(Handling Errors): Document `debug' in handler list.

2 files changed, 38 insertions, 15 deletions

diff --git a/lispref/ChangeLog b/lispref/ChangeLog
index 61bc10c1afc..6a824f8deaf 100644
--- a/lispref/ChangeLog
+++ b/lispref/ChangeLog
@@ -1,3 +1,7 @@
+2007-07-14  Richard Stallman  <rms@gnu.org>
+
+        * control.texi (Handling Errors): Document `debug' in handler list.
+
 2007-07-09  Richard Stallman  <rms@gnu.org>
 
         * files.texi (Magic File Names): Rewrite previous change.
diff --git a/lispref/control.texi b/lispref/control.texi
index 4c469a10368..e99a6329f3e 100644
--- a/lispref/control.texi
+++ b/lispref/control.texi
@@ -893,6 +893,12 @@ establishing an error handler, with the special form
 This deletes the file named @var{filename}, catching any error and
 returning @code{nil} if an error occurs.
 
+  The @code{condition-case} construct is often used to trap errors that
+are predictable, such as failure to open a file in a call to
+@code{insert-file-contents}.  It is also used to trap errors that are
+totally unpredictable, such as when the program evaluates an expression
+read from the user.
+
   The second argument of @code{condition-case} is called the
 @dfn{protected form}.  (In the example above, the protected form is a
 call to @code{delete-file}.)  The error handlers go into effect when
@@ -920,15 +926,33 @@ the two gets to handle it.
   If an error is handled by some @code{condition-case} form, this
 ordinarily prevents the debugger from being run, even if
 @code{debug-on-error} says this error should invoke the debugger.
-@xref{Error Debugging}.  If you want to be able to debug errors that are
-caught by a @code{condition-case}, set the variable
-@code{debug-on-signal} to a non-@code{nil} value.
 
-  When an error is handled, control returns to the handler.  Before this
-happens, Emacs unbinds all variable bindings made by binding constructs
-that are being exited and executes the cleanups of all
-@code{unwind-protect} forms that are exited.  Once control arrives at
-the handler, the body of the handler is executed.
+  If you want to be able to debug errors that are caught by a
+@code{condition-case}, set the variable @code{debug-on-signal} to a
+non-@code{nil} value.  You can also specify that a particular handler
+should let the debugger run first, by writing @code{debug} among the
+conditions, like this:
+
+@example
+@group
+(condition-case nil
+    (delete-file filename)
+  ((debug error) nil))
+@end group
+@end example
+
+@noindent
+The effect of @code{debug} here is only to prevent
+@code{condition-case} from suppressing the call to the debugger.  Any
+given error will invoke the debugger only if @code{debug-on-error} and
+the other usual filtering mechanisms say it should.  @xref{Error Debugging}.
+
+  Once Emacs decides that a certain handler handles the error, it
+returns control to that handler.  To do so, Emacs unbinds all variable
+bindings made by binding constructs that are being exited, and
+executes the cleanups of all @code{unwind-protect} forms that are
+being exited.  Once control arrives at the handler, the body of the
+handler executes normally.
 
   After execution of the handler body, execution returns from the
 @code{condition-case} form.  Because the protected form is exited
@@ -937,12 +961,6 @@ execution at the point of the error, nor can it examine variable
 bindings that were made within the protected form.  All it can do is
 clean up and proceed.
 
-  The @code{condition-case} construct is often used to trap errors that
-are predictable, such as failure to open a file in a call to
-@code{insert-file-contents}.  It is also used to trap errors that are
-totally unpredictable, such as when the program evaluates an expression
-read from the user.
-
   Error signaling and handling have some resemblance to @code{throw} and
 @code{catch} (@pxref{Catch and Throw}), but they are entirely separate
 facilities.  An error cannot be caught by a @code{catch}, and a
@@ -960,7 +978,8 @@ error occurs during @var{protected-form}.
 
 Each of the @var{handlers} is a list of the form @code{(@var{conditions}
 @var{body}@dots{})}.  Here @var{conditions} is an error condition name
-to be handled, or a list of condition names; @var{body} is one or more
+to be handled, or a list of condition names (which can include @code{debug}
+to allow the debugger to run before the handler); @var{body} is one or more
 Lisp expressions to be executed when this handler handles an error.
 Here are examples of handlers: