Merge r21-2-24-utf-2000-0_13-0.

[chise/xemacs-chise.git-] / info / lispref.info-14

This is ../info/lispref.info, produced by makeinfo version 3.12s from
lispref/lispref.texi.

INFO-DIR-SECTION XEmacs Editor
START-INFO-DIR-ENTRY
* Lispref: (lispref).           XEmacs Lisp Reference Manual.
END-INFO-DIR-ENTRY

   Edition History:

   GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU
Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid
Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994
XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995
GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp
Programmer's Manual (for 19.13) Third Edition, July 1995 XEmacs Lisp
Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp
Reference Manual (for 19.15 and 20.1, 20.2, 20.3) v3.2, April, May,
November 1997 XEmacs Lisp Reference Manual (for 21.0) v3.3, April 1998

   Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software
Foundation, Inc.  Copyright (C) 1994, 1995 Sun Microsystems, Inc.
Copyright (C) 1995, 1996 Ben Wing.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU General Public License" is included
exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the section entitled "GNU General Public License"
may be included in a translation approved by the Free Software
Foundation instead of in the original English.

\1f
File: lispref.info,  Node: Trapping Errors,  Next: Edebug Views,  Prev: Breakpoints,  Up: Edebug

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 *Note Edebug Misc::).

   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.

\1f
File: lispref.info,  Node: Edebug Views,  Next: Edebug Eval,  Prev: Trapping Errors,  Up: Edebug

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.

\1f
File: lispref.info,  Node: Edebug Eval,  Next: Eval List,  Prev: Edebug Views,  Up: Edebug

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 *Note The Outside Context:: for details on this
process.  Also see *Note Reading in Edebug:: and *Note 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'.

\1f
File: lispref.info,  Node: Eval List,  Next: Reading in Edebug,  Prev: Edebug Eval,  Up: Edebug

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.

\1f
File: lispref.info,  Node: Reading in Edebug,  Next: Printing in Edebug,  Prev: Eval List,  Up: Edebug

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.

\1f
File: lispref.info,  Node: Printing in Edebug,  Next: Tracing,  Prev: Reading in Edebug,  Up: Edebug

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.

\1f
File: lispref.info,  Node: Tracing,  Next: Coverage Testing,  Prev: Printing in Edebug,  Up: Edebug

Tracing
-------

   In addition to automatic stepping through source code, which is also
called _tracing_ (see *Note 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.)

\1f
File: lispref.info,  Node: Coverage Testing,  Next: The Outside Context,  Prev: Tracing,  Up: Edebug

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.

\1f
File: lispref.info,  Node: The Outside Context,  Next: Instrumenting Macro Calls,  Prev: Coverage Testing,  Up: Edebug

The Outside Context
-------------------

   Edebug tries to be transparent to the program you are debugging.  In
addition, most evaluations you do within Edebug (see *Note Edebug
Eval::) 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.

* Menu:

* Checking Whether to Stop::    When Edebug decides what to do.
* Edebug Display Update::       When Edebug updates the display.
* Edebug Recursive Edit::       When Edebug stops execution.

\1f
File: lispref.info,  Node: Checking Whether to Stop,  Next: Edebug Display Update,  Up: The Outside Context

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'.


\1f
File: lispref.info,  Node: Edebug Display Update,  Next: Edebug Recursive Edit,  Prev: Checking Whether to Stop,  Up: The Outside Context

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.


\1f
File: lispref.info,  Node: Edebug Recursive Edit,  Prev: Edebug Display Update,  Up: The Outside Context

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'.


\1f
File: lispref.info,  Node: Instrumenting Macro Calls,  Next: Edebug Options,  Prev: The Outside Context,  Up: Edebug

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.

* Menu:

* Specification List::          How to specify complex patterns of evaluation.
* Backtracking::                What Edebug does when matching fails.
* Debugging Backquote:: Debugging Backquote
* Specification Examples::      To help understand specifications.

