Edebug
======

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

  * Step through evaluation, stopping before and after each expression.

  * Set conditional or unconditional breakpoints.

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

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

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

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

  * Output trace info on function enter and exit.

  * Stop when an error occurs.

  * Display a backtrace, omitting Edebug's own frames.

  * Specify argument evaluation for macros and defining forms.

  * Obtain rudimentary coverage testing and frequency counts.

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

Using Edebug
------------

To debug a Lisp program with Edebug, you must first 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 C-u C-M-x (eval-defun with a prefix argument).  See 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.  See 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 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 fac (shown below) and then execute (fac 3), here is what you normally see.  Point is at the open-parenthesis before if.

     (defun fac (n)
     =>-!-(if (< 0 n)
           (* n (fac (1- n)))
         1))

The places within a function where Edebug can stop execution are called 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 fac:

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

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 SPC to execute until the next stop point. If you type SPC once after entry to fac, here is the display you will see:

     (defun fac (n)
     =>(if -!-(< 0 n)
           (* n (fac (1- n)))
         1))

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

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

Instrumenting for Edebug
------------------------

In order to use Edebug to debug Lisp code, you must first instrument the code.  Instrumenting code inserts additional code into it, to invoke Edebug at the proper places.

Once you have loaded Edebug, the command C-M-x (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 edebug-all-defs is non-nil, that inverts the meaning of the prefix argument: then C-M-x instruments the definition unless it has a prefix argument.  The default value of edebug-all-defs is nil.  The command M-x edebug-all-defs toggles the value of the variable edebug-all-defs.

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

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

When Edebug is about to instrument code for the first time in a session, it runs the hook edebug-setup-hook, then sets it to 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 I (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, eval-region records the position of every definition it evaluates, even if not instrumenting it.  See also the i command (see Jumping), which steps into the call after instrumenting the function.

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; see Instrumenting Macro Calls, for details.

To remove instrumentation from a definition, simply reevaluate its definition in a way that does not instrument.  There are two ways of evaluating forms that never instrument them: from a file with load, and from the minibuffer with eval-expression (M-ESC).

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

See Edebug Eval, for other evaluation functions available inside of Edebug.

Edebug Execution Modes
----------------------

Edebug supports several execution modes for running the program you are debugging.  We call these alternatives 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 S resume execution of the program, at least for a certain distance.

S
     Stop: don't execute any more of the program for now, just wait for more Edebug commands (edebug-stop).

SPC
     Step: stop at the next stop point encountered (edebug-step-mode).

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

t
     Trace: pause one second at each Edebug stop point (edebug-trace-mode).

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

g
     Go: run until the next breakpoint (edebug-go-mode).  See Breakpoints.

c
     Continue: pause one second at each breakpoint, and then continue (edebug-continue-mode).

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

G
     Go non-stop: ignore breakpoints (edebug-Go-nonstop-mode).  You can still stop the program by typing S, or any editing command.

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.

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 t during execution switches to trace mode at the next stop point.  You can use 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.

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 edebug-continue-kbd-macro option (see Edebug Options).

When you enter a new Edebug level, the initial execution mode comes from the value of the variable 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.

Jumping
-------

The commands described in this section execute until they reach a specified location.  All except 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.  See 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.

h
     Proceed to the stop point near where point is (edebug-goto-here).

f
     Run the program forward over one expression (edebug-forward-sexp).

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

i
     Step into the function or macro called by the form after point.

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

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

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

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

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

The 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, 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.

The 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 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.

Miscellaneous Edebug Commands
-----------------------------

Some miscellaneous Edebug commands are described here.

?
     Display the help message for Edebug (edebug-help).

C-]
     Abort one level back to the previous command level (abort-recursive-edit).

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

Q
     Like q but don't stop even for protected code (top-level-nonstop).

r
     Redisplay the most recently known expression result in the echo area (edebug-previous-result).

d
     Display a backtrace, excluding Edebug's own functions for clarity (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.

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 q or abort one recursive edit level with C-].  You can display a backtrace of all the pending evaluations with d.

