emacs/doc/lispref/edebug.texi

@comment -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1992-1994, 1998-1999, 2001-2019 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.

@c This file can also be used by an independent Edebug User
@c Manual 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
@section Edebug
@cindex Edebug debugging facility

  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 re-evaluate a list of expressions and
display their results each time Edebug updates the display.

@item
Output trace information on function calls and returns.

@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
start using 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.
* Breaks::                      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.
* Edebug and Macros::           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.  Depending on which Edebug execution mode you have selected,
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.  The default execution mode is step,
which stops 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}, and it is temporarily read-only.

  An arrow in the left fringe 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 would 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 use periods to show the stop points 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
@cindex 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, to invoke Edebug at the proper places.

@kindex C-M-x
@findex eval-defun @r{(Edebug)}
  When you invoke command @kbd{C-M-x} (@code{eval-defun}) with a
prefix argument on a function definition, it instruments the
definition before evaluating it.  (This does not modify the source
code itself.)  If the variable @code{edebug-all-defs} is
non-@code{nil}, that inverts the meaning of the prefix argument: in
this case, @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 eval-region @r{(Edebug)}
@findex eval-buffer @r{(Edebug)}
@findex eval-current-buffer @r{(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
@findex edebug-defun
  Another command, @kbd{M-x edebug-eval-top-level-form}, is available to
instrument any top-level form regardless of the values of
@code{edebug-all-defs} and @code{edebug-all-forms}.
@code{edebug-defun} is an alias for @code{edebug-eval-top-level-form}.

  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 it is not already
instrumented.  This is possible only if Edebug knows where to find the
source for that function; for this reason, 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.

  Edebug knows how to instrument all the standard special forms,
@code{interactive} forms with an expression argument, anonymous lambda
expressions, and other defining forms.  However, Edebug cannot determine
on its own what a user-defined macro will do with the arguments of a
macro call, so you must provide that information using Edebug
specifications; for details, @pxref{Edebug and Macros}.

  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 Edebug specifications
associated with a package you are using, but only when you use Edebug.

@cindex edebug, failure to instrument
  If Edebug detects a syntax error while instrumenting, it leaves point
at the erroneous code and signals an @code{invalid-read-syntax} error.
@c FIXME?  I can't see that it "leaves point at the erroneous code".
Example:

@example
@error{} Invalid read syntax: "Expected lambda expression"
@end example

  One potential reason for such a failure to instrument is that some
macro definitions are not yet known to Emacs.  To work around this,
load the file which defines the function you are about to instrument.

@findex eval-expression @r{(Edebug)}
  To remove instrumentation from a definition, simply re-evaluate its
definition in a way that does not instrument.  There are two ways of
evaluating forms that never instrument them: from a file with
@code{load}, and from the minibuffer with @code{eval-expression}
(@kbd{M-:}).

@findex edebug-remove-instrumentation
  If you want to remove Edebug instrumentation from all functions, you
can use the @code{edebug-remove-instrumentation} command.

  @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 execution 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, but wait for more
Edebug commands (@code{edebug-stop}).
@c FIXME Does not work. https://debbugs.gnu.org/9764

@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{Jumping}.

@item t
Trace: pause (normally 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 than the modes later in the list.

When you enter a new Edebug level, Edebug will normally stop at the
first instrumented function it encounters.  If you prefer to stop only
at a break point, or not at all (for example, when gathering coverage
data), change the value of @code{edebug-initial-mode} from its default
@code{step} to @code{go}, or @code{Go-nonstop}, or one of its other
values (@pxref{Edebug Options}).  You can do this readily with
@kbd{C-x C-a C-m} (@code{edebug-set-initial-mode}):

@deffn Command edebug-set-initial-mode
@kindex C-x C-a C-m
This command, bound to @kbd{C-x C-a C-m}, sets
@code{edebug-initial-mode}.  It prompts you for a key to indicate the
mode.  You should enter one of the eight keys listed above, which sets
the corresponding mode.
@end deffn

Note that you may reenter the same Edebug level several times if, for
example, an instrumented function is called several times from one
command.

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 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.  See also the
@code{edebug-continue-kbd-macro} option in @ref{Edebug Options}.

@defopt edebug-sit-for-seconds
This option specifies how many seconds to wait between execution steps
in trace mode or continue mode.  The default is 1 second.
@end defopt

@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,
as that 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 for one expression
(@code{edebug-forward-sexp}).

