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765 lines
30 KiB
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765 lines
30 KiB
Plaintext
@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1998, 1999
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@c Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/debugging
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@node Debugging, Read and Print, Advising Functions, Top
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@chapter Debugging Lisp Programs
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There are three ways to investigate a problem in an Emacs Lisp program,
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depending on what you are doing with the program when the problem appears.
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@itemize @bullet
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@item
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If the problem occurs when you run the program, you can use a Lisp
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debugger to investigate what is happening during execution. In addition
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to the ordinary debugger, Emacs comes with a source level debugger,
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Edebug. This chapter describes both of them.
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@item
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If the problem is syntactic, so that Lisp cannot even read the program,
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you can use the Emacs facilities for editing Lisp to localize it.
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@item
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If the problem occurs when trying to compile the program with the byte
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compiler, you need to know how to examine the compiler's input buffer.
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@end itemize
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@menu
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* Debugger:: How the Emacs Lisp debugger is implemented.
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* Edebug:: A source-level Emacs Lisp debugger.
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* Syntax Errors:: How to find syntax errors.
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* Compilation Errors:: How to find errors that show up in byte compilation.
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@end menu
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Another useful debugging tool is the dribble file. When a dribble
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file is open, Emacs copies all keyboard input characters to that file.
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Afterward, you can examine the file to find out what input was used.
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@xref{Terminal Input}.
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For debugging problems in terminal descriptions, the
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@code{open-termscript} function can be useful. @xref{Terminal Output}.
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@node Debugger
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@section The Lisp Debugger
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@cindex debugger
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@cindex Lisp debugger
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@cindex break
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The ordinary @dfn{Lisp debugger} provides the ability to suspend
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evaluation of a form. While evaluation is suspended (a state that is
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commonly known as a @dfn{break}), you may examine the run time stack,
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examine the values of local or global variables, or change those values.
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Since a break is a recursive edit, all the usual editing facilities of
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Emacs are available; you can even run programs that will enter the
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debugger recursively. @xref{Recursive Editing}.
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@menu
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* Error Debugging:: Entering the debugger when an error happens.
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* Infinite Loops:: Stopping and debugging a program that doesn't exit.
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* Function Debugging:: Entering it when a certain function is called.
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* Explicit Debug:: Entering it at a certain point in the program.
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* Using Debugger:: What the debugger does; what you see while in it.
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* Debugger Commands:: Commands used while in the debugger.
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* Invoking the Debugger:: How to call the function @code{debug}.
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* Internals of Debugger:: Subroutines of the debugger, and global variables.
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@end menu
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@node Error Debugging
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@subsection Entering the Debugger on an Error
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@cindex error debugging
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@cindex debugging errors
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The most important time to enter the debugger is when a Lisp error
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happens. This allows you to investigate the immediate causes of the
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error.
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However, entry to the debugger is not a normal consequence of an
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error. Many commands frequently cause Lisp errors when invoked
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inappropriately (such as @kbd{C-f} at the end of the buffer), and during
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ordinary editing it would be very inconvenient to enter the debugger
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each time this happens. So if you want errors to enter the debugger, set
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the variable @code{debug-on-error} to non-@code{nil}. (The command
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@code{toggle-debug-on-error} provides an easy way to do this.)
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@defopt debug-on-error
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This variable determines whether the debugger is called when an error is
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signaled and not handled. If @code{debug-on-error} is @code{t}, all
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kinds of errors call the debugger (except those listed in
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@code{debug-ignored-errors}). If it is @code{nil}, none call the
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debugger.
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The value can also be a list of error conditions that should call the
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debugger. For example, if you set it to the list
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@code{(void-variable)}, then only errors about a variable that has no
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value invoke the debugger.
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When this variable is non-@code{nil}, Emacs does not create an error
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handler around process filter functions and sentinels. Therefore,
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errors in these functions also invoke the debugger. @xref{Processes}.
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@end defopt
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@defopt debug-ignored-errors
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This variable specifies certain kinds of errors that should not enter
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the debugger. Its value is a list of error condition symbols and/or
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regular expressions. If the error has any of those condition symbols,
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or if the error message matches any of the regular expressions, then
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that error does not enter the debugger, regardless of the value of
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@code{debug-on-error}.