\1f
File: lispref.info,  Node: Specification List,  Next: Backtracking,  Up: Instrumenting Macro Calls

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 *Note
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 *Note 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...)'.

\1f
File: lispref.info,  Node: Backtracking,  Next: Debugging Backquote,  Prev: Specification List,  Up: Instrumenting Macro Calls

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.

\1f
File: lispref.info,  Node: Debugging Backquote,  Next: Specification Examples,  Prev: Backtracking,  Up: Instrumenting Macro Calls

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 (*note Problems with Macros: ()Problems
with Macros.) 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.

\1f
File: lispref.info,  Node: Specification Examples,  Prev: Debugging Backquote,  Up: Instrumenting Macro Calls

Specification Examples
......................

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

   A `let' special form 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))

\1f
File: lispref.info,  Node: Edebug Options,  Prev: Instrumenting Macro Calls,  Up: Edebug

Edebug Options
--------------

   These options affect the behavior of Edebug:

 - User Option: edebug-setup-hook
     Functions to call before Edebug is used.  Each time it is set to a
     new value, Edebug will call those functions once and then
     `edebug-setup-hook' is reset 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.  See *Note Instrumenting::.

 - User Option: edebug-all-defs
     If non-`nil', normal evaluation of any defining forms (e.g.
     `defun' and `defmacro') will instrument them for Edebug.  This
     applies to `eval-defun', `eval-region', and `eval-current-buffer'.

     Use the command `M-x edebug-all-defs' to toggle the value of this
     variable. You may want to make this variable local to each buffer
     by calling `(make-local-variable 'edebug-all-defs)' in your
     `emacs-lisp-mode-hook'.  See *Note Instrumenting::.

 - User Option: edebug-all-forms
     If non-`nil', normal evaluation of any forms by `eval-defun',
     `eval-region', and `eval-current-buffer' will instrument them for
     Edebug.

     Use the command `M-x edebug-all-forms' to toggle the value of this
     option.  See *Note Instrumenting::.

 - User Option: edebug-save-windows
     If non-`nil', save and restore window configuration on Edebug
     calls.  It takes some time to do this, so if your program does not
     care what happens to data about windows, you may want to set this
     variable to `nil'.

     If the value is a list, only the listed windows are saved and
     restored.

     `M-x edebug-toggle-save-windows' may be used to change this
     variable.  This command is bound to `W' in source code buffers.
     See *Note Edebug Display Update::.

 - User Option: edebug-save-displayed-buffer-points
     If non-`nil', save and restore point in all displayed buffers.
     This is necessary if you are debugging code that changes the point
     of a buffer which is displayed in a non-selected window.  If
     Edebug or the user then selects the window, the buffer's point
     will be changed to the window's point.

     This is an expensive operation since it visits each window and
     therefore each displayed buffer twice for each Edebug activation,
     so it is best to avoid it if you can.  See *Note Edebug Display
     Update::.

 - User Option: edebug-initial-mode
     If this variable is non-`nil', it specifies the initial execution
     mode for Edebug when it is first activated.  Possible values are
     `step', `next', `go', `Go-nonstop', `trace', `Trace-fast',
     `continue', and `Continue-fast'.

     The default value is `step'.  See *Note Edebug Execution Modes::.

 - User Option: edebug-trace
     Non-`nil' means display a trace of function entry and exit.
     Tracing output is displayed in a buffer named `*edebug-trace*', one
     function entry or exit per line, indented by the recursion level.

     The default value is `nil'.

     Also see `edebug-tracing'.  See *Note Tracing::.

 - User Option: edebug-test-coverage
     If non-`nil', Edebug tests coverage of all expressions debugged.
     This is done by comparing the result of each expression with the
     previous result. Coverage is considered OK if two different
     results are found.  So to sufficiently test the coverage of your
     code, try to execute it under conditions that evaluate all
     expressions more than once, and produce different results for each
     expression.

     Use `M-x edebug-display-freq-count' to display the frequency count
     and coverage information for a definition.  See *Note Coverage
     Testing::.