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 breakpoints in the program you are testing: points where execution should stop.  You can set a breakpoint at any stop point, as defined in 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:

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

u
     Unset the breakpoint (if any) at the stop point at or after point (edebug-unset-breakpoint).

x CONDITION RET
     Set a conditional breakpoint which stops the program only if CONDITION evaluates to a non-nil value (edebug-set-conditional-breakpoint).  With a prefix argument, the breakpoint is temporary.

B
     Move point to the next breakpoint in the definition (edebug-next-breakpoint).

While in Edebug, you can set a breakpoint with b and unset one with u.  First move point to the Edebug stop point of your choice, then type b or 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 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 nil.  To set a conditional breakpoint, use 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 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 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.

Global Break Condition
......................

A 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-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.

You can set or edit the condition expression, stored in edebug-global-break-condition, using the X command (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 nil when not using it.

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 source breakpoint, which is simply a call to the function edebug in your source code.  You can, of course, make such a call conditional.  For example, in the fac function, insert the first line as shown below to stop when the argument reaches zero:

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

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

If no instrumented code is being executed when edebug is called, that function calls debug.

Trapping Errors
---------------

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

If you change debug-on-error or 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.

Edebug Views
------------

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

v
     Temporarily view the outside window configuration (edebug-view-outside).

p
     Temporarily display the outside current buffer with point at its outside position (edebug-bounce-point).  With a prefix argument N, pause for N seconds instead.

w
     Move point back to the current stop point (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.

W
     Toggle whether Edebug saves and restores the outside window configuration (edebug-toggle-save-windows).

     With a prefix argument, 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 C-x X W from the global keymap.

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

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

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.  See The Outside Context, for details on this process.

e EXP RET
     Evaluate expression EXP in the context outside of Edebug (edebug-eval-expression).  That is, Edebug tries to minimize its interference with the evaluation.

M-ESC EXP RET
     Evaluate expression EXP in the context of Edebug itself.

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

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

Evaluation List Buffer
----------------------

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

E
     Switch to the evaluation list buffer *edebug* (edebug-visit-eval-list).

In the *edebug* buffer you can use the commands of Lisp Interaction mode (see Lisp Interaction (The GNU Emacs Manual)) as well as these special commands:

LFD
     Evaluate the expression before point, in the outside context, and insert the value in the buffer (edebug-eval-print-last-sexp).

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

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

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

C-c C-w
     Switch back to the source code buffer at the current stop point (edebug-where).

You can evaluate expressions in the evaluation list window with LFD or C-x C-e, just as you would in *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 evaluation list consisting of expressions to be evaluated each time execution stops.

To do this, write one or more 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 C-c C-u (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.)

Be careful not to add expressions that execute instrumented code since that would cause an infinite loop.

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 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:

     (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
     ;---------------------------------------------------------------

To delete a group, move point into it and type C-c C-d, or simply delete the text for the group and update the evaluation list with 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 C-c C-u.

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

Printing in Edebug
------------------

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 print-length or print-level to truncate the printing.  Edebug does this for you; it binds print-length and print-level to 50 if they were nil.  (Actually, the variables edebug-print-length and edebug-print-level specify the values to use within Edebug.) See Output Variables.

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

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

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

     (setq a '(x y))
     (setcar a a))

Custom printing prints this as Result: #1=(#1# y).  The #1= notation labels the structure that follows it with the label 1, and the #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 cust-print.el for details.

Trace Buffer
------------

Edebug can record an execution trace, storing it in a buffer named *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 edebug-trace to a non-nil value.

Making a trace buffer is not the same thing as using trace execution mode (see 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 ::::{ followed by the function name and argument values.  A function exit record looks like ::::} followed by the function name and result of the function.

The number of :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.

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

 -- Macro: edebug-tracing string body...
     This macro requests additional trace information around the execution of the BODY forms.  The argument STRING specifies text to put in the trace buffer.  All the arguments are evaluated. edebug-tracing returns the value of the last form in BODY.

 -- Function: edebug-trace format-string &rest format-args
     This function inserts text in the trace buffer.  It computes the text with (apply 'format FORMAT-STRING FORMAT-ARGS). It also appends a newline to separate entries.

edebug-tracing and 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.

