispref / edebug.texi

@comment -*-texinfo-*-

@c This file is intended to be used as a section within the Emacs Lisp 
@c Reference Manual.  It may also be used by an independent Edebug User 
@c Manual, edebug.tex, in which case the Edebug node below should be used 
@c with the following links to the Bugs section and to the top level:

@c , Bugs and Todo List, Top, Top

@node Edebug, Bugs and Todo List, Top, Top
@section Edebug
@cindex Edebug mode

@cindex Edebug
  Edebug is a source-level debugger for Emacs Lisp programs with which
you can:

@itemize @bullet
@item
Step through evaluation, stopping before and after each expression.

@item
Set conditional or unconditional breakpoints.

@item
Stop when a specified condition is true (the global break event).

@item
Trace slow or fast, stopping briefly at each stop point, or
at each breakpoint.

@item
Display expression results and evaluate expressions as if outside of
Edebug.

@item 
Automatically reevaluate a list of expressions and
display their results each time Edebug updates the display.

@item
Output trace info on function enter and exit.

@item
Stop when an error occurs.

@item
Display a backtrace, omitting Edebug's own frames.

@item
Specify argument evaluation for macros and defining forms.

@item
Obtain rudimentary coverage testing and frequency counts.
@end itemize

The first three sections below should tell you enough about Edebug to
enable you to use it.

@menu
* Using Edebug::                Introduction to use of Edebug.
* Instrumenting::               You must instrument your code
                                  in order to debug it with Edebug.
* Modes: Edebug Execution Modes. Execution modes, stopping more or less often.
* Jumping::                     Commands to jump to a specified place.
* Misc: Edebug Misc.            Miscellaneous commands.
* Breakpoints::                 Setting breakpoints to make the program stop.
* Trapping Errors::             trapping errors with Edebug.
* Views: Edebug Views.          Views inside and outside of Edebug.
* Eval: Edebug Eval.                    Evaluating expressions within Edebug.
* Eval List::                   Expressions whose values are displayed
                                  each time you enter Edebug.
* Printing in Edebug::          Customization of printing.
* Trace Buffer::                How to produce trace output in a buffer.
* Coverage Testing::            How to test evaluation coverage.
* The Outside Context::         Data that Edebug saves and restores.
* Instrumenting Macro Calls::   Specifying how to handle macro calls.
* Options: Edebug Options.      Option variables for customizing Edebug.
@end menu

@node Using Edebug
@subsection Using Edebug

  To debug a Lisp program with Edebug, you must first @dfn{instrument}
the Lisp code that you want to debug.  A simple way to do this is to
first move point into the definition of a function or macro and then do
@kbd{C-u C-M-x} (@code{eval-defun} with a prefix argument).  See
@ref{Instrumenting}, for alternative ways to instrument code.

  Once a function is instrumented, any call to the function activates
Edebug.  Activating Edebug may stop execution and let you step through
the function, or it may update the display and continue execution while
checking for debugging commands, depending on which Edebug execution
mode you have selected.  The default execution mode is step, which does
stop execution.  @xref{Edebug Execution Modes}.

  Within Edebug, you normally view an Emacs buffer showing the source of
the Lisp code you are debugging.  This is referred to as the @dfn{source
code buffer}.  This buffer is temporarily read-only.

  An arrow at the left margin indicates the line where the function is
executing.  Point initially shows where within the line the function is
executing, but this ceases to be true if you move point yourself.

  If you instrument the definition of @code{fac} (shown below) and then
execute @code{(fac 3)}, here is what you normally see.  Point is at the
open-parenthesis before @code{if}.

@example
(defun fac (n)
=>@point{}(if (< 0 n)
      (* n (fac (1- n)))
    1))
@end example

@cindex stop points
The places within a function where Edebug can stop execution are called
@dfn{stop points}.  These occur both before and after each subexpression
that is a list, and also after each variable reference.  
Here we show with periods the stop points found in the function
@code{fac}:

@example
(defun fac (n)
  .(if .(< 0 n.).
      .(* n. .(fac (1- n.).).).
    1).)
@end example

The special commands of Edebug are available in the source code buffer
in addition to the commands of Emacs Lisp mode.  For example, you can
type the Edebug command @key{SPC} to execute until the next stop point.
If you type @key{SPC} once after entry to @code{fac}, here is the
display you will see:

@example
(defun fac (n)
=>(if @point{}(< 0 n)
      (* n (fac (1- n)))
    1))
@end example

When Edebug stops execution after an expression, it displays the
expression's value in the echo area. 

Other frequently used commands are @kbd{b} to set a breakpoint at a stop
point, @kbd{g} to execute until a breakpoint is reached, and @kbd{q} to
exit Edebug and return to the top-level command loop.  Type @kbd{?} to
display a list of all Edebug commands.

@node Instrumenting
@subsection Instrumenting for Edebug

  In order to use Edebug to debug Lisp code, you must first
@dfn{instrument} the code.  Instrumenting code inserts additional code
into it, code which invokes Edebug at the proper places.

  Once a function is instrumented, any call to the function activates
Edebug.  This may or may not stop execution, depending on the Edebug
execution mode in use.  Some Edebug modes only update the display to
indicate the progress of the evaluation without stopping execution.

@kindex C-M-x
@findex eval-defun (Edebug)
  Once you have loaded Edebug, the command @kbd{C-M-x}
(@code{eval-defun}) is redefined so that when invoked with a prefix
argument on a definition, it instruments the definition before
evaluating it.  (The source code itself is not modified.)  If the
variable @code{edebug-all-defs} is non-@code{nil}, that inverts the
meaning of the prefix argument: then @kbd{C-M-x} instruments the
definition @emph{unless} it has a prefix argument.  The default value of
@code{edebug-all-defs} is @code{nil}.  The command @kbd{M-x
edebug-all-defs} toggles the value of the variable
@code{edebug-all-defs}.

@findex edebug-all-forms
@findex eval-region (Edebug)
@findex eval-current-buffer (Edebug)
  If @code{edebug-all-defs} is non-@code{nil}, then the commands
@code{eval-region}, @code{eval-current-buffer}, and @code{eval-buffer}
also instrument any definitions they evaluate.  Similarly,
@code{edebug-all-forms} controls whether @code{eval-region} should
instrument @emph{any} form, even non-defining forms.  This doesn't apply
to loading or evaluations in the minibuffer.  The command @kbd{M-x
edebug-all-forms} toggles this option.

@findex edebug-eval-top-level-form
Another command, @kbd{M-x edebug-eval-top-level-form}, is available to
instrument any top-level form regardless of the value of
@code{edebug-all-defs} or @code{edebug-all-forms}.

When Edebug is about to instrument code for the first time in a session,
it runs the hook @code{edebug-setup-hook}, then sets it to @code{nil}.
You can use this to load up Edebug specifications associated with a
package you are using, but only when you also use Edebug.

While Edebug is active, the command @kbd{I}
(@code{edebug-instrument-callee}) instruments the definition of the
function or macro called by the list form after point, if is not already
instrumented.  This is possible only if Edebug knows where to find the
source for that function; after loading Edebug, @code{eval-region}
records the position of every definition it evaluates, even if not
instrumenting it.  See also the @kbd{i} command (@pxref{Jumping}), which
steps into the call after instrumenting the function.

