22. Debugging Lisp Programs

There are three ways to investigate a problem in an XEmacs 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 (either the default debugger or Edebug) to investigate what is happening during execution.
• If the problem is syntactic, so that Lisp cannot even read the program, you can use the XEmacs 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, XEmacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. See section Terminal Input.

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

22.1 The Lisp Debugger

The 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 XEmacs are available; you can even run programs that will enter the debugger recursively. See section Recursive Editing.

22.1.1 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 get Lisp errors when invoked in inappropriate contexts (such as C-f at the end of the buffer) and during ordinary editing it would be very unpleasant to enter the debugger each time this happens. If you want errors to enter the debugger, set the variable debug-on-error to non-nil.

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 errors call the debugger. 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 catch errors that happen in process filter functions and sentinels. Therefore, these errors also can invoke the debugger. See section Processes.

User Option: debug-on-signal

This variable is similar to debug-on-error but breaks whenever an error is signalled, regardless of whether it would be handled.

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.

To debug an error that happens during loading of the ‘.emacs’ file, use the option ‘-debug-init’, which binds debug-on-error to t while ‘.emacs’ is loaded and inhibits use of condition-case to catch init file errors.

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

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

22.1.2 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 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 Quitting.

22.1.3 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.

Please note: if you redefine a function after using debug-on-entry on it, the code to enter the debugger is lost.

debug-on-entry returns function-name.

(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 &optional 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 debugging for all functions.

If cancel-debug-on-entry is called more than once on the same function, the second call does nothing. cancel-debug-on-entry returns function-name.

22.1.4 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. 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 Sequencing).

22.1.5 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 XEmacs 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 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.

22.1.6 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 XEmacs, 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 file 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. This resumes execution of the program as if the debugger had never been entered (aside from the effect of any variables or data structures you may have changed while inside the debugger).

Continuing when an error or quit was signalled will cause the normal action of the signalling to take place. If you do not want this to happen, but instead want the program execution to continue as if the call to signal did not occur, use the r command.

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.

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 outside-the-debugger values so you can examine 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.

q

Terminate the program being debugged; return to top-level XEmacs 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); 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.

If the debugger was entered at the beginning of a function call, r has the same effect as c, and the specified return value does not matter.

If the debugger was entered through a call to signal (i.e. as a result of an error or quit), then returning a value will cause the call to signal itself to return, rather than throwing to top-level or invoking a handler, as is normal. This allows you to correct an error (e.g. the type of an argument was wrong) or continue from a debug-on-quit as if it never happened.

Note that some errors (e.g. any error signalled using the error function, and many errors signalled from a primitive function) are not continuable. If you return a value from them and continue execution, then the error will immediately be signalled again. Other errors (e.g. wrong-type-argument errors) will be continually resignalled until the problem is corrected.

22.1.7 Invoking the Debugger

Here we describe fully the function 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.

If the first of the debugger-args passed to debug is nil (or if it is not one of the special values in the table below), then debug displays the rest of its arguments at the top of the ‘*Backtrace*’ buffer. This mechanism is used to display a message to the user.

However, if the first argument passed to debug is one of the following special values, then it has special significance. Normally, these values are passed to debug only by the internals of XEmacs and the debugger, and not by programmers calling debug.

The special values are:

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:’ on 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.

22.1.8 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). Presumably this function will enter 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 &optional stream detailed

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.

The backtrace is normally printed to standard-output, but this can be changed by specifying a value for stream. If detailed is non-nil, the backtrace also shows places where currently active variable bindings, catches, condition-cases, and unwind-protects were made as well as function calls.

In the following example, a Lisp expression calls backtrace explicitly. This prints the backtrace to the stream standard-output: in this case, to 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 operators 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.

The advantage, for the debugger, of using this variable rather than another 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 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.

22.2 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.

22.2.1 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 insert a close parenthesis at the end of the file and type C-M-b (backward-sexp). This will move you to the beginning of the defun that is unbalanced. (Then type C-<SPC> C-_ C-u C-<SPC> to set the mark there, undo the insertion of the close parenthesis, and finally return to the mark.)

