18. Debugging Lisp Programs

There are three ways to investigate a problem in an Emacs Lisp program, depending on what you are doing with the program when the problem appears.

• If the problem occurs when you run the program, you can use a Lisp debugger to investigate what is happening during execution. In addition to the ordinary debugger, Emacs comes with a source level debugger, Edebug. This chapter describes both of them.

• If the problem is syntactic, so that Lisp cannot even read the program, you can use the Emacs facilities for editing Lisp to localize it.

• If the problem occurs when trying to compile the program with the byte compiler, you need to know how to examine the compiler's input buffer.

Another useful debugging tool is the dribble file. When a dribble file is open, Emacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. See section 40.8 Terminal Input.

For debugging problems in terminal descriptions, the open-termscript function can be useful. See section 40.9 Terminal Output.

18.1 The Lisp Debugger

The ordinary Lisp debugger provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a break), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of Emacs are available; you can even run programs that will enter the debugger recursively. See section 21.12 Recursive Editing.

18.1.1 Entering the Debugger on an Error

The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error.

However, entry to the debugger is not a normal consequence of an error. Many commands frequently cause Lisp errors when invoked inappropriately (such as C-f at the end of the buffer), and during ordinary editing it would be very inconvenient to enter the debugger each time this happens. So if you want errors to enter the debugger, set the variable debug-on-error to non-nil. (The command toggle-debug-on-error provides an easy way to do this.)

User Option: debug-on-error

This variable determines whether the debugger is called when an error is signaled and not handled. If debug-on-error is t, all kinds of errors call the debugger (except those listed in debug-ignored-errors). If it is nil, none call the debugger.

The value can also be a list of error conditions that should call the debugger. For example, if you set it to the list (void-variable), then only errors about a variable that has no value invoke the debugger.

When this variable is non-nil, Emacs does not create an error handler around process filter functions and sentinels. Therefore, errors in these functions also invoke the debugger. See section 37. Processes.

User Option: debug-ignored-errors

This variable specifies certain kinds of errors that should not enter the debugger. Its value is a list of error condition symbols and/or regular expressions. If the error has any of those condition symbols, or if the error message matches any of the regular expressions, then that error does not enter the debugger, regardless of the value of debug-on-error.

The normal value of this variable lists several errors that happen often during editing but rarely result from bugs in Lisp programs. However, "rarely" is not "never"; if your program fails with an error that matches this list, you will need to change this list in order to debug the error. The easiest way is usually to set debug-ignored-errors to nil.

User Option: debug-on-signal

Normally, errors that are caught by condition-case never run the debugger, even if debug-on-error is non-nil. In other words, condition-case gets a chance to handle the error before the debugger gets a chance.

If you set debug-on-signal to a non-nil value, then the debugger gets the first chance at every error; an error will invoke the debugger regardless of any condition-case, if it fits the criteria specified by the values of debug-on-error and debug-ignored-errors.

Warning: This variable is strong medicine! Various parts of Emacs handle errors in the normal course of affairs, and you may not even realize that errors happen there. If you set debug-on-signal to a non-nil value, those errors will enter the debugger.

Warning: debug-on-signal has no effect when debug-on-error is nil.

To debug an error that happens during loading of the init file, use the option `--debug-init'. This binds debug-on-error to t while loading the init file, and bypasses the condition-case which normally catches errors in the init file.

If your init file sets debug-on-error, the effect may not last past the end of loading the init file. (This is an undesirable byproduct of the code that implements the `--debug-init' command line option.) The best way to make the init file set debug-on-error permanently is with after-init-hook, like this:

(add-hook 'after-init-hook
          (lambda () (setq debug-on-error t)))

18.1.2 Debugging Infinite Loops

When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes a quit.

Ordinary quitting gives no information about why the program was looping. To get more information, you can set the variable debug-on-quit to non-nil. Quitting with C-g is not considered an error, and debug-on-error has no effect on the handling of C-g. Likewise, debug-on-quit has no effect on errors.

Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you will probably get enough information to solve the problem.

User Option: debug-on-quit

This variable determines whether the debugger is called when quit is signaled and not handled. If debug-on-quit is non-nil, then the debugger is called whenever you quit (that is, type C-g). If debug-on-quit is nil, then the debugger is not called when you quit. See section 21.10 Quitting.

18.1.3 Entering the Debugger on a Function Call

To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller.