@item o
Run the program until the end of the containing sexp (@code{edebug-step-out}).

@item i
Step into the function or macro called by the form after point
(@code{edebug-step-in}).
@end table

The @kbd{h} command proceeds to the stop point at or after the current
location of point, using a temporary breakpoint.

The @kbd{f} command runs the program forward over one expression.  More
precisely, it sets a temporary breakpoint at the position that
@code{forward-sexp} 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.

You must check that the position @code{forward-sexp} finds is a place
that the program will really get to.  In @code{cond}, for example,
this may not be true.

For flexibility, the @kbd{f} command does @code{forward-sexp} starting
at point, rather than at the stop point.  If you want to execute one
expression @emph{from the current stop point}, first type @kbd{w}
(@code{edebug-where}) 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, @kbd{o} 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.

Normally, the @kbd{h}, @kbd{f}, and @kbd{o} commands display ``Break''
and pause for @code{edebug-sit-for-seconds} before showing the result
of the form just evaluated.  You can avoid this pause by setting
@code{edebug-sit-on-break} to @code{nil}.  @xref{Edebug Options}.

The @kbd{i} command steps into the function or macro called by the list
form after point, and stops at its first stop 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{edebug-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}).

@xref{Backtraces}, for a description of backtraces
and the commands which work on them.

@findex edebug-backtrace-show-instrumentation
@findex edebug-backtrace-hide-instrumentation
If you would like to see Edebug's functions in the backtrace,
use @kbd{M-x edebug-backtrace-show-instrumentation}.  To hide them
again use @kbd{M-x edebug-backtrace-hide-instrumentation}.

If a backtrace frame starts with @samp{>} that means that Edebug knows
where the source code for the frame is located.  Use @kbd{s} to jump
to the source code for the current frame.

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

You can invoke commands from Edebug that activate Edebug again
recursively.  Whenever 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 Breaks
@subsection Breaks

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

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

@node Breakpoints
@subsubsection Edebug Breakpoints

@cindex breakpoints (Edebug)
While using Edebug, you can specify @dfn{breakpoints} in the program you
are testing: these are places 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.  An overlay with the @code{edebug-enabled-breakpoint} or
@code{edebug-disabled-breakpoint} faces is put at the breakpoint.

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

@item U
Unset any breakpoints in the current form
(@code{edebug-unset-breakpoints}).

@item D
Toggle whether to disable the breakpoint near point
(@code{edebug-toggle-disable-breakpoint}).  This command is mostly
useful if the breakpoint is conditional and it would take some work to
recreate the condition.

@item x @var{condition} @key{RET}
Set a conditional breakpoint which stops the program only if
evaluating @var{condition} produces 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 current 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.

Re-evaluating or reinstrumenting a definition removes all of its
previous 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 argument 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 following point, within the same
function, or to the first breakpoint if there are no following
breakpoints.  This command does not continue execution---it just moves
point in the buffer.

@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
  The condition expression is stored in
@code{edebug-global-break-condition}.  You can specify a new expression
using the @kbd{X} command from the source code buffer while Edebug is
active, or using @kbd{C-x X X} from any buffer at any time, as long as
Edebug is loaded (@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.  If you wish 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, you can 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 and
executing instrumented code, 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, and 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}).

@c Edebug should be changed for the following: -- dan
  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.

@node Edebug Views
@subsection Edebug Views

  These Edebug commands let you view aspects of the buffer and window
status as they were before entry to Edebug.  The outside window
configuration is the collection of windows and contents that were in
effect outside of Edebug.

@table @kbd
@item v
Switch to viewing the outside window configuration
(@code{edebug-view-outside}).  Type @kbd{C-x X w} to return to Edebug.

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

@item w
Move point back to the current stop point in the source code buffer
(@code{edebug-where}).

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
@c Its function is not simply to forget the saved configuration -- dan
Toggle whether Edebug saves and restores the outside window
configuration (@code{edebug-toggle-save-windows}).

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 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.
You can do that with @kbd{w} from a source code buffer.  You can jump
back to the stop point in the source code buffer from any buffer using
@kbd{C-x X w}.

  Each time you use @kbd{W} to turn saving @emph{off}, Edebug forgets the
saved outside window configuration---so that even if you turn saving
back @emph{on}, the current window configuration remains unchanged when
you next exit Edebug (by continuing the program).  However, the
automatic redisplay of @file{*edebug*} and @file{*edebug-trace*} may
conflict with the buffers you wish to see unless you have enough windows
open.