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The normal value of this variable lists several errors that happen often
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during editing but rarely result from bugs in Lisp programs. However,
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``rarely'' is not ``never''; if your program fails with an error that
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matches this list, you will need to change this list in order to debug
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the error. The easiest way is usually to set
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@code{debug-ignored-errors} to @code{nil}.
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@end defopt
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@defopt debug-on-signal
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Normally, errors that are caught by @code{condition-case} never run the
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debugger, even if @code{debug-on-error} is non-@code{nil}. In other
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words, @code{condition-case} gets a chance to handle the error before
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the debugger gets a chance.
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If you set @code{debug-on-signal} to a non-@code{nil} value, then the
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debugger gets the first chance at every error; an error will invoke the
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debugger regardless of any @code{condition-case}, if it fits the
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criteria specified by the values of @code{debug-on-error} and
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@code{debug-ignored-errors}.
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@strong{Warning:} This variable is strong medicine! Various parts of
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Emacs handle errors in the normal course of affairs, and you may not
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even realize that errors happen there. If you set
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@code{debug-on-signal} to a non-@code{nil} value, those errors will
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enter the debugger.
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@strong{Warning:} @code{debug-on-signal} has no effect when
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@code{debug-on-error} is @code{nil}.
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@end defopt
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To debug an error that happens during loading of the init
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file, use the option @samp{--debug-init}. This binds
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@code{debug-on-error} to @code{t} while loading the init file, and
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bypasses the @code{condition-case} which normally catches errors in the
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init file.
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If your init file sets @code{debug-on-error}, the effect may
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not last past the end of loading the init file. (This is an undesirable
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byproduct of the code that implements the @samp{--debug-init} command
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line option.) The best way to make the init file set
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@code{debug-on-error} permanently is with @code{after-init-hook}, like
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this:
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@example
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(add-hook 'after-init-hook
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(lambda () (setq debug-on-error t)))
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@end example
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@node Infinite Loops
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@subsection Debugging Infinite Loops
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@cindex infinite loops
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@cindex loops, infinite
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@cindex quitting from infinite loop
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@cindex stopping an infinite loop
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When a program loops infinitely and fails to return, your first
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problem is to stop the loop. On most operating systems, you can do this
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with @kbd{C-g}, which causes a @dfn{quit}.
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Ordinary quitting gives no information about why the program was
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looping. To get more information, you can set the variable
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@code{debug-on-quit} to non-@code{nil}. Quitting with @kbd{C-g} is not
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considered an error, and @code{debug-on-error} has no effect on the
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handling of @kbd{C-g}. Likewise, @code{debug-on-quit} has no effect on
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errors.
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Once you have the debugger running in the middle of the infinite loop,
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you can proceed from the debugger using the stepping commands. If you
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step through the entire loop, you will probably get enough information
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to solve the problem.
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@defopt debug-on-quit
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This variable determines whether the debugger is called when @code{quit}
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is signaled and not handled. If @code{debug-on-quit} is non-@code{nil},
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then the debugger is called whenever you quit (that is, type @kbd{C-g}).
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If @code{debug-on-quit} is @code{nil}, then the debugger is not called
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when you quit. @xref{Quitting}.
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@end defopt
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@node Function Debugging
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@subsection Entering the Debugger on a Function Call
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@cindex function call debugging
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@cindex debugging specific functions
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To investigate a problem that happens in the middle of a program, one
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useful technique is to enter the debugger whenever a certain function is
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called. You can do this to the function in which the problem occurs,
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and then step through the function, or you can do this to a function
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called shortly before the problem, step quickly over the call to that
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function, and then step through its caller.
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@deffn Command debug-on-entry function-name
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This function requests @var{function-name} to invoke the debugger each time
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it is called. It works by inserting the form @code{(debug 'debug)} into
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the function definition as the first form.
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Any function defined as Lisp code may be set to break on entry,
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regardless of whether it is interpreted code or compiled code. If the
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function is a command, it will enter the debugger when called from Lisp
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and when called interactively (after the reading of the arguments). You
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can't debug primitive functions (i.e., those written in C) this way.
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When @code{debug-on-entry} is called interactively, it prompts for
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@var{function-name} in the minibuffer. If the function is already set
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up to invoke the debugger on entry, @code{debug-on-entry} does nothing.
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@code{debug-on-entry} always returns @var{function-name}.
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@strong{Note:} if you redefine a function after using
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@code{debug-on-entry} on it, the code to enter the debugger is discarded
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by the redefinition. In effect, redefining the function cancels
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the break-on-entry feature for that function.
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@example
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@group
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(defun fact (n)
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(if (zerop n) 1
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(* n (fact (1- n)))))
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@result{} fact
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@end group
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@group
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(debug-on-entry 'fact)
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@result{} fact
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@end group
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@group
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(fact 3)
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@end group
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@group
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------ Buffer: *Backtrace* ------
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Entering:
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* fact(3)
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eval-region(4870 4878 t)
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byte-code("...")
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eval-last-sexp(nil)
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(let ...)
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eval-insert-last-sexp(nil)
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* call-interactively(eval-insert-last-sexp)
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------ Buffer: *Backtrace* ------
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@end group
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@group
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(symbol-function 'fact)
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@result{} (lambda (n)
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(debug (quote debug))
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(if (zerop n) 1 (* n (fact (1- n)))))
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@end group
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@end example
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@end deffn
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@deffn Command cancel-debug-on-entry function-name
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This function undoes the effect of @code{debug-on-entry} on
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@var{function-name}. When called interactively, it prompts for
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@var{function-name} in the minibuffer. If @var{function-name} is
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@code{nil} or the empty string, it cancels break-on-entry for all
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functions.
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Calling @code{cancel-debug-on-entry} does nothing to a function which is
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not currently set up to break on entry. It always returns
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@var{function-name}.
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@end deffn
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@node Explicit Debug
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@subsection Explicit Entry to the Debugger
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You can cause the debugger to be called at a certain point in your
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program by writing the expression @code{(debug)} at that point. To do
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this, visit the source file, insert the text @samp{(debug)} at the
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proper place, and type @kbd{C-M-x}. @strong{Warning:} if you do this
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for temporary debugging purposes, be sure to undo this insertion before
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you save the file!
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The place where you insert @samp{(debug)} must be a place where an
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additional form can be evaluated and its value ignored. (If the value
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of @code{(debug)} isn't ignored, it will alter the execution of the
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program!) The most common suitable places are inside a @code{progn} or
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an implicit @code{progn} (@pxref{Sequencing}).
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@node Using Debugger
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@subsection Using the Debugger
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When the debugger is entered, it displays the previously selected
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buffer in one window and a buffer named @samp{*Backtrace*} in another
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window. The backtrace buffer contains one line for each level of Lisp
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function execution currently going on. At the beginning of this buffer
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is a message describing the reason that the debugger was invoked (such
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as the error message and associated data, if it was invoked due to an
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error).
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The backtrace buffer is read-only and uses a special major mode,
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Debugger mode, in which letters are defined as debugger commands. The
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usual Emacs editing commands are available; thus, you can switch windows
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to examine the buffer that was being edited at the time of the error,
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switch buffers, visit files, or do any other sort of editing. However,
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the debugger is a recursive editing level (@pxref{Recursive Editing})
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and it is wise to go back to the backtrace buffer and exit the debugger
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(with the @kbd{q} command) when you are finished with it. Exiting
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the debugger gets out of the recursive edit and kills the backtrace
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buffer.
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@cindex current stack frame
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The backtrace buffer shows you the functions that are executing and
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their argument values. It also allows you to specify a stack frame by
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moving point to the line describing that frame. (A stack frame is the
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place where the Lisp interpreter records information about a particular
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invocation of a function.) The frame whose line point is on is
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considered the @dfn{current frame}. Some of the debugger commands
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operate on the current frame.
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If a function name is underlined, that means the debugger knows
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where its source code is located. You can click @kbd{Mouse-2} on that
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name, or move to it and type @key{RET}, to visit the source code.
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The debugger itself must be run byte-compiled, since it makes
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assumptions about how many stack frames are used for the debugger
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itself. These assumptions are false if the debugger is running
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interpreted.
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@need 3000
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@node Debugger Commands
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@subsection Debugger Commands
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@cindex debugger command list
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The debugger buffer (in Debugger mode) provides special commands in
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addition to the usual Emacs commands. The most important use of
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debugger commands is for stepping through code, so that you can see
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how control flows. The debugger can step through the control
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structures of an interpreted function, but cannot do so in a
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byte-compiled function. If you would like to step through a
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byte-compiled function, replace it with an interpreted definition of
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the same function. (To do this, visit the source for the function and
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type @kbd{C-M-x} on its definition.)