Command: debug-on-entry function-name

This function requests function-name to invoke the debugger each time it is called. It works by inserting the form (debug 'debug) into the function definition as the first form.

Any function defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. If the function is a command, it will enter the debugger when called from Lisp and when called interactively (after the reading of the arguments). You can't debug primitive functions (i.e., those written in C) this way.

When debug-on-entry is called interactively, it prompts for function-name in the minibuffer. If the function is already set up to invoke the debugger on entry, debug-on-entry does nothing. debug-on-entry always returns function-name.

Note: if you redefine a function after using debug-on-entry on it, the code to enter the debugger is discarded by the redefinition. In effect, redefining the function cancels the break-on-entry feature for that function.

(defun fact (n) (if (zerop n) 1 (* n (fact (1- n))))) => fact (debug-on-entry 'fact) => fact (fact 3) ------ Buffer: *Backtrace* ------ Entering: * fact(3) eval-region(4870 4878 t) byte-code("...") eval-last-sexp(nil) (let ...) eval-insert-last-sexp(nil) * call-interactively(eval-insert-last-sexp) ------ Buffer: *Backtrace* ------ (symbol-function 'fact) => (lambda (n) (debug (quote debug)) (if (zerop n) 1 (* n (fact (1- n)))))

Command: cancel-debug-on-entry function-name

This function undoes the effect of debug-on-entry on function-name. When called interactively, it prompts for function-name in the minibuffer. If function-name is nil or the empty string, it cancels break-on-entry for all functions.

Calling cancel-debug-on-entry does nothing to a function which is not currently set up to break on entry. It always returns function-name.

18.1.4 Explicit Entry to the Debugger

You can cause the debugger to be called at a certain point in your program by writing the expression (debug) at that point. To do this, visit the source file, insert the text `(debug)' at the proper place, and type C-M-x. Warning: if you do this for temporary debugging purposes, be sure to undo this insertion before you save the file!

The place where you insert `(debug)' must be a place where an additional form can be evaluated and its value ignored. (If the value of (debug) isn't ignored, it will alter the execution of the program!) The most common suitable places are inside a progn or an implicit progn (see section 10.1 Sequencing).

18.1.5 Using the Debugger

When the debugger is entered, it displays the previously selected buffer in one window and a buffer named `*Backtrace*' in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error).

The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual Emacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (see section 21.12 Recursive Editing) and it is wise to go back to the backtrace buffer and exit the debugger (with the q command) when you are finished with it. Exiting the debugger gets out of the recursive edit and kills the backtrace buffer.

The backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the current frame. Some of the debugger commands operate on the current frame.

The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted.

18.1.6 Debugger Commands

Inside the debugger (in Debugger mode), these special commands are available in addition to the usual cursor motion commands. (Keep in mind that all the usual facilities of Emacs, such as switching windows or buffers, are still available.)

The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source for the function and type C-M-x on its definition.)

Here is a list of Debugger mode commands:

c

Exit the debugger and continue execution. When continuing is possible, it resumes execution of the program as if the debugger had never been entered (aside from any side-effects that you caused by changing variable values or data structures while inside the debugger).

Continuing is possible after entry to the debugger due to function entry or exit, explicit invocation, or quitting. You cannot continue if the debugger was entered because of an error.

d

Continue execution, but enter the debugger the next time any Lisp function is called. This allows you to step through the subexpressions of an expression, seeing what values the subexpressions compute, and what else they do.

The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the u command to cancel this flag.

b

Flag the current frame so that the debugger will be entered when the frame is exited. Frames flagged in this way are marked with stars in the backtrace buffer.

u

Don't enter the debugger when the current frame is exited. This cancels a b command on that frame. The visible effect is to remove the star from the line in the backtrace buffer.

e

Read a Lisp expression in the minibuffer, evaluate it, and print the value in the echo area. The debugger alters certain important variables, and the current buffer, as part of its operation; e temporarily restores their values from outside the debugger, so you can examine and change them. This makes the debugger more transparent. By contrast, M-: does nothing special in the debugger; it shows you the variable values within the debugger.

R

Like e, but also save the result of evaluation in the buffer `*Debugger-record*'.

q

Terminate the program being debugged; return to top-level Emacs command execution.