The next step is to determine precisely what is wrong. There is no way to be sure of this except to study 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.

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.

22.2.2 Excess Close Parentheses

To deal with an excess close parenthesis, first insert an open parenthesis at the beginning of the file, back up over it, and type C-M-f to find the end of the unbalanced defun. (Then type C-<SPC> C-_ C-u C-<SPC> to set the mark there, undo the insertion of the open parenthesis, and finally return to the mark.)

Then find the actual matching close parenthesis by typing C-M-f at the beginning of the 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 fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.

22.3 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.

22.4 Edebug

Edebug is a source-level debugger for XEmacs Lisp programs that provides the following features:

• Step through evaluation, stopping before and after each expression.
• Set conditional or unconditional breakpoints, install embedded breakpoints, or a global break event.
• Trace slow or fast stopping briefly at each stop point, or each breakpoint.
• Display expression results and evaluate expressions as if outside of Edebug. Interface with the custom printing package for printing circular structures.
• Automatically reevaluate a list of expressions and display their results each time Edebug updates the display.
• Output trace info on function enter and exit.
• Errors stop before the source causing the error.
• Display backtrace without Edebug calls.
• Allow specification of argument evaluation for macros and defining forms.
• Provide rudimentary coverage testing and display of frequency counts.

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

22.4.1 Using Edebug

To debug an XEmacs Lisp program with Edebug, you must first instrument the Lisp code that you want to debug. If you want to just try it now, load ‘edebug.el’, move point into a definition and do C-u C-M-x (eval-defun with a prefix argument). See Instrumenting for Edebug 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 the selected Edebug execution mode. The initial execution mode is step, by default, which does stop execution. See section Edebug Execution Modes.

Within Edebug, you normally view an XEmacs buffer showing the source of the Lisp function you are debugging. This is referred to as the source code buffer—but note that it is not always the same buffer depending on which function is currently being executed.

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 you can 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).)

While the source code buffer is selected, the special commands of Edebug are available in it, in addition to the commands of XEmacs Lisp mode. (The buffer is temporarily made read-only, however.) 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 to the top-level command loop. Type ? to display a list of all Edebug commands.

22.4.2 Instrumenting for Edebug

In order to use Edebug to debug Lisp code, you must first instrument the code. Instrumenting a form inserts additional code into it which invokes Edebug at the proper places. Furthermore, if Edebug detects a syntax error while instrumenting, point is left at the erroneous code and an invalid-read-syntax error is signaled.

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.

Just before Edebug instruments any code, it calls any functions in the variable edebug-setup-hook and resets its value 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. For example, ‘my-specs.el’ may be loaded automatically when you use my-package with Edebug by including the following code in ‘my-package.el’.

(add-hook 'edebug-setup-hook
  (function (lambda () (require 'my-specs))))

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. If the location of the definition is not known to Edebug, this command cannot be used. After loading Edebug, eval-region records the position of every definition it evaluates, even if not instrumenting it. Also see the command i (Jumping) which steps into the callee.

Edebug knows how to instrument all the standard special operators, an interactive form with an expression argument, anonymous lambda expressions, and other defining forms. (Specifications for macros defined by ‘cl.el’ (version 2.03) are provided in ‘cl-specs.el’.) 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 the details.

Note that a couple ways remain to evaluate expressions without instrumenting them. Loading a file via the load subroutine does not instrument expressions for Edebug. Evaluations in the minibuffer via eval-expression (M-ESC) are not instrumented.

To remove instrumentation from a definition, simply reevaluate it with one of the non-instrumenting commands, or reload the file.

See Evaluation for other evaluation functions available inside of Edebug.

22.4.3 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 Edebug displays the progress of the evaluation, whether it stops at each stop point, or continues to the next breakpoint, for example.

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 Miscellaneous.

t

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

T

Rapid trace: update 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 Breakpoints.

c

Continue: pause for one second at each breakpoint, but don’t stop (edebug-continue-mode).

C

Rapid continue: update at each breakpoint, but don’t actually pause (edebug-Continue-fast-mode).

G

Go non-stop: ignore breakpoints (edebug-Go-nonstop-mode). You can still stop the program by hitting any key.