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Here is a list of Debugger mode commands:
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@table @kbd
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@item c
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Exit the debugger and continue execution. When continuing is possible,
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it resumes execution of the program as if the debugger had never been
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entered (aside from any side-effects that you caused by changing
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variable values or data structures while inside the debugger).
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Continuing is possible after entry to the debugger due to function entry
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or exit, explicit invocation, or quitting. You cannot continue if the
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debugger was entered because of an error.
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@item d
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Continue execution, but enter the debugger the next time any Lisp
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function is called. This allows you to step through the
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subexpressions of an expression, seeing what values the subexpressions
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compute, and what else they do.
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The stack frame made for the function call which enters the debugger in
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this way will be flagged automatically so that the debugger will be
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called again when the frame is exited. You can use the @kbd{u} command
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to cancel this flag.
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@item b
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Flag the current frame so that the debugger will be entered when the
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frame is exited. Frames flagged in this way are marked with stars
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in the backtrace buffer.
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@item u
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Don't enter the debugger when the current frame is exited. This
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cancels a @kbd{b} command on that frame. The visible effect is to
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remove the star from the line in the backtrace buffer.
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@item e
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Read a Lisp expression in the minibuffer, evaluate it, and print the
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value in the echo area. The debugger alters certain important
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variables, and the current buffer, as part of its operation; @kbd{e}
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temporarily restores their values from outside the debugger, so you can
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examine and change them. This makes the debugger more transparent. By
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contrast, @kbd{M-:} does nothing special in the debugger; it shows you
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the variable values within the debugger.
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@item R
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Like @kbd{e}, but also save the result of evaluation in the
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buffer @samp{*Debugger-record*}.
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@item q
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Terminate the program being debugged; return to top-level Emacs
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command execution.
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If the debugger was entered due to a @kbd{C-g} but you really want
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to quit, and not debug, use the @kbd{q} command.
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@item r
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Return a value from the debugger. The value is computed by reading an
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expression with the minibuffer and evaluating it.
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The @kbd{r} command is useful when the debugger was invoked due to exit
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from a Lisp call frame (as requested with @kbd{b} or by entering the
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frame with @kbd{d}); then the value specified in the @kbd{r} command is
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used as the value of that frame. It is also useful if you call
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@code{debug} and use its return value. Otherwise, @kbd{r} has the same
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effect as @kbd{c}, and the specified return value does not matter.
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You can't use @kbd{r} when the debugger was entered due to an error.
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@end table
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@node Invoking the Debugger
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@subsection Invoking the Debugger
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Here we describe in full detail the function @code{debug} that is used
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to invoke the debugger.
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@defun debug &rest debugger-args
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This function enters the debugger. It switches buffers to a buffer
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named @samp{*Backtrace*} (or @samp{*Backtrace*<2>} if it is the second
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recursive entry to the debugger, etc.), and fills it with information
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about the stack of Lisp function calls. It then enters a recursive
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edit, showing the backtrace buffer in Debugger mode.
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The Debugger mode @kbd{c} and @kbd{r} commands exit the recursive edit;
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then @code{debug} switches back to the previous buffer and returns to
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whatever called @code{debug}. This is the only way the function
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@code{debug} can return to its caller.
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The use of the @var{debugger-args} is that @code{debug} displays the
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rest of its arguments at the top of the @samp{*Backtrace*} buffer, so
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that the user can see them. Except as described below, this is the
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@emph{only} way these arguments are used.
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However, certain values for first argument to @code{debug} have a
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special significance. (Normally, these values are used only by the
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internals of Emacs, and not by programmers calling @code{debug}.) Here
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is a table of these special values:
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@table @code
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@item lambda
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@cindex @code{lambda} in debug
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A first argument of @code{lambda} means @code{debug} was called because
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of entry to a function when @code{debug-on-next-call} was
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non-@code{nil}. The debugger displays @samp{Entering:} as a line of
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text at the top of the buffer.