@cindex special forms (Edebug)
@cindex interactive commands (Edebug)
@cindex anonymous lambda expressions (Edebug)
@cindex Common Lisp (Edebug)
@pindex cl.el (Edebug)
@pindex cl-specs.el
  Edebug knows how to instrument all the standard special forms, an
interactive form with an expression argument, anonymous lambda
expressions, and other defining forms.  Edebug cannot know what a
user-defined macro will do with the arguments of a macro call, so you
must tell it; @xref{Instrumenting Macro Calls}, for details.

@findex eval-expression (Edebug)
  To remove instrumentation from a definition, simply reevaluate its
definition in a way that does not instrument.  There are two ways of
evaluating forms without instrumenting them: from a file with
@code{load}, and from the minibuffer with @code{eval-expression}
(@kbd{M-ESC}).

  If Edebug detects a syntax error while instrumenting, it leaves point
at the erroneous code and signals an @code{invalid-read-syntax} error.

  @xref{Edebug Eval}, for other evaluation functions available
inside of Edebug.

@node Edebug Execution Modes
@subsection Edebug Execution Modes

@cindex Edebug execution modes
Edebug supports several execution modes for running the program you are
debugging.  We call these alternatives @dfn{Edebug execution modes}; do
not confuse them with major or minor modes.  The current Edebug mode
determines how far Edebug continues execution before stopping---whether
it stops at each stop point, or continues to the next breakpoint, for
example---and how much Edebug displays the progress of the evaluation
before it stops.

Normally, you specify the Edebug execution mode by typing a command to
continue the program in a certain mode.  Here is a table of these
commands.  All except for @kbd{S} resume execution of the program, at
least for a certain distance.

@table @kbd
@item S
Stop: don't execute any more of the program for now, just wait for more
Edebug commands (@code{edebug-stop}).

@item @key{SPC}
Step: stop at the next stop point encountered (@code{edebug-step-mode}).

@item n
Next: stop at the next stop point encountered after an expression
(@code{edebug-next-mode}).  Also see @code{edebug-forward-sexp} in
@ref{Edebug Misc}.

@item t
Trace: pause one second at each Edebug stop point (@code{edebug-trace-mode}).

@item T
Rapid trace: update the display at each stop point, but don't actually
pause (@code{edebug-Trace-fast-mode}).

@item g
Go: run until the next breakpoint (@code{edebug-go-mode}).  @xref{Breakpoints}.

@item c
Continue: pause one second at each breakpoint, and then continue
(@code{edebug-continue-mode}).

@item C
Rapid continue: move point to each breakpoint, but don't pause
(@code{edebug-Continue-fast-mode}).

@item G
Go non-stop: ignore breakpoints (@code{edebug-Go-nonstop-mode}).  You
can still stop the program by typing @kbd{S}, or any editing command.
@end table

In general, the execution modes earlier in the above list run the
program more slowly or stop sooner.

While executing or tracing, you can interrupt the execution by typing
any Edebug command.  Edebug stops the program at the next stop point and
then executes the command that you typed.  For example, typing @kbd{t}
during execution switches to trace mode at the next stop point.  You can
use @kbd{S} to stop execution without doing anything else.

If your function happens to read input, a character you type intending
to interrupt execution may be read by the function instead.  You can
avoid such unintended results by paying attention to when your program
wants input.

@cindex keyboard macros (Edebug)
Keyboard macros containing the commands in this section do not
completely work: exiting from Edebug, to resume the program, loses track
of the keyboard macro.  This is not easy to fix.  Also, defining or
executing a keyboard macro outside of Edebug does not affect commands
inside Edebug.  This is usually an advantage.  But see the
@code{edebug-continue-kbd-macro} option (@pxref{Edebug Options}).

When you enter a new Edebug level, the initial execution mode comes from
the value of the variable @code{edebug-initial-mode}.  By default, this
specifies step mode.  Note that you may reenter the same Edebug level
several times if, for example, an instrumented function is called
several times from one command.


@node Jumping
@subsection Jumping

  The commands described in this section execute until they reach a
specified location.  All except @kbd{i} make a temporary breakpoint to
establish the place to stop, then switch to go mode.  Any other
breakpoint reached before the intended stop point will also stop
execution.  @xref{Breakpoints}, for the details on breakpoints.

  These commands may fail to work as expected in case of nonlocal exit,
because a nonlocal exit can bypass the temporary breakpoint where you
expected the program to stop.

@table @kbd
@item h
Proceed to the stop point near where point is (@code{edebug-goto-here}).

@item f
Run the program forward over one expression
(@code{edebug-forward-sexp}).

@item o
Run the program until the end of the containing sexp.

@item i
Step into the function or macro called by the form after point.
@end table

The @kbd{h} command proceeds to the stop point near the current location
if point, using a temporary breakpoint.  See @ref{Breakpoints}, for more
about breakpoints.

The @kbd{f} command runs the program forward over one expression.  More
precisely, it sets a temporary breakpoint at the position that
@kbd{C-M-f} would reach, then executes in go mode so that the program
will stop at breakpoints.

With a prefix argument @var{n}, the temporary breakpoint is placed
@var{n} sexps beyond point.  If the containing list ends before @var{n}
more elements, then the place to stop is after the containing
expression.

Be careful that the position @kbd{C-M-f} finds is a place that the
program will really get to; this may not be true in a
@code{cond}, for example.

The @kbd{f} command does @code{forward-sexp} starting at point, rather
than at the stop point, for flexibility.  If you want to execute one
expression @emph{from the current stop point}, type @kbd{w} first, to
move point there, and then type @kbd{f}.

The @kbd{o} command continues ``out of'' an expression.  It places a
temporary breakpoint at the end of the sexp containing point.  If the
containing sexp is a function definition itself, it continues until just
before the last sexp in the definition.  If that is where you are now,
it returns from the function and then stops.  In other words, this
command does not exit the currently executing function unless you are
positioned after the last sexp.

The @kbd{i} command steps into the function or macro called by the list
form after point.  Note that the form need not be the one about to be
evaluated.  But if the form is a function call about to be evaluated,
remember to use this command before any of the arguments are evaluated,
since otherwise it will be too late.

The @kbd{i} command instruments the function or macro it's supposed to
step into, if it isn't instrumented already.  This is convenient, but keep
in mind that the function or macro remains instrumented unless you explicitly
arrange to deinstrument it.

@node Edebug Misc
@subsection Miscellaneous Edebug Commands

  Some miscellaneous Edebug commands are described here.

@table @kbd
@item ?
Display the help message for Edebug (@code{edebug-help}).

@item C-]
Abort one level back to the previous command level
(@code{abort-recursive-edit}).

@item q
Return to the top level editor command loop (@code{top-level}).  This
exits all recursive editing levels, including all levels of Edebug
activity.  However, instrumented code protected with
@code{unwind-protect} or @code{condition-case} forms may resume
debugging.

@item Q
Like @kbd{q} but don't stop even for protected code
(@code{top-level-nonstop}).

@item r
Redisplay the most recently known expression result in the echo area
(@code{edebug-previous-result}).

@item d
Display a backtrace, excluding Edebug's own functions for clarity
(@code{edebug-backtrace}).