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

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.

While executing or tracing, you can interrupt the execution by typing any Edebug command. Edebug stops the program at the next stop point and then executes the command that you typed. For example, typing 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 hit 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 Edebug commands do not work; when you exit from Edebug, to resume the program, whether you are defining or executing a keyboard macro is forgotten. Also, defining or executing a keyboard macro outside of Edebug does not affect the command loop inside Edebug. This is usually an advantage. But see edebug-continue-kbd-macro.

22.4.4 Jumping

Commands described here let you jump to a specified location. All, except i, use temporary breakpoints to establish the stop point and 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.

f

Run the program forward over one expression (edebug-forward-sexp). More precisely, set a temporary breakpoint at the position that C-M-f would reach, then execute 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.

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

o

Continue “out of” an expression (edebug-step-out). It places a temporary breakpoint at the end of the sexp containing point.

If the containing sexp is a function definition itself, it continues until just before the last sexp in the definition. If that is where you are now, it returns from the function and then stops. In other words, this command does not exit the currently executing function unless you are positioned after the last sexp.

I

Step into the function or macro after point after first ensuring that it is instrumented. It does this by calling edebug-on-entry and then switching to go mode.

Although the automatic instrumentation is convenient, it is not later automatically uninstrumented.

h

Proceed to the stop point near where point is using a temporary breakpoint (edebug-goto-here).

All the commands in this section 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.

22.4.5 Miscellaneous

Some miscellaneous 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.

22.4.6 Breakpoints

There are three more ways to stop execution once it has started: breakpoints, the global break condition, and embedded 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 the current 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). If you use a prefix argument, the breakpoint is temporary (it turns off the first time it stops the program).

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 you must move point to a position at or before the desired Edebug stop point, then hit the key to change the breakpoint. Unsetting a breakpoint that has not been set does nothing.

Reevaluating or reinstrumenting a definition clears all its breakpoints.

A conditional breakpoint tests a condition each time the program gets there. To set a conditional breakpoint, use x, and specify the condition expression in the minibuffer. Setting a conditional breakpoint at a stop point that already has a conditional breakpoint puts the current condition expression in the minibuffer so you can edit it.

You can make both conditional and unconditional breakpoints temporary by using a prefix arg to the command to set the breakpoint. After breaking at a temporary breakpoint, it is automatically cleared.

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 B, which moves point to the next breakpoint in the definition following point, or to the first breakpoint if there are no following breakpoints. This command does not continue execution—it just moves point in the buffer.

22.4.6.1 Global Break Condition

In contrast to breaking when execution reaches specified locations, you can also cause a break when a certain event occurs. The global break condition is a condition that is repeatedly evaluated at every stop point. If it evaluates to a non-nil value, then execution is stopped or paused depending on the execution mode, just like a breakpoint. Any errors that might occur as a result of evaluating the condition are ignored, as if the result were nil.

You can set or edit the condition expression, stored in edebug-global-break-condition, using X (edebug-set-global-break-condition).

Using the global break condition is perhaps the fastest way to find where in your code some event occurs, but since it is rather expensive you should reset the condition to nil when not in use.

22.4.6.2 Embedded Breakpoints

Since all breakpoints in a definition are cleared each time you reinstrument it, you might rather create an embedded breakpoint which is simply a call to the function edebug. 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, Edebug will stop before the call to edebug. Depending on the execution mode, Edebug will stop or pause.

However, if no instrumented code is being executed, calling edebug will instead invoke debug. Calling debug will always invoke the standard backtrace debugger.

22.4.7 Trapping Errors

An error may be signaled by subroutines or XEmacs Lisp code. If a signal is not handled by a condition-case, this indicates an unrecognized situation has occurred. If Edebug is not active when an unhandled error is signaled, debug is run normally (if debug-on-error is non-nil). But while Edebug is active, debug-on-error and debug-on-quit are bound to edebug-on-error and edebug-on-quit, which are both t by default. Actually, if debug-on-error already has a non-nil value, that value is still used.