If the debugger was entered due to a C-g but you really want to quit, and not debug, use the q command.

r

Return a value from the debugger. The value is computed by reading an expression with the minibuffer and evaluating it.

The r command is useful when the debugger was invoked due to exit from a Lisp call frame (as requested with b or by entering the frame with d); then the value specified in the r command is used as the value of that frame. It is also useful if you call debug and use its return value. Otherwise, r has the same effect as c, and the specified return value does not matter.

You can't use r when the debugger was entered due to an error.

18.1.7 Invoking the Debugger

Here we describe in full detail the function debug that is used to invoke the debugger.

Function: debug &rest debugger-args

This function enters the debugger. It switches buffers to a buffer named `*Backtrace*' (or `*Backtrace*<2>' if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode.

The Debugger mode c and r commands exit the recursive edit; then debug switches back to the previous buffer and returns to whatever called debug. This is the only way the function debug can return to its caller.

The use of the debugger-args is that debug displays the rest of its arguments at the top of the `*Backtrace*' buffer, so that the user can see them. Except as described below, this is the only way these arguments are used.

However, certain values for first argument to debug have a special significance. (Normally, these values are used only by the internals of Emacs, and not by programmers calling debug.) Here is a table of these special values:

lambda

A first argument of lambda means debug was called because of entry to a function when debug-on-next-call was non-nil. The debugger displays `Entering:' as a line of text at the top of the buffer.

debug

debug as first argument indicates a call to debug because of entry to a function that was set to debug on entry. The debugger displays `Entering:', just as in the lambda case. It also marks the stack frame for that function so that it will invoke the debugger when exited.

t

When the first argument is t, this indicates a call to debug due to evaluation of a list form when debug-on-next-call is non-nil. The debugger displays the following as the top line in the buffer:

Beginning evaluation of function call form:

exit

When the first argument is exit, it indicates the exit of a stack frame previously marked to invoke the debugger on exit. The second argument given to debug in this case is the value being returned from the frame. The debugger displays `Return value:' in the top line of the buffer, followed by the value being returned.

error

When the first argument is error, the debugger indicates that it is being entered because an error or quit was signaled and not handled, by displaying `Signaling:' followed by the error signaled and any arguments to signal. For example,

(let ((debug-on-error t)) (/ 1 0)) ------ Buffer: *Backtrace* ------ Signaling: (arith-error) /(1 0) ... ------ Buffer: *Backtrace* ------

If an error was signaled, presumably the variable debug-on-error is non-nil. If quit was signaled, then presumably the variable debug-on-quit is non-nil.

nil

Use nil as the first of the debugger-args when you want to enter the debugger explicitly. The rest of the debugger-args are printed on the top line of the buffer. You can use this feature to display messages--for example, to remind yourself of the conditions under which debug is called.

18.1.8 Internals of the Debugger

This section describes functions and variables used internally by the debugger.

Variable: debugger

The value of this variable is the function to call to invoke the debugger. Its value must be a function of any number of arguments, or, more typically, the name of a function. This function should invoke some kind of debugger. The default value of the variable is debug.

The first argument that Lisp hands to the function indicates why it was called. The convention for arguments is detailed in the description of debug.

Command: backtrace

This function prints a trace of Lisp function calls currently active. This is the function used by debug to fill up the `*Backtrace*' buffer. It is written in C, since it must have access to the stack to determine which function calls are active. The return value is always nil.

In the following example, a Lisp expression calls backtrace explicitly. This prints the backtrace to the stream standard-output, which, in this case, is the buffer `backtrace-output'.

Each line of the backtrace represents one function call. The line shows the values of the function's arguments if they are all known; if they are still being computed, the line says so. The arguments of special forms are elided.

(with-output-to-temp-buffer "backtrace-output" (let ((var 1)) (save-excursion (setq var (eval '(progn (1+ var) (list 'testing (backtrace)))))))) => nil ----------- Buffer: backtrace-output ------------ backtrace() (list ...computing arguments...) (progn ...) eval((progn (1+ var) (list (quote testing) (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) eval-region(1973 2142 #<buffer *scratch*>) byte-code("... for eval-print-last-sexp ...") eval-print-last-sexp(nil) * call-interactively(eval-print-last-sexp) ----------- Buffer: backtrace-output ------------

The character `*' indicates a frame whose debug-on-exit flag is set.