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@item debug
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@code{debug} as first argument indicates a call to @code{debug} because
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of entry to a function that was set to debug on entry. The debugger
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displays @samp{Entering:}, just as in the @code{lambda} case. It also
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marks the stack frame for that function so that it will invoke the
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debugger when exited.
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@item t
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When the first argument is @code{t}, this indicates a call to
|
|
@code{debug} due to evaluation of a list form when
|
|
@code{debug-on-next-call} is non-@code{nil}. The debugger displays the
|
|
following as the top line in the buffer:
|
|
|
|
@smallexample
|
|
Beginning evaluation of function call form:
|
|
@end smallexample
|
|
|
|
@item exit
|
|
When the first argument is @code{exit}, it indicates the exit of a stack
|
|
frame previously marked to invoke the debugger on exit. The second
|
|
argument given to @code{debug} in this case is the value being returned
|
|
from the frame. The debugger displays @samp{Return value:} in the top
|
|
line of the buffer, followed by the value being returned.
|
|
|
|
@item error
|
|
@cindex @code{error} in debug
|
|
When the first argument is @code{error}, the debugger indicates that
|
|
it is being entered because an error or @code{quit} was signaled and not
|
|
handled, by displaying @samp{Signaling:} followed by the error signaled
|
|
and any arguments to @code{signal}. For example,
|
|
|
|
@example
|
|
@group
|
|
(let ((debug-on-error t))
|
|
(/ 1 0))
|
|
@end group
|
|
|
|
@group
|
|
------ Buffer: *Backtrace* ------
|
|
Signaling: (arith-error)
|
|
/(1 0)
|
|
...
|
|
------ Buffer: *Backtrace* ------
|
|
@end group
|
|
@end example
|
|
|
|
If an error was signaled, presumably the variable
|
|
@code{debug-on-error} is non-@code{nil}. If @code{quit} was signaled,
|
|
then presumably the variable @code{debug-on-quit} is non-@code{nil}.
|
|
|
|
@item nil
|
|
Use @code{nil} as the first of the @var{debugger-args} when you want
|
|
to enter the debugger explicitly. The rest of the @var{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 @code{debug} is called.
|
|
@end table
|
|
@end defun
|
|
|
|
@node Internals of Debugger
|
|
@subsection Internals of the Debugger
|
|
|
|
This section describes functions and variables used internally by the
|
|
debugger.
|
|
|
|
@defvar 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
|
|
@code{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 @code{debug}.
|
|
@end defvar
|
|
|
|
@deffn Command backtrace
|
|
@cindex run time stack
|
|
@cindex call stack
|
|
This function prints a trace of Lisp function calls currently active.
|
|
This is the function used by @code{debug} to fill up the
|
|
@samp{*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 @code{nil}.
|
|
|
|
In the following example, a Lisp expression calls @code{backtrace}
|
|
explicitly. This prints the backtrace to the stream
|
|
@code{standard-output}, which, in this case, is the buffer
|
|
@samp{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.
|
|
|
|
@smallexample
|
|
@group
|
|
(with-output-to-temp-buffer "backtrace-output"
|
|
(let ((var 1))
|
|
(save-excursion
|
|
(setq var (eval '(progn
|
|
(1+ var)
|
|
(list 'testing (backtrace))))))))
|
|
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
----------- Buffer: backtrace-output ------------
|
|
backtrace()
|
|
(list ...computing arguments...)
|
|
@end group
|
|
(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 ...")
|
|
@group
|
|
eval-print-last-sexp(nil)
|
|
* call-interactively(eval-print-last-sexp)
|
|
----------- Buffer: backtrace-output ------------
|
|
@end group
|
|
@end smallexample
|
|
|
|
The character @samp{*} indicates a frame whose debug-on-exit flag is
|
|
set.
|
|
@end deffn
|
|
|
|
@ignore @c Not worth mentioning
|
|
@defopt stack-trace-on-error
|
|
@cindex stack trace
|
|
This variable controls whether Lisp automatically displays a
|
|
backtrace buffer after every error that is not handled. A quit signal
|
|
counts as an error for this variable. If it is non-@code{nil} then a
|
|
backtrace is shown in a pop-up buffer named @samp{*Backtrace*} on every
|
|
error. If it is @code{nil}, then a backtrace is not shown.