You cannot use debugger commands in the backtrace buffer in Edebug as
you would in the standard debugger.

The backtrace buffer is killed automatically when you continue
execution.
@end table

>From the Edebug recursive edit, you may invoke commands that activate
Edebug again recursively.  Any time Edebug is active, you can quit to
the top level with @kbd{q} or abort one recursive edit level with
@kbd{C-]}.  You can display a backtrace of all the 
pending evaluations with @kbd{d}.

@node Breakpoints
@subsection Breakpoints

@cindex breakpoints
Edebug's step mode stops execution at the next stop point reached.
There are three other ways to stop Edebug execution once it has started:
breakpoints, the global break condition, and source breakpoints.

While using Edebug, you can specify @dfn{breakpoints} in the program you
are testing: points where execution should stop.  You can set a
breakpoint at any stop point, as defined in @ref{Using Edebug}.  For
setting and unsetting breakpoints, the stop point that is affected is
the first one at or after point in the source code buffer.  Here are the
Edebug commands for breakpoints:

@table @kbd
@item b
Set a breakpoint at the stop point at or after point
(@code{edebug-set-breakpoint}).  If you use a prefix argument, the
breakpoint is temporary (it turns off the first time it stops the
program).

@item u
Unset the breakpoint (if any) at the stop point at or after 
point (@code{edebug-unset-breakpoint}).

@item x @var{condition} @key{RET}
Set a conditional breakpoint which stops the program only if
@var{condition} evaluates to a non-@code{nil} value
(@code{edebug-set-conditional-breakpoint}).  With a prefix argument, the
breakpoint is temporary.

@item B
Move point to the next breakpoint in the definition
(@code{edebug-next-breakpoint}).
@end table

While in Edebug, you can set a breakpoint with @kbd{b} and unset one
with @kbd{u}.  First move point to the Edebug stop point of your choice,
then type @kbd{b} or @kbd{u} to set or unset a breakpoint there.
Unsetting a breakpoint where none has been set has no effect.

Reevaluating or reinstrumenting a definition forgets all its breakpoints.

A @dfn{conditional breakpoint} tests a condition each time the program
gets there.  Any errors that occur as a result of evaluating the
condition are ignored, as if the result were @code{nil}.  To set a
conditional breakpoint, use @kbd{x}, and specify the condition
expression in the minibuffer.  Setting a conditional breakpoint at a
stop point that has a previously established conditional breakpoint puts
the previous condition expression in the minibuffer so you can edit it.

You can make a conditional or unconditional breakpoint
@dfn{temporary} by using a prefix arg with the command to set the
breakpoint.  When a temporary breakpoint stops the program, it is
automatically unset.

Edebug always stops or pauses at a breakpoint except when the Edebug
mode is Go-nonstop.  In that mode, it ignores breakpoints entirely.

To find out where your breakpoints are, use the @kbd{B} command, which
moves point to the next breakpoint in the definition following point, or
to the first breakpoint if there are no following breakpoints.  This
command does not continue execution---it just moves point in the buffer.

@menu
* Global Break Condition::      Breaking on an event. 
* Source Breakpoints::          Embedding breakpoints in source code.
@end menu


@node Global Break Condition
@subsubsection Global Break Condition

@cindex stopping on events
@cindex global break condition
  A @dfn{global break condition} stops execution when a specified
condition is satisfied, no matter where that may occur.  Edebug
evaluates the global break condition at every stop point.  If it
evaluates to a non-@code{nil} value, then execution stops or pauses
depending on the execution mode, as if a breakpoint had been hit.  If
evaluating the condition gets an error, execution does not stop.

@findex edebug-set-global-break-condition
@vindex edebug-global-break-condition
  You can set or edit the condition expression, stored in
@code{edebug-global-break-condition}, using the @kbd{X} command
(@code{edebug-set-global-break-condition}).

  The global break condition is the simplest way to find where in your
code some event occurs, but it makes code run much more slowly.  So you
should reset the condition to @code{nil} when not using it.

@node Source Breakpoints
@subsubsection Source Breakpoints

@findex edebug
@cindex source breakpoints
  All breakpoints in a definition are forgotten each time you
reinstrument it.  To make a breakpoint that won't be forgotten, you can
write a @dfn{source breakpoint}, which is simply a call to the function
@code{edebug} in your source code.  You can, of course, make such a call
conditional.  For example, in the @code{fac} function, insert the first
line as shown below to stop when the argument reaches zero:

@example
(defun fac (n)
  (if (= n 0) (edebug))
  (if (< 0 n)
      (* n (fac (1- n)))
    1))
@end example

When the @code{fac} definition is instrumented and the function is
called, the call to @code{edebug} acts as a breakpoint.  Depending on
the execution mode, Edebug stops or pauses there.

If no instrumented code is being executed when @code{edebug} is called,
that function calls @code{debug}.
@c This may not be a good idea anymore.

@node Trapping Errors
@subsection Trapping Errors

Emacs normally displays an error message when an error is signaled and
not handled with @code{condition-case}.  While Edebug is active, it
normally responds to all unhandled errors.  You can customize this with
the options @code{edebug-on-error} and @code{edebug-on-quit}; see
@ref{Edebug Options}.

When Edebug responds to an error, it shows the last stop point
encountered before the error.  This may be the location of a call to a
function which was not instrumented, within which the error actually
occurred.  For an unbound variable error, the last known stop point
might be quite distant from the offending variable reference.  In that
case you might want to display a full backtrace (@pxref{Edebug Misc}).

If you change @code{debug-on-error} or @code{debug-on-quit} while
Edebug is active, these changes will be forgotten when Edebug becomes
inactive.  Furthermore, during Edebug's recursive edit, these variables
are bound to the values they had outside of Edebug.

@ignore @c I don't want to document something that works only partly -- rms.
Edebug can also trap signals even if they are handled.  If
@code{debug-on-error} is a list of signal names, Edebug will stop when
any of these errors are signaled.  Edebug shows you the last known stop
point just as for unhandled errors.  After you continue execution, the
error is signaled again (but without being caught by Edebug).  Edebug
can only trap errors that are handled if they are signaled in Lisp code
(not subroutines) since it does so by temporarily replacing the
@code{signal} function.
@end ignore

@node Edebug Views
@subsection Edebug Views

These Edebug commands let you view aspects of the buffer and window
status that obtained before entry to Edebug.

@table @kbd
@item v
View the outside window configuration (@code{edebug-view-outside}).

@item p
Temporarily display the outside current buffer with point at its outside
position (@code{edebug-bounce-point}).  With a prefix argument @var{n},
pause for @var{n} seconds instead.

@item w
Move point back to the current stop point (@code{edebug-where}) in the
source code buffer.  Also, if you use this command in a different window
displaying the same buffer, that window will be used instead to display
the current definition in the future.

@item W
Forget the saved outside window configuration---so that the current
window configuration will remain unchanged when you next exit Edebug (by
continuing the program).  Also toggle the @code{edebug-save-windows}
variable.
@ignore  @c This text is implementation-oriented and doesn't emphasize
            what users really want to know.
Toggle the @code{edebug-save-windows} variable which indicates whether
the outside window configuration is saved and restored
(@code{edebug-toggle-save-windows}).  Also, each time it is toggled on,
make the outside window configuration the same as the current window
configuration.
@end ignore
@end table

You can view the outside window configuration with @kbd{v} or just
bounce to the point in the current buffer with @kbd{p}, even if
it is not normally displayed.  After moving point, you may wish to jump
back to the stop point with @kbd{w} from a source code buffer.

