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.