|
|
|
|
When a backtrace is shown, that buffer is not selected. If either
|
|
@code{debug-on-quit} or @code{debug-on-error} is also non-@code{nil}, then
|
|
a backtrace is shown in one buffer, and the debugger is popped up in
|
|
another buffer with its own backtrace.
|
|
|
|
We consider this feature to be obsolete and superseded by the debugger
|
|
itself.
|
|
@end defopt
|
|
@end ignore
|
|
|
|
@defvar debug-on-next-call
|
|
@cindex @code{eval}, and debugging
|
|
@cindex @code{apply}, and debugging
|
|
@cindex @code{funcall}, and debugging
|
|
If this variable is non-@code{nil}, it says to call the debugger before
|
|
the next @code{eval}, @code{apply} or @code{funcall}. Entering the
|
|
debugger sets @code{debug-on-next-call} to @code{nil}.
|
|
|
|
The @kbd{d} command in the debugger works by setting this variable.
|
|
@end defvar
|
|
|
|
@defun backtrace-debug level flag
|
|
This function sets the debug-on-exit flag of the stack frame @var{level}
|
|
levels down the stack, giving it the value @var{flag}. If @var{flag} is
|
|
non-@code{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.
|
|
@end defun
|
|
|
|
@defvar 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 @code{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.
|
|
@end defvar
|
|
|
|
@defun backtrace-frame frame-number
|
|
The function @code{backtrace-frame} is intended for use in Lisp
|
|
debuggers. It returns information about what computation is happening
|
|
in the stack frame @var{frame-number} levels down.
|
|
|
|
If that frame has not evaluated the arguments yet, or is a special
|
|
form, the value is @code{(nil @var{function} @var{arg-forms}@dots{})}.
|
|
|
|
If that frame has evaluated its arguments and called its function
|
|
already, the return value is @code{(t @var{function}
|
|
@var{arg-values}@dots{})}.
|
|
|
|
In the return value, @var{function} is whatever was supplied as the
|
|
@sc{car} of the evaluated list, or a @code{lambda} expression in the
|
|
case of a macro call. If the function has a @code{&rest} argument, that
|
|
is represented as the tail of the list @var{arg-values}.
|
|
|
|
If @var{frame-number} is out of range, @code{backtrace-frame} returns
|
|
@code{nil}.
|
|
@end defun
|
|
|
|
@include edebug.texi
|
|
|
|
@node Syntax Errors
|
|
@section 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 @kbd{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.)
|
|
|
|
@menu
|
|
* Excess Open:: How to find a spurious open paren or missing close.
|
|
* Excess Close:: How to find a spurious close paren or missing open.
|
|
@end menu
|
|
|
|
@node Excess Open
|
|
@subsection 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 @kbd{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 @kbd{C-M-q}
|
|
and see what moves. @strong{But don't do this yet!} Keep reading,
|
|
first.
|
|
|
|
Before you do this, make sure the defun has enough close parentheses.
|
|
Otherwise, @kbd{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 @kbd{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 @kbd{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 @kbd{C-M-q} with @kbd{C-_}, since the old
|
|
indentation is probably appropriate to the intended parentheses.
|
|
|
|
After you think you have fixed the problem, use @kbd{C-M-q} again. If
|
|
the old indentation actually fit the intended nesting of parentheses,
|
|
and you have put back those parentheses, @kbd{C-M-q} should not change
|
|
anything.
|
|
|
|
@node Excess Close
|
|
@subsection Excess Close Parentheses
|
|
|
|
To deal with an excess close parenthesis, first go to the beginning of
|
|
the file, then type @kbd{C-u -1 C-M-u} to find the end of the unbalanced
|
|
defun.
|
|
|
|
Then find the actual matching close parenthesis by typing @kbd{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 @kbd{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 @kbd{C-M-q} with @kbd{C-_}, since the
|
|
old indentation is probably appropriate to the intended parentheses.
|
|
|
|
After you think you have fixed the problem, use @kbd{C-M-q} again. If
|
|
the old indentation actually fits the intended nesting of parentheses,
|
|
and you have put back those parentheses, @kbd{C-M-q} should not change
|
|
anything.
|
|
|
|
@node Compilation Errors
|
|
@section 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 @samp{*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 @w{@samp{ *Compiler Input*}}.
|
|
(Note that the buffer name starts with a space, so it does not show
|
|
up in @kbd{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 @emph{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.
|