It is best to change the values of edebug-on-error or edebug-on-quit when Edebug is not active since their values won’t be used until the next time Edebug is invoked at a deeper command level. If you only 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 shows you the last stop point that it knew about before the error was signaled. 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. If the cause of the error is not obvious at first, note that you can also get a full backtrace inside of Edebug (see Miscellaneous).

Edebug can also trap signals even if they are handled. If debug-on-error is a list of signal names, Edebug will stop when any of these errors are signaled. Edebug shows you the last known stop point just as for unhandled errors. After you continue execution, the error is signaled again (but without being caught by Edebug). Edebug can only trap errors that are handled if they are signaled in Lisp code (not subroutines) since it does so by temporarily replacing the signal function.

22.4.8 Edebug Views

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

v

View the outside window configuration (edebug-view-outside).

p

Temporarily display the outside current buffer with point at its outside position (edebug-bounce-point). If prefix arg is supplied, sit for that many 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 another window displaying the same buffer, this window will be used instead to display the buffer in the future.

W

Toggle the edebug-save-windows variable which indicates whether the outside window configuration is saved and restored (edebug-toggle-save-windows). Also, each time it is toggled on, make the outside window configuration the same as the current window configuration.

With a prefix argument, edebug-toggle-save-windows 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-xXW from the global keymap.

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

By using W twice, Edebug again saves and restores the outside window configuration, but to the current configuration. This is a convenient way to, for example, add another buffer to be displayed whenever Edebug is active. However, the automatic redisplay of ‘*edebug*’ and ‘*edebug-trace*’ may conflict with the buffers you wish to see unless you have enough windows open.

22.4.9 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 things that Edebug explicitly saves and restores. See The Outside Context for details on this process. Also see Reading in Edebug and Printing in Edebug for topics related to evaluation.

e exp <RET>

Evaluate expression exp in the context outside of Edebug (edebug-eval-expression). In other words, Edebug tries to avoid altering the effect of exp.

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.

22.4.10 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 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 first expression of each group, reevaluate and redisplay (edebug-update-eval-list). Groups are separated by comment lines.

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 unless you add them to the evaluation list with C-c C-u. This command builds a new list from the first expression of each evaluation list group. Groups are separated by comment lines. Be careful not to add expressions that execute instrumented code otherwise an infinite loop will result.

When the evaluation list is redisplayed, each expression is displayed followed by the result of evaluating it, and a comment line. If an error occurs during an evaluation, the error message is displayed in a string as if it were the result. Therefore expressions that, for example, 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. When you add a new group, be sure it is separated from its neighbors by a comment line.

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.

22.4.11 Reading in Edebug

To instrument a form, Edebug first reads the whole form. Edebug replaces the standard Lisp Reader with its own reader that remembers the positions of expressions. This reader is used by the Edebug replacements for eval-region, eval-defun, eval-buffer, and eval-current-buffer.

Another package, ‘cl-read.el’, replaces the standard reader with one that understands Common Lisp reader macros. If you use that package, Edebug will automatically load ‘edebug-cl-read.el’ to provide corresponding reader macros that remember positions of expressions. If you define new reader macros, you will have to define similar reader macros for Edebug.

22.4.12 Printing in Edebug

If the result of an expression in your program contains a circular reference, you may get an error when Edebug attempts to print it. You can set print-length to a non-zero value to limit the print length of lists (the number of cdrs), and in Emacs 19, set print-level to a non-zero value to limit the print depth of lists. But you can print such 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. You can also activate custom printing for printing in any Lisp code; see the package for details.

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

(progn
  (edebug-install-custom-print)
  (setq a '(x y))
  (setcar a a))

Edebug will print the result of the setcar as ‘Result: #1=(#1# y)’. The ‘#1=’ notation names the structure that follows it, and the ‘#1#’ notation references the previously named structure. This notation is used for any shared elements of lists or vectors.

Independent of whether ‘cust-print’ is active, while printing results Edebug binds print-length, print-level, and print-circle to edebug-print-length (50), edebug-print-level (50), and edebug-print-circle (t) respectively, if these values are non-nil. Also, print-readably is bound to nil since some objects simply cannot be printed readably.

22.4.13 Tracing

In addition to automatic stepping through source code, which is also called tracing (see Edebug Execution Modes), Edebug can produce a traditional trace listing of execution in a separate buffer, ‘*edebug-trace*’.