@ignore I don't understand this -- rms
If you type @kbd{W} twice, Edebug continues saving and restoring an
outside window configuration, but updates it to match the current
configuration.  You can use this to add another buffer to be displayed
whenever Edebug is active.  However, the automatic redisplay of
@samp{*edebug*} and @samp{*edebug-trace*} may conflict with the buffers
you wish to see unless you have enough windows open.

With a prefix argument, @code{W} only toggles saving and restoring of
the selected window.  To specify a window that is not displaying the
source code buffer, you must use @kbd{C-x X W} from the global keymap.
@end ignore

@node Edebug Eval
@subsection Evaluation

While within Edebug, you can evaluate expressions ``as if'' Edebug were
not running.  Edebug tries to be invisible to the expression's
evaluation and printing.  Evaluation of expressions that cause side
effects will work as expected except for things that Edebug explicitly
saves and restores.  @xref{The Outside Context}, for details on this
process.

@table @kbd
@item e @var{exp} @key{RET}
Evaluate expression @var{exp} in the context outside of Edebug
(@code{edebug-eval-expression}).  That is, Edebug tries to minimize its
interference with the evaluation.

@item M-@key{ESC} @var{exp} @key{RET}
Evaluate expression @var{exp} in the context of Edebug itself.

@item C-x C-e
Evaluate the expression before point, in the context outside of Edebug
(@code{edebug-eval-last-sexp}).
@end table

@cindex lexical binding (Edebug)
Edebug supports evaluation of expressions containing references to
lexically bound symbols created by the following constructs in
@file{cl.el} (version 2.03 or later): @code{lexical-let},
@code{macrolet}, and @code{symbol-macrolet}.


@node Eval List
@subsection Evaluation List Buffer

You can use the @dfn{evaluation list buffer}, called @samp{*edebug*}, to
evaluate expressions interactively.  You can also set up the
@dfn{evaluation list} of expressions to be evaluated automatically each
time Edebug updates the display.

@table @kbd
@item E
Switch to the evaluation list buffer @samp{*edebug*}
(@code{edebug-visit-eval-list}).
@end table

In the @samp{*edebug*} buffer you can use the commands of Lisp
Interaction mode (@pxref{Lisp Interaction,,, emacs, The GNU Emacs
Manual}) as well as these special commands:

@table @kbd
@item LFD
Evaluate the expression before point, in the outside context, and insert
the value in the buffer (@code{edebug-eval-print-last-sexp}).

@item C-x C-e
Evaluate the expression before point, in the context outside of Edebug
(@code{edebug-eval-last-sexp}).

@item C-c C-u
Build a new evaluation list from contents of the buffer
(@code{edebug-update-eval-list}).

@item C-c C-d
Delete the evaluation list group that point is in
(@code{edebug-delete-eval-item}).

@item C-c C-w
Switch back to the source code buffer at the current stop point
(@code{edebug-where}).
@end table

You can evaluate expressions in the evaluation list window with
@kbd{LFD} or @kbd{C-x C-e}, just as you would in @samp{*scratch*};
but they are evaluated in the context outside of Edebug.

The expressions you enter interactively (and their results) are lost
when you continue execution; but you can set up an @dfn{evaluation list}
consisting of expressions to be evaluated each time execution stops. 

@cindex evaluation list group
To do this, write one or more @dfn{evaluation list groups} in the
evaluation list buffer.  An evaluation list group consists of one or
more Lisp expressions.  Groups are separated by comment lines.

The command @kbd{C-c C-u} (@code{edebug-update-eval-list}) rebuilds the
evaluation list, scanning the buffer and using the first expression of
each group.

Be careful not to add expressions that execute instrumented code since
that would cause an infinite loop.
@c There ought to be a way to fix this.

Redisplaying the evaluation list works by inserting each expression in
the buffer, followed by its current value.  It also inserts comment
lines so that each expression becomes its own group.  Thus, if you type
@kbd{C-c C-u} again without changing the buffer text, the evaluation
list is effectively unchanged.

If an error occurs during an evaluation from the evaluation list, the
error message is displayed in a string as if it were the result.
Therefore, expressions that use variables not currently valid do not
interrupt your debugging.

Here is an example of what the evaluation list window looks like after
several expressions have been added to it:

@smallexample
(current-buffer)
#<buffer *scratch*>
;---------------------------------------------------------------
(selected-window)
#<window 16 on *scratch*>
;---------------------------------------------------------------
(point)
196
;---------------------------------------------------------------
bad-var
"Symbol's value as variable is void: bad-var"
;---------------------------------------------------------------
(recursion-depth)
0
;---------------------------------------------------------------
this-command
eval-last-sexp
;---------------------------------------------------------------
@end smallexample

To delete a group, move point into it and type @kbd{C-c C-d}, or simply
delete the text for the group and update the evaluation list with
@kbd{C-c C-u}.  To add a new expression to the evaluation list, insert
the expression at a suitable place, and insert a new comment line.  (You
need not insert dashes in the comment line---its contents don't matter.)
Then type @kbd{C-c C-u}.

After selecting @samp{*edebug*}, you can return to the source code
buffer with @kbd{C-c C-w}.  The @samp{*edebug*} buffer is killed when
you continue execution, and recreated next time it is needed.


@node Printing in Edebug
@subsection Printing in Edebug

@cindex printing (Edebug)
@cindex printing circular structures
@pindex cust-print
  If an expression in your program produces a value containing circular
list structure, you may get an error when Edebug attempts to print it.

@vindex edebug-print-length
@vindex edebug-print-level
  One way to cope with circular structure is to set @code{print-length}
or @code{print-level} to truncate the printing.  Edebug does this for
you; it binds @code{print-length} and @code{print-level} to 50 if they
were @code{nil}.  (Actually, the variables @code{edebug-print-length}
and @code{edebug-print-level} specify the values to use within Edebug.)
@xref{Output Variables}.

  You can also print circular structures and structures that share
elements more informatively by using the @file{cust-print} package.

  To load @file{cust-print} and activate custom printing only for
Edebug, simply use the command @kbd{M-x edebug-install-custom-print}.
To restore the standard print functions, use @kbd{M-x
edebug-uninstall-custom-print}.

  Here is an example of code that creates a circular structure:

@example
(setq a '(x y))
(setcar a a))
@end example

@noindent
Custom printing prints this as @samp{Result: #1=(#1# y)}.  The
@samp{#1=} notation labels the structure that follows it with the label
@samp{1}, and the @samp{#1#} notation references the previously labelled
structure.  This notation is used for any shared elements of lists or
vectors.

  Other programs can also use custom printing; see @file{cust-print.el}
for details.

@node Trace Buffer
@subsection Trace Buffer
@cindex trace buffer

  Edebug can record an execution trace in a buffer named
@samp{*edebug-trace*}.  This is a log of function calls and returns,
showing the function names and their arguments and values.  To enable
trace recording, set @code{edebug-trace} to a non-@code{nil} value.