@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 changes to data 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-: @var{exp} @key{RET}
Evaluate expression @var{exp} in the context of Edebug itself
(@code{eval-expression}).

@item C-x C-e
Evaluate the expression before point, in the context outside of Edebug
(@code{edebug-eval-last-sexp}).  With the prefix argument of zero
(@kbd{C-u 0 C-x C-e}), don't shorten long items (like strings and
lists).
@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}: @code{lexical-let}, @code{macrolet}, and
@code{symbol-macrolet}.
@c FIXME?  What about lexical-binding = t?

@node Eval List
@subsection Evaluation List Buffer

  You can use the @dfn{evaluation list buffer}, called @file{*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 @file{*edebug*}
(@code{edebug-visit-eval-list}).
@end table

  In the @file{*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 C-j
Evaluate the expression before point, in the outside context, and
insert the value in the buffer (@code{edebug-eval-print-last-sexp}).
With prefix argument of zero (@kbd{C-u 0 C-j}), don't shorten long
items (like strings and lists).

@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 the 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{C-j} or @kbd{C-x C-e}, just as you would in @file{*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.  (The idea is that the second expression of the group is the
value previously computed and displayed.)

  Each entry to Edebug redisplays the evaluation list 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 using variables that are 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, insert a new comment line, then type
@kbd{C-c C-u}.  You need not insert dashes in the comment line---its
contents don't matter.

After selecting @file{*edebug*}, you can return to the source code
buffer with @kbd{C-c C-w}.  The @file{*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.

  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 the values
of the variables @code{edebug-print-length} and
@code{edebug-print-level} (so long as they have non-@code{nil}
values).  @xref{Output Variables}.

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

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

  You can also print circular structures and structures that share
elements more informatively by binding @code{print-circle}
to a non-@code{nil} value.

  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 labeled
structure.  This notation is used for any shared elements of lists or
vectors.

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

  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, storing it in a buffer named
@file{*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 consists of
@samp{::::@{}, followed by the function name and argument values.  A
function exit record consists of @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, after the @samp{@{} or @samp{@}}.  All
the arguments are evaluated, and @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 appends a newline to separate entries.
@end defun

  @code{edebug-tracing} and @code{edebug-trace} insert lines in the
trace buffer whenever they are called, even if Edebug is not active.
Adding text to the trace buffer also scrolls its window to show the last
lines inserted.

@node Coverage Testing
@subsection Coverage Testing

@cindex coverage testing (Edebug)
@cindex frequency counts
@cindex performance analysis (Edebug)
  Edebug provides rudimentary coverage testing and display of execution
frequency.

  Coverage testing works by comparing the result of each expression with
the previous result; each form in the program is considered covered
if it has returned two different values since you began testing coverage
in the current Emacs session.  Thus, to do coverage testing on your
program, execute it under various conditions and note whether it behaves
correctly; Edebug will tell you when you have tried enough different
conditions that each form has returned two different values.

  Coverage testing makes execution slower, so it is only done if
@code{edebug-test-coverage} is non-@code{nil}.  Frequency counting is
performed for all executions of an instrumented function, even if the
execution mode is Go-nonstop, and regardless of whether coverage testing
is enabled.

@kindex C-x X =
@findex edebug-temp-display-freq-count
  Use @kbd{C-x X =} (@code{edebug-display-freq-count}) to display both
the coverage information and the frequency counts for a definition.
Just @kbd{=} (@code{edebug-temp-display-freq-count}) displays the same
information temporarily, only until you type another key.

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

It inserts frequency counts as comment lines after each line of code.
You can undo all insertions with one @code{undo} command.  The counts
appear under the @samp{(} before an expression or the @samp{)} after
an expression, or on the last character of a variable.  To simplify
the display, a count is not shown if it is equal to the count of an
earlier expression 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.
In other words, it is not yet covered for coverage testing purposes.

To clear the frequency count and coverage data for a definition,
simply reinstrument it with @code{eval-defun}.
@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      = =5
  (if (< 0 n)
;#5         =
      (* n (fac (1- n)))
;#    5               0
    1))
;#   0
@end example

The comment lines show that @code{fac} was called 6 times.  The
first @code{if} statement 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} did not return 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.