If the variable edebug-trace is non-nil, each function entry and exit adds lines to the trace buffer. On function entry, Edebug prints ‘::::{’ followed by the function name and argument values. On function exit, Edebug prints ‘::::}’ followed by the function name and result of the function. The number of ‘:’s is computed from the recursion depth. The balanced braces in the trace buffer can be used to find the matching beginning or end of function calls. These displays may be customized by replacing the functions edebug-print-trace-before and edebug-print-trace-after, which take an arbitrary message string to print.

The macro edebug-tracing provides tracing similar to function enter and exit tracing, but for arbitrary expressions. This macro should be explicitly inserted by you around expressions you wish to trace the execution of. The first argument is a message string (evaluated), and the rest are expressions to evaluate. The result of the last expression is returned.

Finally, you can insert arbitrary strings into the trace buffer with explicit calls to edebug-trace. The arguments of this function are the same as for message, but a newline is always inserted after each string printed in this way.

edebug-tracing and edebug-trace insert lines in the trace buffer even if Edebug is not active. Every time the trace buffer is added to, the window is scrolled to show the last lines inserted. (There may be some display problems if you use tracing along with the evaluation list.)

22.4.14 Coverage Testing

Edebug provides a rudimentary coverage tester and display of execution frequency. Frequency counts are always accumulated, both before and after evaluation of each instrumented expression, even if the execution mode is Go-nonstop. Coverage testing is only done if the option edebug-test-coverage is non-nil because this is relatively expensive. Both data sets are displayed by M-x edebug-display-freq-count.

Command: edebug-display-freq-count

Display the frequency count data for each line of the current definition. The frequency counts are inserted as comment lines after each line, and you can undo all insertions with one undo command. The counts are inserted starting under the ( before an expression or the ) after an expression, or on the last char of a symbol. The counts are only displayed when they differ from previous counts on the same line.

If coverage is being tested, whenever all known results of an expression are eq, the char = will be appended after the count for that expression. Note that this is always the case for an expression only evaluated once.

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

For example, after evaluating (fac 5) with an embedded breakpoint, and setting edebug-test-coverage to t, when the breakpoint is reached, the frequency data is 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 has been called 6 times. The first if statement has returned 5 times with the same result each time, and the same is true for the condition on the second if. The recursive call of fac has not returned at all.

22.4.15 The Outside Context

Edebug tries to be transparent to the program you are debugging. In addition, most evaluations you do within Edebug (see Evaluation) occur in the same outside context which is temporarily restored for the evaluation. But Edebug is not completely successful and this section explains precisely how it fails. Edebug operation unavoidably alters some data in XEmacs, and this can interfere with debugging certain programs. Also notice that Edebug’s protection against change of outside data means that any side effects intended by the user in the course of debugging will be defeated.

22.4.15.1 Checking Whether to Stop

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

• 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.

22.4.15.2 Edebug Display Update

When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from “outside” Edebug. When you exit Edebug (by continuing the program), it restores the previous window configuration.

XEmacs 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, XEmacs 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 where point and mark are in the current buffer are saved and restored.
• The Edebug Display Update, is saved and restored if edebug-save-windows is non-nil. It 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.

• 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.

22.4.15.3 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, for whichever buffer was current.
• 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.

  For Emacs 18, Edebug cannot save and restore the value of unread-command-char. 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.

22.4.16 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 may 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 macro call arguments by using def-edebug-spec to define an Edebug specification for each 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.

Unless you are using Emacs 19 or XEmacs, this macro is only defined in Edebug, so you may want to use the following which is equivalent: (put 'macro 'edebug-form-spec 'specification)

Here is a simple example that defines the specification for the for macro described in the XEmacs 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.

22.4.16.1 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 keyword specifications, are symbols beginning with ‘&’ (e.g. &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. Keyword specifications only apply to the remainder of the sublist or group they are contained in and there is an implicit grouping around a keyword specification and all following elements in the sublist or group.

If a specification list fails at some level, then backtracking may be invoked to find some alternative at a higher level, or if no alternatives remain, an error will be signaled. See Backtracking for more details.