Coverage Testing
----------------

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 edebug-test-coverage is non-nil.  The command M-x edebug-display-freq-count displays both the frequency data and the coverage data (if recorded).

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

     The frequency counts appear as comment lines after each line of code, and you can undo all insertions with one undo command.  The counts appear under the ( before an expression or the ) 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 = 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.

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

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

The comment lines show that fac was called 6 times.  The first if statement returned 5 times with the same result each time; the same is true of the condition on the second if. The recursive call of fac did not return at all.

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 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.

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.

  * max-lisp-eval-depth and 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.

  * The state of keyboard macro execution is saved and restored.  While Edebug is active, executing-macro is bound to edebug-continue-kbd-macro.

Edebug Display Update
.....................

When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from ``outside'' Edebug (see Window Configurations).  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.

  * Which buffer is current, and the positions of point and the mark in the current buffer, are saved and restored.

  * The outside window configuration is saved and restored if edebug-save-windows is non-nil (see Edebug Display Update).

     The window configuration is not restored on error or quit, but the outside selected window is reselected even on error or quit in case a save-excursion is active.  If the value of 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.

  * The value of point in each displayed buffer is saved and restored if edebug-save-displayed-buffer-points is non-nil.

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

  * cursor-in-echo-area is locally bound to nil so that the cursor shows up in the window.

Edebug Recursive Edit
.....................

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

  * The current match data.  See Match Data.

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

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

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

  * Complex commands executed while in Edebug are added to the variable command-history.  In rare cases this can alter execution.

  * 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.

  * standard-output and standard-input are bound to nil by the recursive-edit, but Edebug temporarily restores them during evaluations.

  * The state of keyboard macro definition is saved and restored.  While Edebug is active, defining-kbd-macro is bound to edebug-continue-kbd-macro.

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 def-edebug-spec to define the format of calls to a given macro.

 -- Macro: def-edebug-spec macro specification
     Specify which expressions of a call to macro 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.

     The MACRO argument may actually be any symbol, not just a macro name.

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

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

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

Here is a table of the possibilities for SPECIFICATION and how each directs processing of arguments.

t
     All arguments are instrumented for evaluation.

0
     None of the arguments is instrumented.

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.

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.

Specification List
..................

A 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 specification keywords, are symbols beginning with & (such as &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. See 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:

sexp
     A single Lisp object, not unevaluated.

form
     A single evaluated expression, which is instrumented.

place
     A place to store a value, as in the Common Lisp setf construct.

body
     Short for &rest form.  See &rest below.

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 quote rather than function, since it instruments the body of the lambda expression either way.

lambda-expr
     A lambda expression with no quoting.

&optional
     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 [&optional SPECS...].  To specify that several elements must all match or none, use &optional [SPECS...].  See the defun example below.

&rest
     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 [&rest SPECS...]. To specify several elements that must all match on every repetition, use &rest [SPECS...].

&or
     Each of the following elements in the specification list is an alternative.  One of the alternatives must match, or the &or specification fails.

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

&not
     Each of the following elements is matched as alternatives as if by using &or, but if any of them match, the specification fails.  If none of them match, nothing is matched, but the &not specification succeeds.

&define
     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 &define keyword should be the first element in a list specification.

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.

gate
     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 Backtracking, for more details.  Also see the let example below.

OTHER-SYMBOL
     Any other symbol in a specification list may be a predicate or an indirect specification.

     If the symbol has an Edebug specification, this 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 def-edebug-spec just as for macros. See the 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 nil.  In either case, that argument is not instrumented.

     Some suitable predicates include symbolp, integerp, stringp, vectorp, and atom.

[ELEMENTS...]
     A vector of elements groups the elements into a single group specification.  Its meaning has nothing to do with vectors.

"STRING"
     The argument should be a symbol named STRING.  This specification is equivalent to the quoted symbol, 'SYMBOL, where the name of SYMBOL is the STRING, but the string form is preferred.

(vector ELEMENTS...)
     The argument should be a vector whose elements must match the ELEMENTS in the specification.  See the backquote example below.

(ELEMENTS...)
     Any other list is a sublist specification and the argument must be a list whose elements match the specification ELEMENTS.