@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, it needs to save and restore certain data
before even deciding whether to make trace information or stop the
program.

@itemize @bullet
@item
@vindex edebug-max-depth
@code{max-lisp-eval-depth} (@pxref{Eval}) and @code{max-specpdl-size}
(@pxref{Local Variables}) are both increased to reduce Edebug's impact
on the stack.  You could, however, still run out of stack space when
using Edebug.  You can also enlarge the value of
@code{edebug-max-depth} if Edebug reaches the limit of recursion depth
instrumenting code that contains very large quoted lists.

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


@node Edebug Display Update
@subsubsection Edebug Display Update

@c This paragraph is not filled, because LaLiberte's conversion script
@c needs an xref to be on just one line.
When Edebug needs to display something (e.g., in trace mode), it saves
the current window configuration from outside Edebug
(@pxref{Window Configurations}).  When you exit Edebug, it restores
the previous window configuration.

Emacs redisplays only when it pauses.  Usually, when you continue
execution, the program re-enters 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.
Consequently, 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 (though some of them 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 Options}).

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

Executing commands within Edebug can change the key sequence that
would be returned by @code{this-command-keys}, and there is no way to
reset the key sequence from Lisp.

Edebug cannot save and restore the value of
@code{unread-command-events}.  Entering Edebug while this variable has a
nontrivial value can interfere with execution of the program you are
debugging.

@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 Edebug and Macros
@subsection Edebug and Macros

To make Edebug properly instrument expressions that call macros, some
extra care is needed.  This subsection explains the details.

@menu
* Instrumenting Macro Calls::   The basic problem.
* Specification List::          How to specify complex patterns of evaluation.
* Backtracking::                What Edebug does when matching fails.
* Specification Examples::      To help understand specifications.
@end menu

@node Instrumenting Macro Calls
@subsubsection Instrumenting Macro Calls

  When Edebug instruments an expression that calls a Lisp macro, it needs
additional information about the macro to do the job properly.  This is
because there is no a-priori 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.)

  Therefore, you must define an Edebug specification for each macro
that Edebug will encounter, to explain the format of calls to that
macro.  To do this, add a @code{debug} declaration to the macro
definition.  Here is a simple example that shows the specification for
the @code{for} example macro (@pxref{Argument Evaluation}).

@smallexample
(defmacro for (var from init to final do &rest body)
  "Execute a simple \"for\" loop.
For example, (for i from 1 to 10 do (print i))."
  (declare (debug (symbolp "from" form "to" form "do" &rest form)))
  ...)
@end smallexample

  The Edebug specification says which parts of a call to the macro are
forms to be evaluated.  For simple macros, the specification
often looks very similar to the formal argument list of the macro
definition, but specifications are much more general than macro
arguments.  @xref{Defining Macros}, for more explanation of
the @code{declare} form.

@c See, e.g., https://debbugs.gnu.org/10577
@c FIXME  Maybe there should be an Edebug option to get it to
@c automatically load the entire source file containing the function
@c being instrumented.  That would avoid this.
  Take care to ensure that the specifications are known to Edebug when
you instrument code.  If you are instrumenting a function which uses a
macro defined in another file, you may first need to either evaluate
the @code{require} forms in the file containing your function, or
explicitly load the file containing the macro.  If the definition of a
macro is wrapped by @code{eval-when-compile}, you may need to evaluate
it.

  You can also define an edebug specification for a macro separately
from the macro definition with @code{def-edebug-spec}.  Adding
@code{debug} declarations is preferred, and more convenient, for macro
definitions in Lisp, but @code{def-edebug-spec} makes it possible to
define Edebug specifications for special forms implemented in C.

@defmac def-edebug-spec macro specification
Specify which expressions of a call to macro @var{macro} are forms to be
evaluated.  @var{specification} should be the edebug specification.
Neither argument is evaluated.

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

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

@table @asis
@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 from 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

If a macro has no Edebug specification, neither through a @code{debug}
declaration nor through a @code{def-edebug-spec} call, the variable
@code{edebug-eval-macro-args} comes into play.

@defopt edebug-eval-macro-args
This controls the way Edebug treats macro arguments with no explicit
Edebug specification.  If it is @code{nil} (the default), none of the
arguments is instrumented for evaluation.  Otherwise, all arguments
are instrumented.
@end defopt

@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 apply only 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.  For more
details, @pxref{Backtracking}.