  Making a trace buffer is not the same thing as using trace execution
mode (@pxref{Edebug Execution Modes}).

  When trace recording is enabled, each function entry and exit adds
lines to the trace buffer.  A function entry record looks like
@samp{::::@{} followed by the function name and argument values.  A
function exit record looks like @samp{::::@}} followed by the function
name and result of the function.

  The number of @samp{:}s in an entry shows its recursion depth.  You
can use the braces in the trace buffer to find the matching beginning or
end of function calls.

@findex edebug-print-trace-before
@findex edebug-print-trace-after
  You can customize trace recording for function entry and exit by
redefining the functions @code{edebug-print-trace-before} and
@code{edebug-print-trace-after}.

@defmac edebug-tracing string body@dots{}
This macro requests additional trace information around the execution
of the @var{body} forms.  The argument @var{string} specifies text
to put in the trace buffer.  All the arguments are evaluated.
@code{edebug-tracing} returns the value of the last form in @var{body}.
@end defmac

@defun edebug-trace format-string &rest format-args
This function inserts text in the trace buffer.  It computes the text
with @code{(apply 'format @var{format-string} @var{format-args})}.
It also inserts a newline to separate entries.
@end defun

  @code{edebug-tracing} and @code{edebug-trace} insert lines in the trace
buffer even if Edebug is not active.

  Adding text to the trace buffer also scrolls its window to show the
last lines inserted.

@ignore  @c too vague
There may be some display problems if you use
tracing along with the evaluation list.
@end ignore

@node Coverage Testing
@subsection Coverage Testing

@cindex coverage testing
@cindex frequency counts
@cindex performance analysis
Edebug provides rudimentary coverage testing and display of execution
frequency.  All execution of an instrumented function accumulates
frequency counts, both before and after evaluation of each instrumented
expression, even if the execution mode is Go-nonstop.  Coverage testing
is more expensive, so it is only done if @code{edebug-test-coverage} is
non-@code{nil}.  The command @kbd{M-x edebug-display-freq-count}
displays both the frequency data and the coverage data (if recorded).

@deffn Command edebug-display-freq-count
This command displays the frequency count data for each line of the
current definition.

The frequency counts appear comment lines after each line of code, and
you can undo all insertions with one @code{undo} command.  The counts
are appear under the @kbd{(} before an expression or the @kbd{)} after
an expression, or on the last character of a symbol.  Values do not appear if
they are equal to the previous count on the same line.

The character @samp{=} following the count for an expression says that
the expression has returned the same value each time it was evaluated
This is the only coverage information that Edebug records.

To clear the frequency count and coverage data for a definition,
reinstrument it.
@end deffn

For example, after evaluating @code{(fac 5)} with a source
breakpoint, and setting @code{edebug-test-coverage} to @code{t}, when
the breakpoint is reached, the frequency data looks like this:

@example
(defun fac (n)
  (if (= n 0) (edebug))
;#6           1      0 =5 
  (if (< 0 n)
;#5         = 
      (* n (fac (1- n)))
;#    5               0  
    1))
;#   0 
@end example

The comment lines show that @code{fac} has been called 6 times.  The
first @code{if} statement has returned 5 times with the same result each
time; the same is true of the condition on the second @code{if}.
The recursive call of @code{fac} has not returned at all.


@node The Outside Context
@subsection The Outside Context

Edebug tries to be transparent to the program you are debugging, but it
does not succeed completely.  Edebug also tries to be transparent when
you evaluate expressions with @kbd{e} or with the evaluation list
buffer, by temporarily restoring the outside context.  This section
explains precisely what context Edebug restores, and how Edebug fails to
be completely transparent.

@c This can be fixed and should be
The same mechanism that avoids masking certain variable's outside values
also currently makes it impossible to set these variables within Edebug.

@menu
* Checking Whether to Stop::    When Edebug decides what to do.
* Edebug Display Update::       When Edebug updates the display.
* Edebug Recursive Edit::       When Edebug stops execution.
@end menu

@node Checking Whether to Stop
@subsubsection Checking Whether to Stop

Whenever Edebug is entered just to think about whether to take some
action, it needs to save and restore certain data.

@itemize @bullet
@item 
@code{max-lisp-eval-depth} and @code{max-specpdl-size} are both
incremented one time to reduce Edebug's impact on the stack.
You could, however, still run out of stack space when using Edebug.

@item 
The state of keyboard macro execution is saved and restored.  While
Edebug is active, @code{executing-macro} is bound to
@code{edebug-continue-kbd-macro}.

@end itemize


@node Edebug Display Update
@subsubsection Edebug Display Update

When Edebug needs to display something (e.g., in trace mode), it saves
the current window configuration from ``outside'' Edebug (@pxref{Window
Configurations,,, elisp, GNU Emacs Lisp Reference Manual}).  When
you exit Edebug (by continuing the program), it restores the previous
window configuration.

Emacs redisplays only when it pauses.  Usually, when you continue
execution, the program comes back into Edebug at a breakpoint or after
stepping without pausing or reading input in between.  In such cases,
Emacs never gets a chance to redisplay the ``outside'' configuration.
What you see is the same window configuration as the last time Edebug
was active, with no interruption.

Entry to Edebug for displaying something also saves and restores the
following data, but some of these are deliberately not restored if an
error or quit signal occurs.

@itemize @bullet
@item 
@cindex current buffer point and mark (Edebug)
Which buffer is current, and the positions of point and the mark in the
current buffer, are saved and restored.

@item 
@cindex window configuration (Edebug)
The outside window configuration is saved and restored if
@code{edebug-save-windows} is non-@code{nil} (@pxref{Edebug Display Update}).

The window configuration is not restored on error or quit, but the
outside selected window @emph{is} reselected even on error or quit in
case a @code{save-excursion} is active.  If the value of
@code{edebug-save-windows} is a list, only the listed windows are saved
and restored.

The window start and horizontal scrolling of the source code buffer are
not restored, however, so that the display remains coherent within Edebug.

@item
@vindex edebug-save-displayed-buffer-points
The value of point in each displayed buffer is saved and restored if
@code{edebug-save-displayed-buffer-points} is non-@code{nil}.

@item
The variables @code{overlay-arrow-position} and
@code{overlay-arrow-string} are saved and restored.  So you can safely
invoke Edebug from the recursive edit elsewhere in the same buffer.

@item 
@code{cursor-in-echo-area} is locally bound to @code{nil} so that
the cursor shows up in the window.
@end itemize

@node Edebug Recursive Edit
@subsubsection Edebug Recursive Edit

When Edebug is entered and actually reads commands from the user, it
saves (and later restores) these additional data:

@itemize @bullet
@item
The current match data.  @xref{Match Data}.

@item
@code{last-command}, @code{this-command}, @code{last-command-char},
@code{last-input-char}, @code{last-input-event},
@code{last-command-event}, @code{last-event-frame},
@code{last-nonmenu-event}, and @code{track-mouse}.  Commands used within
Edebug do not affect these variables outside of Edebug.

The key sequence returned by @code{this-command-keys} is changed by
executing commands within Edebug and there is no way to reset
the key sequence from Lisp.