Variable: debug-on-next-call

If this variable is non-nil, it says to call the debugger before the next eval, apply or funcall. Entering the debugger sets debug-on-next-call to nil.

The d command in the debugger works by setting this variable.

Function: backtrace-debug level flag

This function sets the debug-on-exit flag of the stack frame level levels down the stack, giving it the value flag. If flag is non-nil, this will cause the debugger to be entered when that frame later exits. Even a nonlocal exit through that frame will enter the debugger.

This function is used only by the debugger.

Variable: command-debug-status

This variable records the debugging status of the current interactive command. Each time a command is called interactively, this variable is bound to nil. The debugger can set this variable to leave information for future debugger invocations during the same command invocation.

The advantage of using this variable rather than an ordinary global variable is that the data will never carry over to a subsequent command invocation.

Function: backtrace-frame frame-number

The function backtrace-frame is intended for use in Lisp debuggers. It returns information about what computation is happening in the stack frame frame-number levels down.

If that frame has not evaluated the arguments yet, or is a special form, the value is (nil function arg-forms...).

If that frame has evaluated its arguments and called its function already, the return value is (t function arg-values...).

In the return value, function is whatever was supplied as the CAR of the evaluated list, or a lambda expression in the case of a macro call. If the function has a &rest argument, that is represented as the tail of the list arg-values.

If frame-number is out of range, backtrace-frame returns nil.

18.2 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 re-evaluate 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.

18.2.1 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 18.2.2 Instrumenting for Edebug, 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. See section 18.2.3 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, and it 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 would 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 use periods to show the stop points 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.

18.2.2 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: in this case, 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 values of edebug-all-defs and edebug-all-forms.

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; for this reading, after loading Edebug, eval-region records the position of every definition it evaluates, even if not instrumenting it. See also the i command (see section 18.2.4 Jumping), which steps into the call after instrumenting the function.

Edebug knows how to instrument all the standard special forms, 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; see 18.2.15 Instrumenting Macro Calls, for details.

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 Edebug specifications (see section 18.2.15 Instrumenting Macro Calls) associated with a package you are using, but only when you use 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 load, and from the minibuffer with eval-expression (M-:).

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

See section 18.2.9 Evaluation, for other evaluation functions available inside of Edebug.

18.2.3 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, but 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 18.2.5 Miscellaneous Edebug Commands.

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 section 18.2.6 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. See also the edebug-continue-kbd-macro option (see section 18.2.16 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.

18.2.4 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 section 18.2.6 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.

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 of point, using a temporary breakpoint. See 18.2.6 Breakpoints, for more information 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.

You must check that the position C-M-f finds is a place that the program will really get to. In cond, for example, this may not be true.

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

18.2.5 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.

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

18.2.6 Breakpoints

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.

While using Edebug, you can specify 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 18.2.1 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 current 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.

Re-evaluating or reinstrumenting a definition removes all of its previous 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 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 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.

18.2.6.1 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.

The condition expression is stored in edebug-global-break-condition. You can specify a new expression 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.

18.2.6.2 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 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, you can 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.

18.2.7 Trapping Errors

Emacs normally displays an error message when an error is signaled and not handled with condition-case. While Edebug is active and executing instrumented code, it normally responds to all unhandled errors. You can customize this with the options edebug-on-error and edebug-on-quit; see 18.2.16 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 (see section 18.2.5 Miscellaneous Edebug Commands).

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.

18.2.8 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.

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 in the source code buffer (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.

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.

18.2.9 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. See section 18.2.14 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-: 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.

18.2.10 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 section `Lisp Interaction' in The GNU Emacs Manual) as well as these special commands:

C-j

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 C-j 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.)

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, insert a new comment line, then type C-c C-u. You need not insert dashes in the comment line--its contents don't matter.

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.

18.2.11 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 section 19.6 Variables Affecting Output.

User Option: edebug-print-length

If non-nil, Edebug binds print-length to this value while printing results. The default value is 50.

User Option: edebug-print-level

If non-nil, Edebug binds print-level to this value while printing results. The default value is 50.

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

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

User Option: edebug-print-circle

If non-nil, Edebug binds print-circle to this value while printing results. The default value is nil.

Other programs can also use custom printing; see `cust-print.el' for details.

18.2.12 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 section 18.2.3 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 `::::{', followed by the function name and argument values. A function exit record consists of `::::}', 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, and 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 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.