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 (see @ref{Specification Examples}, for the referenced
examples):

@table @code
@item sexp
A single unevaluated Lisp object, which is not instrumented.
@c an "expression" is not necessarily intended for evaluation.

@item form
A single evaluated expression, which is instrumented.  If your macro
wraps the expression with @code{lambda} before it is evaluated, use
@code{def-form} instead.  See @code{def-form} below.

@item place
A generalized variable.  @xref{Generalized Variables}.

@item body
Short for @code{&rest form}.  See @code{&rest} below.  If your macro
wraps its body of code with @code{lambda} before it is evaluated, use
@code{def-body} instead.  See @code{def-body} 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
@c @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.

@item &rest
@c @kindex &rest @r{(Edebug)}
All following elements in the specification list are repeated zero or
more times.  In the last repetition, however, it is not a problem if the
expression runs out before matching all of the elements of the
specification list.

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
@c @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
@c @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.

@c FIXME &key?

@item &define
@c @kindex &define @r{(Edebug)}
Indicates that the specification is for a defining form.  Edebug's
definition of a defining form is a form containing one or more code
forms which are saved and executed later, after the execution of the
defining form.

The defining form itself is not instrumented (that is, 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.

@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.

@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.

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

Some suitable predicates include @code{symbolp}, @code{integerp},
@code{stringp}, @code{vectorp}, and @code{atom}.

@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.

@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.

@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.  Also see the description of a @code{nil} specification
above for terminating such recursion.

Note that a sublist specification written as @code{(specs .  nil)}
is equivalent to @code{(specs)}, and @code{(specs .
(sublist-elements@dots{}))} is equivalent to @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 appear only after
@code{&define}.  See the @code{defun} example.

@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{&})
are not allowed.

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

@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 it is used 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.
@end table

@node Backtracking
@subsubsection Backtracking in Specifications

@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.

When a syntax error is detected, it might not be reported until much
later, after higher-level alternatives have been exhausted, and with the
point positioned further from the real error.  But if backtracking is
disabled when an error occurs, it can be reported immediately.  Note
that backtracking is also reenabled automatically in several situations;
when a new alternative is established by @code{&optional},
@code{&rest}, or @code{&or}, or at the start of processing a sublist,
group, or indirect specification.  The effect of enabling or disabling
backtracking is limited to the remainder of the level currently being
processed and lower levels.

Backtracking is disabled while matching any of the
form specifications (that is, @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.

Backtracking is also disabled after successfully matching a quoted
symbol or string specification, since this usually indicates a
recognized construct.  But 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] ...]}.

Most needs are satisfied by these two ways that backtracking is
automatically disabled, but occasionally it is useful to explicitly
disable backtracking by using the @code{gate} specification.  This is
useful when you know that no higher alternatives could apply.  See the
example of the @code{let} specification.

@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 expression.  In the specification below, notice the @code{gate}
inside of the sublist to prevent backtracking once a sublist is found.

@ignore
@c FIXME?  The actual definition in edebug.el looks like this (and always
@c has AFAICS).  In fact, nothing in edebug.el uses gate.  So maybe
@c this is just an example for illustration?
(def-edebug-spec let
  ((&rest
    &or (symbolp &optional form) symbolp)
   body))
@end ignore
@example
(def-edebug-spec let
  ((&rest
    &or symbolp (gate symbolp &optional form))
   body))
@end example

Edebug uses the following specifications for @code{defun} and the
associated argument list and @code{interactive} specifications.  It is
necessary to handle interactive forms specially since an expression
argument is actually evaluated outside of the function body.  (The
specification for @code{defmacro} is very similar to that for
@code{defun}, but allows for the @code{declare} statement.)

@smallexample
(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 smallexample

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 is a little different, and does not
support dotted lists because doing so causes very deep recursion that
could fail.)

@smallexample
(def-edebug-spec \` (backquote-form))   ; @r{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 smallexample


@node Edebug Options
@subsection Edebug Options

  These options affect the behavior of Edebug:
@c Previously defopt'd:
@c edebug-sit-for-seconds, edebug-print-length, edebug-print-level
@c edebug-print-circle, edebug-eval-macro-args

@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
reset @code{edebug-setup-hook} 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}, @code{eval-buffer},
and @code{eval-current-buffer}.

