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The `edebug' Source Level Debugger
==================================

   Edebug is a source level debugger.  Edebug normally displays the
source of the code you are debugging, with an arrow at the left that
shows which line you are currently executing.

   You can walk through the execution of a function, line by line, or
run quickly until reaching a "breakpoint" where execution stops.

   Edebug is described in Note: Edebug.

   Here is a bugged function definition for `triangle-recursively'.
Note: Recursion in place of a counter, for
a review of it.

     (defun triangle-recursively-bugged (number)
       "Return sum of numbers 1 through NUMBER inclusive.
     Uses recursion."
       (if (= number 1)
           1
         (+ number
            (triangle-recursively-bugged
             (1= number)))))               ; Error here.

Normally, you would install this definition by positioning your cursor
after the function's closing parenthesis and typing `C-x C-e'
(`eval-last-sexp') or else by positioning your cursor within the
definition and typing `C-M-x' (`eval-defun').  (By default, the
`eval-defun' command works only in Emacs Lisp mode or in Lisp
Interactive mode.)

   However, to prepare this function definition for Edebug, you must
first "instrument" the code using a different command.  You can do this
by positioning your cursor within the definition and typing

     M-x edebug-defun RET

This will cause Emacs to load Edebug automatically if it is not already
loaded, and properly instrument the function.

   After instrumenting the function, place your cursor after the
following expression and type `C-x C-e' (`eval-last-sexp'):

     (triangle-recursively-bugged 3)

You will be jumped back to the source for `triangle-recursively-bugged'
and the cursor positioned at the beginning of the `if' line of the
function.  Also, you will see an arrowhead at the left hand side of
that line.  The arrowhead marks the line where the function is
executing.  (In the following examples, we show the arrowhead with
`=>'; in a windowing system, you may see the arrowhead as a solid
triangle in the window `fringe'.)

     =>-!-(if (= number 1)

In the example, the location of point is displayed as `-!-' (in a
printed book, it is displayed with a five pointed star).

If you now press <SPC>, point will move to the next expression to be
executed; the line will look like this:

     =>(if -!-(= number 1)

As you continue to press <SPC>, point will move from expression to
expression.  At the same time, whenever an expression returns a value,
that value will be displayed in the echo area.  For example, after you
move point past `number', you will see the following:

     Result: 3 = C-c

This means the value of `number' is 3, which is ASCII `control-c' (the
third letter of the alphabet).

   You can continue moving through the code until you reach the line
with the error.  Before evaluation, that line looks like this:

     =>        -!-(1= number)))))               ; Error here.

When you press <SPC> once again, you will produce an error message that
says:

     Symbol's function definition is void: 1=

This is the bug.

   Press `q' to quit Edebug.

   To remove instrumentation from a function definition, simply
re-evaluate it with a command that does not instrument it.  For
example, you could place your cursor after the definition's closing
parenthesis and type `C-x C-e'.

   Edebug does a great deal more than walk with you through a function.
You can set it so it races through on its own, stopping only at an
error or at specified stopping points; you can cause it to display the
changing values of various expressions; you can find out how many times
a function is called, and more.

   Edebug is described in Note: Edebug.