 - User Option: edebug-continue-kbd-macro
     If non-`nil', continue defining or executing any keyboard macro
     that is executing outside of Edebug.   Use this with caution since
     it is not debugged.  See *Note Edebug Execution Modes::.

 - User Option: edebug-print-length
     If non-`nil', bind `print-length' to this while printing results
     in Edebug.  The default value is `50'.  See *Note Printing in
     Edebug::.

 - User Option: edebug-print-level
     If non-`nil', bind `print-level' to this while printing results in
     Edebug.  The default value is `50'.

 - User Option: edebug-print-circle
     If non-`nil', bind `print-circle' to this while printing results
     in Edebug.  The default value is `nil'.

 - User Option: edebug-on-error
     `debug-on-error' is bound to this while Edebug is active.  See
     *Note Trapping Errors::.

 - User Option: edebug-on-quit
     `debug-on-quit' is bound to this while Edebug is active.  See
     *Note Trapping Errors::.

 - User Option: edebug-unwrap-results
     Non-`nil' if Edebug should unwrap results of expressions.  This is
     useful when debugging macros where the results of expressions are
     instrumented expressions.  But don't do this when results might be
     circular or an infinite loop will result.  See *Note Debugging
     Backquote::.

 - User Option: edebug-global-break-condition
     If non-`nil', an expression to test for at every stop point.  If
     the result is non-nil, then break.  Errors are ignored.  See *Note
     Global Break Condition::.

\1f
File: lispref.info,  Node: Read and Print,  Next: Minibuffers,  Prev: Debugging,  Up: Top

Reading and Printing Lisp Objects
*********************************

   "Printing" and "reading" are the operations of converting Lisp
objects to textual form and vice versa.  They use the printed
representations and read syntax described in *Note Lisp Data Types::.

   This chapter describes the Lisp functions for reading and printing.
It also describes "streams", which specify where to get the text (if
reading) or where to put it (if printing).

* Menu:

* Streams Intro::     Overview of streams, reading and printing.
* Input Streams::     Various data types that can be used as input streams.
* Input Functions::   Functions to read Lisp objects from text.
* Output Streams::    Various data types that can be used as output streams.
* Output Functions::  Functions to print Lisp objects as text.
* Output Variables::  Variables that control what the printing functions do.

\1f
File: lispref.info,  Node: Streams Intro,  Next: Input Streams,  Up: Read and Print

Introduction to Reading and Printing
====================================

   "Reading" a Lisp object means parsing a Lisp expression in textual
form and producing a corresponding Lisp object.  This is how Lisp
programs get into Lisp from files of Lisp code.  We call the text the
"read syntax" of the object.  For example, the text `(a . 5)' is the
read syntax for a cons cell whose CAR is `a' and whose CDR is the
number 5.

   "Printing" a Lisp object means producing text that represents that
object--converting the object to its printed representation.  Printing
the cons cell described above produces the text `(a . 5)'.

   Reading and printing are more or less inverse operations: printing
the object that results from reading a given piece of text often
produces the same text, and reading the text that results from printing
an object usually produces a similar-looking object.  For example,
printing the symbol `foo' produces the text `foo', and reading that text
returns the symbol `foo'.  Printing a list whose elements are `a' and
`b' produces the text `(a b)', and reading that text produces a list
(but not the same list) with elements `a' and `b'.

   However, these two operations are not precisely inverses.  There are
three kinds of exceptions:

   * Printing can produce text that cannot be read.  For example,
     buffers, windows, frames, subprocesses and markers print into text
     that starts with `#'; if you try to read this text, you get an
     error.  There is no way to read those data types.

   * One object can have multiple textual representations.  For example,
     `1' and `01' represent the same integer, and `(a b)' and `(a .
     (b))' represent the same list.  Reading will accept any of the
     alternatives, but printing must choose one of them.

   * Comments can appear at certain points in the middle of an object's
     read sequence without affecting the result of reading it.