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

@defopt edebug-all-forms
If this is non-@code{nil}, the commands @code{eval-defun},
@code{eval-region}, @code{eval-buffer}, and @code{eval-current-buffer}
instrument all forms, even those that don't define anything.
This doesn't apply to loading or evaluations in the minibuffer.

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

@defopt edebug-eval-macro-args
When this is non-@code{nil}, all macro arguments will be instrumented
in the generated code.  For any macro, an @code{edebug-form-spec}
overrides this option.  So to specify exceptions for macros that have
some arguments evaluated and some not, use @code{def-edebug-spec} to
specify an @code{edebug-form-spec}.
@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 this is non-@code{nil}, Edebug saves and restores point in all
displayed buffers.

Saving and restoring point in other buffers is necessary if you are
debugging code that changes the point of a buffer that is displayed in
a non-selected window.  If Edebug or the user then selects the window,
point in that buffer will move to the window's value of 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}.  This variable can be set
interactively with @kbd{C-x C-a C-m} (@code{edebug-set-initial-mode}).
@xref{Edebug Execution Modes}.
@end defopt

@defopt edebug-trace
If this is non-@code{nil}, trace each function entry and exit.
Tracing output is displayed in a buffer named @file{*edebug-trace*}, one
function entry or exit per line, indented by the recursion level.

Also see @code{edebug-tracing}, in @ref{Trace Buffer}.
@end defopt

@defopt edebug-test-coverage
If non-@code{nil}, Edebug tests coverage of all expressions debugged.
@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}, the default value of @code{print-length} for
printing results in Edebug.  @xref{Output Variables}.
@end defopt

@defopt edebug-print-level
If non-@code{nil}, the default value of @code{print-level} for
printing results in Edebug.  @xref{Output Variables}.
@end defopt

@defopt edebug-print-circle
If non-@code{nil}, the default value of @code{print-circle} for
printing results in Edebug.  @xref{Output Variables}.
@end defopt

@defopt edebug-unwrap-results
If non-@code{nil}, Edebug tries to remove any of its own
instrumentation when showing the results of expressions.  This is
relevant when debugging macros where the results of expressions are
themselves instrumented expressions.  As a very artificial example,
suppose that the example function @code{fac} has been instrumented,
and consider a macro of the form:

@c FIXME find a less silly example.
@smallexample
(defmacro test () "Edebug example."
  (if (symbol-function 'fac)
      @dots{}))
@end smallexample

If you instrument the @code{test} macro and step through it, then by
default the result of the @code{symbol-function} call has numerous
@code{edebug-after} and @code{edebug-before} forms, which can make it
difficult to see the actual result.  If
@code{edebug-unwrap-results} is non-@code{nil}, Edebug tries to remove
these forms from the result.
@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 via a new command.
@c Not necessarily a deeper command level.
@c A new command is not precisely true, but that is close enough -- dan

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

@defopt edebug-sit-for-seconds
Number of seconds to pause when a breakpoint is reached and the execution
mode is trace or continue.  @xref{Edebug Execution Modes}.
@end defopt

@defopt edebug-sit-on-break
Whether or not to pause for @code{edebug-sit-for-seconds} on reaching
a breakpoint.  Set to @code{nil} to prevent the pause, non-@code{nil}
to allow it.
@end defopt

@defopt edebug-behavior-alist
By default, this alist contains one entry with the key @code{edebug}
and a list of three functions, which are the default implementations
of the functions inserted in instrumented code: @code{edebug-enter},
@code{edebug-before} and @code{edebug-after}.  To change Edebug's
behavior globally, modify the default entry.

Edebug's behavior may also be changed on a per-definition basis by
adding an entry to this alist, with a key of your choice and three
functions.  Then set the @code{edebug-behavior} symbol property of an
instrumented definition to the key of the new entry, and Edebug will
call the new functions in place of its own for that definition.
@end defopt

@defopt edebug-new-definition-function
A function run by Edebug after it wraps the body of a definition
or closure.  After Edebug has initialized its own data, this function
is called with one argument, the symbol associated with the
definition, which may be the actual symbol defined or one generated by
Edebug.  This function may be used to set the @code{edebug-behavior}
symbol property of each definition instrumented by Edebug.
@end defopt

@defopt edebug-after-instrumentation-function
To inspect or modify Edebug's instrumentation before it is used, set
this variable to a function which takes one argument, an instrumented
top-level form, and returns either the same or a replacement form,
which Edebug will then use as the final result of instrumentation.
@end defopt