Edebug specifications provide at least the power of regular expression matching. Some context-free constructs are also supported: the matching of sublists with balanced parentheses, recursive processing of forms, and recursion via indirect specifications.

Each element of a specification list may be one of the following, with the corresponding type of argument:

sexp

A single unevaluated expression.

form

A single evaluated expression, which is instrumented.

place

A place as in the Common Lisp setf place argument. It will be instrumented just like a form, but the macro is expected to strip the instrumentation. Two functions, edebug-unwrap and edebug-unwrap*, are provided to strip the instrumentation one level or recursively at all levels.

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 function arguments might be quoted with quote rather than function since the body of a lambda expression will be instrumented either way.

lambda-expr

An unquoted anonymous lambda expression.

&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 should all succeed together, 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 all elements must match on every repetition, use &rest [specs…].

&or

Each of the following elements in the specification list is an alternative, processed left to right until one matches. One of the alternatives must match otherwise the &or specification fails.

Each list element following &or is a single alternative even if it is a keyword specification. (This breaks the implicit grouping rule.) 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.

Additional specifications that may only appear after &define are described here. See the defun example below.

name

The argument, a symbol, is the name of the defining form. But a defining form need not be named at all, in which case a unique name will be created for it.

The name specification may be used more than once in the specification and each subsequent use will append the corresponding symbol argument to the previous name with ‘@’ between them. This is useful for generating unique but meaningful names for definitions such as defadvice and defmethod.

:name

The element following :name should be a symbol; it is used as an additional name component for the definition. This is useful to add a unique, static component to the name of the definition. It may be used more than once. No argument is matched.

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. See lambda-list and the example below.

lambda-list

This matches the whole argument list of an XEmacs Lisp lambda expression, which is a list of symbols and the keywords &optional and &rest

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.

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 fails. The argument is not instrumented.

Predicates that may be used include: symbolp, integerp, stringp, vectorp, atom (which matches a number, string, symbol, or vector), keywordp, and lambda-list-keywordp. The last two, defined in ‘edebug.el’, test whether the argument is a symbol starting with ‘:’ and ‘&’ respectively.

[elements…]

Rather than matching a vector argument, a vector treats the elements as a single group specification.

"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.

'symbol or (quote symbol)

The argument should be the symbol symbol. But use a string specification instead.

(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 of the form (specs . nil) means the same as (specs), and (specs . (sublist-elements…)) means the same as (specs sublist-elements…).

22.4.16.2 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.

22.4.16.3 Debugging Backquote

Backquote (`) is a macro that results in an expression that may or may not be evaluated. It is often used to simplify the definition of a macro to return an expression that is evaluated, but Edebug does not know when this is the case. However, the forms inside unquotes (, and ,@) are evaluated and Edebug instruments them.

Nested backquotes are supported by Edebug, but there is a limit on the support of quotes inside of backquotes. Quoted forms (with ') are not normally evaluated, but if the quoted form appears immediately within , and ,@ forms, Edebug treats this as a backquoted form at the next higher level (even if there is not a next higher level - this is difficult to fix).

If the backquoted forms happen to be code intended to be evaluated, you can have Edebug instrument them by using edebug-` instead of the regular `. Unquoted forms can always appear inside edebug-` anywhere a form is normally allowed. But (, form) may be used in two other places specially recognized by Edebug: wherever a predicate specification would match, and at the head of a list form in place of a function name or lambda expression. The form inside a spliced unquote, (,@ form), will be wrapped, but the unquote form itself will not be wrapped since this would interfere with the splicing.

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

For example, the for macro (see section ‘Problems with Macros’ in XEmacs Lisp Reference Manual) is shown here but with edebug-` substituted for regular `.

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

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

Here is the corresponding modified Edebug specification and some code that calls the macro:

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

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

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

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

22.4.16.4 Specification Examples

Here we provide several examples of Edebug specifications to show many of its capabilities.

A let special operator 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.

(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 the expression argument of an interactive form specially since 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 provided 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))

This document was generated by Aidan Kehoe on December 27, 2016 using texi2html 1.82.