     A sublist specification may be a dotted list and the corresponding list argument may then be a dotted list.  Alternatively, the last CDR of a dotted list specification may be another sublist specification (via a grouping or an indirect specification, e.g. (spec .  [(more specs...)])) 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 nil specification above for terminating such recursion.

     Note that a sublist specification written as (specs .  nil) is equivalent to (specs), and (specs . (sublist-elements...)) is equivalent to (specs sublist-elements...).

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

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.

:name
     This construct does not actually match an argument.  The element following :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.

arg
     The argument, a symbol, is the name of an argument of the defining form. However, lambda list keywords (symbols starting with &) are not allowed.

lambda-list
     This matches a lambda list---the argument list of a lambda expression.

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

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

Backtracking
............

If a specification fails to match at some point, this does not necessarily mean a syntax error will be signaled; instead, 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 &optional, &rest, or &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. form, body, def-form, and 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., ["foo" &or [first case] [second case] ...].

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

Specification Examples
......................

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

A 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 gate inside of the sublist to prevent backtracking once a sublist is found.

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

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

     (def-edebug-spec defmacro defun) ; Indirect ref to defun spec.
     (def-edebug-spec defun
       (&define name lambda-list
                [&optional stringp]   ; 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))    ; Notice: def-form

The specification for backquote below illustrates how to match dotted lists and use 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.)

     (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))

Edebug Options
--------------

These options affect the behavior of Edebug:

 -- User Option: 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 edebug-setup-hook is reset to nil.  You could use this to load up Edebug specifications associated with a package you are using but only when you also use Edebug. See Instrumenting.

 -- User Option: edebug-all-defs
     If this is non-nil, normal evaluation of defining forms such as defun and defmacro instruments them for Edebug.  This applies to eval-defun, eval-region, eval-buffer, and eval-current-buffer.

     Use the command M-x edebug-all-defs to toggle the value of this option.  See Instrumenting.

 -- User Option: edebug-all-forms
     If this is non-nil, the commands eval-defun, eval-region, eval-buffer, and 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 M-x edebug-all-forms to toggle the value of this option.  See Instrumenting.

 -- User Option: edebug-save-windows
     If this is non-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 nil.

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

     You can use the W command in Edebug to change this variable interactively.  See Edebug Display Update.

 -- User Option: edebug-save-displayed-buffer-points
     If this is non-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 which 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.  See Edebug Display Update.

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

     The default value is step. See Edebug Execution Modes.

 -- User Option: edebug-trace
     Non-nil means display a trace of function entry and exit. Tracing output is displayed in a buffer named *edebug-trace*, one function entry or exit per line, indented by the recursion level.

     The default value is nil.

     Also see edebug-tracing, in see Trace Buffer.

 -- User Option: edebug-test-coverage
     If non-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 M-x edebug-display-freq-count to display the frequency count and coverage information for a definition. See Coverage Testing.

 -- User Option: edebug-continue-kbd-macro
     If non-nil, continue defining or executing any keyboard macro that is executing outside of Edebug.   Use this with caution since it is not debugged. See Edebug Execution Modes.

 -- User Option: edebug-print-length
     If non-nil, bind print-length to this while printing results in Edebug.  The default value is 50. See Printing in Edebug.

 -- User Option: edebug-print-level
     If non-nil, bind print-level to this while printing results in Edebug.  The default value is 50.

 -- User Option: edebug-print-circle
     If non-nil, bind print-circle to this while printing results in Edebug.  The default value is nil.

 -- User Option: edebug-on-error
     Edebug binds debug-on-error to this value, if debug-on-error was previously nil.  See Trapping Errors.

 -- User Option: edebug-on-quit
     Edebug binds debug-on-quit to this value, if debug-on-quit was previously nil.  See Trapping Errors.

If you change the values of edebug-on-error or edebug-on-quit while Edebug is active, their values won't be used until the next time Edebug is invoked via a new command.

 -- User Option: edebug-global-break-condition
     If non-nil, an expression to test for at every stop point. If the result is non-nil, then break.  Errors are ignored. See Global Break Condition.