@item
Complex commands executed while in Edebug are added to the variable
@code{command-history}.  In rare cases this can alter execution.

@item
Within Edebug, the recursion depth appears one deeper than the recursion
depth outside Edebug.  This is not true of the automatically updated
evaluation list window.

@item
@code{standard-output} and @code{standard-input} are bound to @code{nil}
by the @code{recursive-edit}, but Edebug temporarily restores them during
evaluations.

@item 
The state of keyboard macro definition is saved and restored.  While
Edebug is active, @code{defining-kbd-macro} is bound to
@code{edebug-continue-kbd-macro}.
@end itemize

@node Instrumenting Macro Calls
@subsection Instrumenting Macro Calls

When Edebug instruments an expression that calls a Lisp macro, it needs
additional advice to do the job properly.  This is because there is no
way to tell which subexpressions of the macro call are forms to be
evaluated.  (Evaluation may occur explicitly in the macro body, or when
the resulting expansion is evaluated, or any time later.)  You must
explain the format of calls to each macro to enable Edebug to handle it.
To do this, use @code{def-edebug-form-spec} to define the format of
calls to a given macro.

@deffn Macro def-edebug-spec macro specification
Specify which expressions of a call to macro @var{macro} are forms to be
evaluated.  For simple macros, the @var{specification} often looks very
similar to the formal argument list of the macro definition, but
specifications are much more general than macro arguments.

The @var{macro} argument may actually be any symbol, not just a macro
name.
@end deffn

Here is a simple example that defines the specification for the
@code{for} macro described in the Emacs Lisp Reference Manual, followed
by an alternative, equivalent specification.

@example
(def-edebug-spec for
  (symbolp "from" form "to" form "do" &rest form))

(def-edebug-spec for
  (symbolp ['from form] ['to form] ['do body]))
@end example

Here is a table of the possibilities for @var{specification} and how each
directs processing of arguments.

@table @bullet

@item @code{t}
All arguments are instrumented for evaluation.

@item @code{0}
None of the arguments is instrumented.

@item a symbol
The symbol must have an Edebug specification which is used instead.
This indirection is repeated until another kind of specification is
found.  This allows you to inherit the specification for another macro.

@item a list
The elements of the list describe the types of the arguments of a
calling form.  The possible elements of a specification list are
described in the following sections.
@end table

@menu
* Specification List::          How to specify complex patterns of evaluation.
* Backtracking::                What Edebug does when matching fails.
@c * Debugging Backquote::      Debugging Backquote
* Specification Examples::      To help understand specifications.
@end menu


@node Specification List
@subsubsection Specification List

@cindex Edebug specification list
A @dfn{specification list} is required for an Edebug specification if
some arguments of a macro call are evaluated while others are not.  Some
elements in a specification list match one or more arguments, but others
modify the processing of all following elements.  The latter, called
@dfn{specification keywords}, are symbols beginning with @samp{&} (such
as @code{&optional}).

A specification list may contain sublists which match arguments that are
themselves lists, or it may contain vectors used for grouping.  Sublists
and groups thus subdivide the specification list into a hierarchy of
levels.  Specification keywords only apply to the remainder of the
sublist or group they are contained in.

When a specification list involves alternatives or repetition, matching
it against an actual macro call may require backtracking.
@xref{Backtracking}, for more details.

Edebug specifications provide the power of regular expression matching,
plus some context-free grammar constructs: the matching of sublists with
balanced parentheses, recursive processing of forms, and recursion via
indirect specifications.

Here's a table of the possible elements of a specification list, with
their meanings:

@table @code
@item sexp
A single unevaluated Lisp object object.

@item form
A single evaluated expression, which is instrumented.

@item place
@findex edebug-unwrap
A place to store a value, as in the Common Lisp @code{setf} construct.

@item body
Short for @code{&rest form}.  See @code{&rest} below.

@item function-form
A function form: either a quoted function symbol, a quoted lambda
expression, or a form (that should evaluate to a function symbol or
lambda expression).  This is useful when an argument that's a lambda
expression might be quoted with @code{quote} rather than
@code{function}, since it instruments the body of the lambda expression
either way.

@item lambda-expr
A lambda expression with no quoting.

@item &optional
@kindex &optional @r{(Edebug)}
All following elements in the specification list are optional; as soon
as one does not match, Edebug stops matching at this level.  

To make just a few elements optional followed by non-optional elements,
use @code{[&optional @var{specs}@dots{}]}.  To specify that several
elements must all match or none, use @code{&optional
[@var{specs}@dots{}]}.  See the @code{defun} example below.

@item &rest
@kindex &rest @r{(Edebug)}
All following elements in the specification list are repeated zero or
more times.  All the elements need not match in the last repetition,
however.

To repeat only a few elements, use @code{[&rest @var{specs}@dots{}]}.
To specify several elements that must all match on every repetition, use
@code{&rest [@var{specs}@dots{}]}.

@item &or
@kindex &or @r{(Edebug)}
Each of the following elements in the specification list is an
alternative.  One of the alternatives must match, or the @code{&or}
specification fails.

Each list element following @code{&or} is a single alternative.  To
group two or more list elements as a single alternative, enclose them in
@code{[@dots{}]}.

@item &not
@kindex &not @r{(Edebug)}
Each of the following elements is matched as alternatives as if by using
@code{&or}, but if any of them match, the specification fails.  If none
of them match, nothing is matched, but the @code{&not} specification
succeeds.

@item &define 
@kindex &define @r{(Edebug)}
Indicates that the specification is for a defining form.  The defining
form itself is not instrumented (i.e. Edebug does not stop before and
after the defining form), but forms inside it typically will be
instrumented.  The @code{&define} keyword should be the first element in
a list specification.

@item nil
This is successful when there are no more arguments to match at the
current argument list level; otherwise it fails.  See sublist
specifications and the backquote example below.

@item gate
@cindex preventing backtracking
No argument is matched but backtracking through the gate is disabled
while matching the remainder of the specifications at this level.  This
is primarily used to generate more specific syntax error messages.  See
@ref{Backtracking}, for more details.  Also see the @code{let} example
below.

@item @var{other-symbol}
@cindex indirect specifications
Any other symbol in a specification list may be a predicate or an
indirect specification.

If the symbol has an Edebug specification, this @dfn{indirect
specification} should be either a list specification that is used in
place of the symbol, or a function that is called to process the
arguments.  The specification may be defined with @code{def-edebug-spec}
just as for macros. See the @code{defun} example below.

Otherwise, the symbol should be a predicate.  The predicate is called
with the argument and the specification fails if the predicate returns
@code{nil}.  In either case, that argument is not instrumented.

@findex keywordp
@findex lambda-list-keywordp
Some suitable predicates include @code{symbolp}, @code{integerp},
@code{stringp}, @code{vectorp}, and @code{atom}.
@ignore
, @code{keywordp}, and
@code{lambda-list-keywordp}.  The last two, defined in @file{edebug.el},
test whether the argument is a symbol starting with @samp{@code{:}} and
@samp{@code{&}} respectively.
@end ignore

@item [@var{elements}@dots{}]
@cindex [@dots{}] (Edebug)
A vector of elements groups the elements into a single @dfn{group
specification}.  Its meaning has nothing to do with vectors.