18.2.13 Coverage Testing

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 edebug-test-coverage is non-nil. Frequency counting is performed for all execution of an instrumented function, even if the execution mode is Go-nonstop, and regardless of whether coverage testing is enabled.

Use M-x edebug-display-freq-count to display both the coverage information and the frequency counts for a definition.

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 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 `=' 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 eval-defun.

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.

18.2.14 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.

18.2.14.1 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 once 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.

18.2.14.2 Edebug Display Update

When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from "outside" Edebug (see section 28.17 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 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).

• 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 section 18.2.14.2 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.

18.2.14.3 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 section 34.6 The 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.

18.2.15 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, use def-edebug-spec.

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 can actually be any symbol, not just a macro name.

Here is a simple example that defines the specification for the for example macro (see section 13.6.2 Evaluating Macro Arguments Repeatedly), 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 from 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.

18.2.15.1 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 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. See section 18.2.15.2 Backtracking in Specifications, 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 unevaluated Lisp object, which is not instrumented.

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. 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 [&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 (that is, 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 18.2.15.2 Backtracking in Specifications, 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 appear only 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.

18.2.15.2 Backtracking in Specifications

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. 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; it is reenabled when a new alternative is established by &optional, &rest, or &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, 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.

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., ["foo" &or [first case] [second case] ...].

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

18.2.15.3 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 expression. 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))

18.2.16 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 section 18.2.2 Instrumenting for Edebug.

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 section 18.2.2 Instrumenting for Edebug.

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 section 18.2.2 Instrumenting for Edebug.

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 section 18.2.14.2 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 section 18.2.14.2 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 section 18.2.3 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 18.2.12 Trace Buffer.

User Option: edebug-test-coverage

If non-nil, Edebug tests coverage of all expressions debugged. See section 18.2.13 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 section 18.2.3 Edebug Execution Modes.

User Option: edebug-on-error

Edebug binds debug-on-error to this value, if debug-on-error was previously nil. See section 18.2.7 Trapping Errors.

User Option: edebug-on-quit

Edebug binds debug-on-quit to this value, if debug-on-quit was previously nil. See section 18.2.7 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 section 18.2.6.1 Global Break Condition.

18.3 Debugging Invalid Lisp Syntax

The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error "End of file during parsing" in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, "Invalid read syntax: ")"" indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change?

If the problem is not simply an imbalance of parentheses, a useful technique is to try C-M-e at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun.

However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. (In addition, just moving point through the code with Show Paren mode enabled might find the mismatch.)

18.3.1 Excess Open Parentheses

The first step is to find the defun that is unbalanced. If there is an excess open parenthesis, the way to do this is to go to the end of the file and type C-u C-M-u. This will move you to the beginning of the defun that is unbalanced.

The next step is to determine precisely what is wrong. There is no way to be sure of this except by studying the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with C-M-q and see what moves. But don't do this yet! Keep reading, first.

Before you do this, make sure the defun has enough close parentheses. Otherwise, C-M-q will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don't use C-M-e to move there, since that too will fail to work until the defun is balanced.

Now you can go to the beginning of the defun and type C-M-q. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.

After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.

18.3.2 Excess Close Parentheses

To deal with an excess close parenthesis, first go to the beginning of the file, then type C-u -1 C-M-u to find the end of the unbalanced defun.

Then find the actual matching close parenthesis by typing C-M-f at the beginning of that defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity.

If you don't see a problem at that point, the next thing to do is to type C-M-q at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.

After you think you have fixed the problem, use C-M-q again. If the old indentation actually fits the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.

18.4 Debugging Problems in Compilation

When an error happens during byte compilation, it is normally due to invalid syntax in the program you are compiling. The compiler prints a suitable error message in the `*Compile-Log*' buffer, and then stops. The message may state a function name in which the error was found, or it may not. Either way, here is how to find out where in the file the error occurred.

What you should do is switch to the buffer ` *Compiler Input*'. (Note that the buffer name starts with a space, so it does not show up in M-x list-buffers.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read.

If the error was due to invalid Lisp syntax, point shows exactly where the invalid syntax was detected. The cause of the error is not necessarily near by! Use the techniques in the previous section to find the error.

If the error was detected while compiling a form that had been read successfully, then point is located at the end of the form. In this case, this technique can't localize the error precisely, but can still show you which function to check.