@item "@var{string}"
The argument should be a symbol named @var{string}.  This specification
is equivalent to the quoted symbol, @code{'@var{symbol}}, where the name
of @var{symbol} is the @var{string}, but the string form is preferred.

@ignore
@item '@var{symbol} @r{or} (quote @var{symbol})
The argument should be the symbol @var{symbol}.  But use a string
specification instead.
@end ignore

@item (vector @var{elements}@dots{})
The argument should be a vector whose elements must match the
@var{elements} in the specification.  See the backquote example below.

@item (@var{elements}@dots{})
Any other list is a @dfn{sublist specification} and the argument must be
a list whose elements match the specification @var{elements}.

@cindex dotted lists (Edebug)
A sublist specification may be a dotted list and the corresponding list
argument may then be a dotted list.  Alternatively, the last @sc{cdr} of a
dotted list specification may be another sublist specification (via a
grouping or an indirect specification, e.g. @code{(spec .  [(more
specs@dots{})])}) whose elements match the non-dotted list arguments.
This is useful in recursive specifications such as in the backquote
example below.  Also see the description of a @code{nil} specification
above for terminating such recursion.

Note that a sublist specification of the form @code{(specs .  nil)}
means the same as @code{(specs)}, and @code{(specs .
(sublist-elements@dots{}))} means the same as @code{(specs
sublist-elements@dots{})}.
@end table

@c Need to document extensions with &symbol and :symbol

Here is a list of additional specifications that may only appear after
@code{&define}.  See the @code{defun} example below.

@table @code
@item name
The argument, a symbol, is the name of the defining form. 

A defining form is not required to have a name field; and it may have
multiple name fields.

@item :name
This construct does not actually match an argument.  The element
following @code{:name} should be a symbol; it is used as an additional
name component for the definition.  You can use this to add a unique,
static component to the name of the definition.  It may be used more
than once.

@item arg
The argument, a symbol, is the name of an argument of the defining form.
However, lambda list keywords (symbols starting with @samp{@code{&}})
are not allowed.  See @code{lambda-list} and the example below.

@item lambda-list
@cindex lambda-list (Edebug)
This matches a lambda list---the argument list of a lambda expression.
The argument should be a list of symbols.

@item def-body
The argument is the body of code in a definition.  This is like
@code{body}, described above, but a definition body must be instrumented
with a different Edebug call that looks up information associated with
the definition.  Use @code{def-body} for the highest level list of forms
within the definition.

@item def-form
The argument is a single, highest-level form in a definition.  This is
like @code{def-body}, except use this to match a single form rather than
a list of forms.  As a special case, @code{def-form} also means that
tracing information is not output when the form is executed.  See the
@code{interactive} example below.
@end table

@node Backtracking
@subsubsection Backtracking

@cindex backtracking
@cindex syntax error (Edebug)
If a specification fails to match at some point, this does not
necessarily mean a syntax error will be signaled; instead,
@dfn{backtracking} will take place until all alternatives have been
exhausted.  Eventually every element of the argument list must be
matched by some element in the specification, and every required element
in the specification must match some argument.

Backtracking is disabled for the remainder of a sublist or group when
certain conditions occur, described below.  Backtracking is reenabled
when a new alternative is established by @code{&optional}, @code{&rest},
or @code{&or}.  It is also reenabled initially when processing a
sublist or group specification or an indirect specification.

You might want to disable backtracking to commit to some alternative so
that Edebug can provide a more specific syntax error message.  Normally,
if no alternative matches, Edebug reports that none matched, but if one
alternative is committed to, Edebug can report how it failed to match.

First, backtracking is disabled while matching any of the form
specifications (i.e. @code{form}, @code{body}, @code{def-form}, and
@code{def-body}).  These specifications will match any form so any error
must be in the form itself rather than at a higher level.

Second, backtracking is disabled after successfully matching a quoted
symbol or string specification, since this usually indicates a
recognized construct.  If you have a set of alternative constructs that
all begin with the same symbol, you can usually work around this
constraint by factoring the symbol out of the alternatives, e.g.,
@code{["foo" &or [first case] [second case] ...]}.

Third, backtracking may be explicitly disabled by using the
@code{gate} specification.  This is useful when you know that
no higher alternatives may apply.

@ignore
@node Debugging Backquote
@subsubsection Debugging Backquote

@findex ` (Edebug)
@cindex backquote (Edebug)
Backquote (@kbd{`}) is a macro that results in an expression that may or
may not be evaluated.  It is often used to simplify the definition of a
macro to return an expression to be evaluated, but Edebug cannot know
whether the resyult of backquote will be used in any other way.

The forms inside unquotes (@code{,} and @code{,@@}) are evaluated, and
Edebug instruments them.

Edebug supports nested backquotes, but there is a limit on the support
of quotes inside of backquotes.  Forms quoted with @code{'} are not
normally evaluated, but if the quoted form appears immediately within
@code{,} and @code{,@@} forms, Edebug treats this as a backquoted form
at the next higher level (even if there is not a next higher level; this
is difficult to fix).

@findex edebug-`
If the backquoted forms are code to be evaluated, you can have Edebug
instrument them by using @code{edebug-`} instead of the regular
@code{`}.  Unquoting forms can be used inside @code{edebug-`} anywhere a
form is normally allowed.  But @code{(, @var{form})} may be used in two
other places specially recognized by Edebug: wherever a predicate
specification would match, and at the head of a list form where the
function name normally appears.  The @var{form} inside a spliced
unquote, @code{(,@@ @var{form})}, will be instrumented, but the unquote
form itself will not be instrumented since this would interfere with the
splicing.

There is one other complication with using @code{edebug-`}.  If the
@code{edebug-`} call is in a macro and the macro may be called from code
that is also instrumented, and if unquoted forms contain any macro
arguments bound to instrumented forms, then you should modify the
specification for the macro as follows: the specifications for those
arguments must use @code{def-form} instead of @code{form}.  (This is to
reestablish the Edebugging context for those external forms.)

For example, the @code{for} macro (@pxref{Problems with Macros,,, elisp,
Emacs Lisp Reference Manual}) is shown here but with @code{edebug-`}
substituted for regular @code{`}.

@example
(defmacro inc (var)
  (list 'setq var (list '1+ var)))

(defmacro for (var from init to final do &rest body)
  (let ((tempvar (make-symbol "max")))
    (edebug-` (let (((, var) (, init))
                    ((, tempvar) (, final)))
                (while (<= (, var) (, tempvar))
                  (,@ body)
                  (inc (, var)))))))
@end example

Here is the corresponding modified Edebug specification and a
call of the macro:

@example
(def-edebug-spec for
  (symbolp "from" def-form "to" def-form "do" &rest def-form))

(let ((n 5))
  (for i from n to (* n (+ n 1)) do
    (message "%s" i)))
@end example

After instrumenting the @code{for} macro and the macro call, Edebug
first steps to the beginning of the macro call, then into the macro
body, then through each of the unquoted expressions in the backquote
showing the expressions that will be embedded.  Then when the macro
expansion is evaluated, Edebug will step through the @code{let} form and
each time it gets to an unquoted form, it will jump back to an argument
of the macro call to step through that expression.  Finally stepping
will continue after the macro call.  Even more convoluted execution
paths may result when using anonymous functions.

@vindex edebug-unwrap-results
When the result of an expression is an instrumented expression, it is
difficult to see the expression inside the instrumentation.  So
you may want to set the option @code{edebug-unwrap-results} to a
non-@code{nil} value while debugging such expressions, but it would slow
Edebug down to always do this.

@end ignore
@node Specification Examples
@subsubsection Specification Examples

It may be easier to understand Edebug specifications by studying
the examples provided here.

A @code{let} special form has a sequence of bindings and a body.  Each
of the bindings is either a symbol or a sublist with a symbol and
optional value.  In the specification below, notice the @code{gate}
inside of the sublist to prevent backtracking once a sublist is found.

@example
(def-edebug-spec let
  ((&rest
    &or symbolp (gate symbolp &optional form))
   body))
@end example

Edebug uses the following specifications for @code{defun} and
@code{defmacro} and the associated argument list and @code{interactive}
specifications.  It is necessary to handle interactive forms specially
since an expression argument it is actually evaluated outside of the
function body.

@example
(def-edebug-spec defmacro defun)      ; @r{Indirect ref to @code{defun} spec}
(def-edebug-spec defun 
  (&define name lambda-list 
           [&optional stringp]        ; @r{Match the doc string, if present.}
           [&optional ("interactive" interactive)]
           def-body))

(def-edebug-spec lambda-list
  (([&rest arg]
    [&optional ["&optional" arg &rest arg]]
    &optional ["&rest" arg]
    )))

(def-edebug-spec interactive
  (&optional &or stringp def-form))    ; @r{Notice: @code{def-form}}
@end example

The specification for backquote below illustrates how to match
dotted lists and use @code{nil} to terminate recursion.  It also
illustrates how components of a vector may be matched.  (The actual
specification defined by Edebug does not support dotted lists because
doing so causes very deep recursion that could fail.)

@example
(def-edebug-spec ` (backquote-form))  ;; alias just for clarity

(def-edebug-spec backquote-form
  (&or ([&or "," ",@@"] &or ("quote" backquote-form) form)
       (backquote-form . [&or nil backquote-form])
       (vector &rest backquote-form)
       sexp))
@end example


@node Edebug Options
@subsection Edebug Options

  These options affect the behavior of Edebug:

@defopt edebug-setup-hook
Functions to call before Edebug is used.  Each time it is set to a new
value, Edebug will call those functions once and then
@code{edebug-setup-hook} is reset to @code{nil}.  You could use this to
load up Edebug specifications associated with a package you are using
but only when you also use Edebug.
@xref{Instrumenting}.
@end defopt

@defopt edebug-all-defs
If this is non-@code{nil}, normal evaluation of defining forms such as
@code{defun} and @code{defmacro} instruments them for Edebug.  This
applies to @code{eval-defun}, @code{eval-region}, and
@code{eval-current-buffer}.  @xref{Instrumenting}.
@end defopt

@defopt edebug-all-forms
If this is non-@code{nil}, the commands @code{eval-defun}, @code{eval-region},
and @code{eval-current-buffer} instrument all forms, even those that
don't define anything.

Use the command @kbd{M-x edebug-all-forms} to toggle the value of this
option.
@xref{Instrumenting}.
@end defopt

@defopt edebug-save-windows
If this is non-@code{nil}, Edebug saves and restores the window
configuration.  That takes some time, so if your program does not care
what happens to the window configurations, it is better to set this
variable to @code{nil}.

If the value is a list, only the listed windows are saved and
restored.  

You can use the @kbd{W} command in Edebug to change this variable
interactively.  @xref{Edebug Display Update}.
@end defopt

@defopt edebug-save-displayed-buffer-points
If non-@code{nil}, Edebug saves and restores point in all buffers.

Saving and restoring point in other buffers is necessary if you are
debugging code that changes the point of a buffer which is displayed in
a non-selected window.  If Edebug or the user then selects the window,
the buffer's point will change to the window's point.

Saving and restoring point in all buffers is expensive, since it
requires selecting each window twice, so enable this only if you need
it.  @xref{Edebug Display Update}.
@end defopt

@defopt edebug-initial-mode
If this variable is non-@code{nil}, it specifies the initial execution
mode for Edebug when it is first activated.  Possible values are
@code{step}, @code{next}, @code{go}, @code{Go-nonstop}, @code{trace},
@code{Trace-fast}, @code{continue}, and @code{Continue-fast}.

The default value is @code{step}.  
@xref{Edebug Execution Modes}.
@end defopt

@defopt edebug-trace
@findex edebug-print-trace-before
@findex edebug-print-trace-after
Non-@code{nil} means display a trace of function entry and exit.
Tracing output is displayed in a buffer named @samp{*edebug-trace*}, one
function entry or exit per line, indented by the recursion level.  

The default value is @code{nil}.  

Also see @code{edebug-tracing}.
@xref{Tracing}.
@end defopt

@defopt edebug-test-coverage 
If non-@code{nil}, Edebug tests coverage of all expressions debugged.
This is done by comparing the result of each expression
with the previous result. Coverage is considered OK if two different
results are found.  So to sufficiently test the coverage of your code,
try to execute it under conditions that evaluate all expressions more
than once, and produce different results for each expression.

Use @kbd{M-x edebug-display-freq-count} to display the frequency count
and coverage information for a definition.
@xref{Coverage Testing}.
@end defopt

@defopt edebug-continue-kbd-macro 
If non-@code{nil}, continue defining or executing any keyboard macro
that is executing outside of Edebug.   Use this with caution since it is not
debugged.
@xref{Edebug Execution Modes}.
@end defopt

@defopt edebug-print-length
If non-@code{nil}, bind @code{print-length} to this while printing
results in Edebug.  The default value is @code{50}.
@xref{Printing in Edebug}.
@end defopt

@defopt edebug-print-level 
If non-@code{nil}, bind @code{print-level} to this while printing
results in Edebug.  The default value is @code{50}.
@end defopt

@defopt edebug-print-circle 
If non-@code{nil}, bind @code{print-circle} to this while printing
results in Edebug.  The default value is @code{nil}.
@end defopt

@defopt edebug-on-error
Edebug binds @code{debug-on-error} to this value, if
@code{debug-on-error} was previously @code{nil}.  @xref{Trapping
Errors}.
@end defopt

@defopt edebug-on-quit
Edebug binds @code{debug-on-quit} to this value, if
@code{debug-on-quit} was previously @code{nil}.  @xref{Trapping
Errors}.
@end defopt

  If you change the values of @code{edebug-on-error} or
@code{edebug-on-quit} while Edebug is active, their values won't be used
until the @emph{next} time Edebug is invoked at a deeper command level.

@ignore
@defopt edebug-unwrap-results
Non-@code{nil} if Edebug should unwrap results of expressions.  This is
useful when debugging macros where the results of expressions are
instrumented expressions.  But don't do this when results might be
circular, or an infinite loop will result.  @xref{Debugging Backquote}.
@end defopt
@end ignore

@defopt edebug-global-break-condition
If non-@code{nil}, an expression to test for at every stop point.
If the result is non-nil, then break.  Errors are ignored.
@xref{Global Break Condition}.
@end defopt