Improve documentation of byte-code objects

[gnu-emacs] / doc / lispref / compile.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1994, 2001-2016 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Byte Compilation
@chapter Byte Compilation
@cindex byte compilation
@cindex byte-code
@cindex compilation (Emacs Lisp)

  Emacs Lisp has a @dfn{compiler} that translates functions written
in Lisp into a special representation called @dfn{byte-code} that can be
executed more efficiently.  The compiler replaces Lisp function
definitions with byte-code.  When a byte-code function is called, its
definition is evaluated by the @dfn{byte-code interpreter}.

  Because the byte-compiled code is evaluated by the byte-code
interpreter, instead of being executed directly by the machine's
hardware (as true compiled code is), byte-code is completely
transportable from machine to machine without recompilation.  It is not,
however, as fast as true compiled code.

  In general, any version of Emacs can run byte-compiled code produced
by recent earlier versions of Emacs, but the reverse is not true.

@vindex no-byte-compile
  If you do not want a Lisp file to be compiled, ever, put a file-local
variable binding for @code{no-byte-compile} into it, like this:

@example
;; -*-no-byte-compile: t; -*-
@end example

@menu
* Speed of Byte-Code::          An example of speedup from byte compilation.
* Compilation Functions::       Byte compilation functions.
* Docs and Compilation::        Dynamic loading of documentation strings.
* Dynamic Loading::             Dynamic loading of individual functions.
* Eval During Compile::         Code to be evaluated when you compile.
* Compiler Errors::             Handling compiler error messages.
* Byte-Code Objects::           The data type used for byte-compiled functions.
* Disassembly::                 Disassembling byte-code; how to read byte-code.
@end menu

@node Speed of Byte-Code
@section Performance of Byte-Compiled Code

  A byte-compiled function is not as efficient as a primitive function
written in C, but runs much faster than the version written in Lisp.
Here is an example:

@example
@group
(defun silly-loop (n)
  "Return the time, in seconds, to run N iterations of a loop."
  (let ((t1 (float-time)))
    (while (> (setq n (1- n)) 0))
    (- (float-time) t1)))
@result{} silly-loop
@end group

@group
(silly-loop 50000000)
@result{} 10.235304117202759
@end group

@group
(byte-compile 'silly-loop)
@result{} @r{[Compiled code not shown]}
@end group

@group
(silly-loop 50000000)
@result{} 3.705854892730713
@end group
@end example

  In this example, the interpreted code required 10 seconds to run,
whereas the byte-compiled code required less than 4 seconds.  These
results are representative, but actual results may vary.

@node Compilation Functions
@section Byte-Compilation Functions
@cindex compilation functions

  You can byte-compile an individual function or macro definition with
the @code{byte-compile} function.  You can compile a whole file with
@code{byte-compile-file}, or several files with
@code{byte-recompile-directory} or @code{batch-byte-compile}.

  Sometimes, the byte compiler produces warning and/or error messages
(@pxref{Compiler Errors}, for details).  These messages are recorded
in a buffer called @file{*Compile-Log*}, which uses Compilation mode.
@xref{Compilation Mode,,,emacs, The GNU Emacs Manual}.

@cindex macro compilation
  Be careful when writing macro calls in files that you intend to
byte-compile.  Since macro calls are expanded when they are compiled,
the macros need to be loaded into Emacs or the byte compiler will not
do the right thing.  The usual way to handle this is with
@code{require} forms which specify the files containing the needed
macro definitions (@pxref{Named Features}).  Normally, the
byte compiler does not evaluate the code that it is compiling, but it
handles @code{require} forms specially, by loading the specified
libraries.  To avoid loading the macro definition files when someone
@emph{runs} the compiled program, write @code{eval-when-compile}
around the @code{require} calls (@pxref{Eval During Compile}).  For
more details, @xref{Compiling Macros}.

  Inline (@code{defsubst}) functions are less troublesome; if you
compile a call to such a function before its definition is known, the
call will still work right, it will just run slower.

@defun byte-compile symbol
This function byte-compiles the function definition of @var{symbol},
replacing the previous definition with the compiled one.  The function
definition of @var{symbol} must be the actual code for the function;
@code{byte-compile} does not handle function indirection.  The return
value is the byte-code function object which is the compiled
definition of @var{symbol} (@pxref{Byte-Code Objects}).

@example
@group
(defun factorial (integer)
  "Compute factorial of INTEGER."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
@result{} factorial
@end group

@group
(byte-compile 'factorial)
@result{}
#[(integer)
  "^H\301U\203^H^@@\301\207\302^H\303^HS!\"\207"
  [integer 1 * factorial]
  4 "Compute factorial of INTEGER."]
@end group
@end example

If @var{symbol}'s definition is a byte-code function object,
@code{byte-compile} does nothing and returns @code{nil}.  It does not
compile the symbol's definition again, since the original
(non-compiled) code has already been replaced in the symbol's function
cell by the byte-compiled code.

The argument to @code{byte-compile} can also be a @code{lambda}
expression.  In that case, the function returns the corresponding
compiled code but does not store it anywhere.
@end defun

@deffn Command compile-defun &optional arg
This command reads the defun containing point, compiles it, and
evaluates the result.  If you use this on a defun that is actually a
function definition, the effect is to install a compiled version of that
function.

@code{compile-defun} normally displays the result of evaluation in the
echo area, but if @var{arg} is non-@code{nil}, it inserts the result
in the current buffer after the form it compiled.
@end deffn

@deffn Command byte-compile-file filename &optional load
This function compiles a file of Lisp code named @var{filename} into a
file of byte-code.  The output file's name is made by changing the
@samp{.el} suffix into @samp{.elc}; if @var{filename} does not end in
@samp{.el}, it adds @samp{.elc} to the end of @var{filename}.

Compilation works by reading the input file one form at a time.  If it
is a definition of a function or macro, the compiled function or macro
definition is written out.  Other forms are batched together, then each
batch is compiled, and written so that its compiled code will be
executed when the file is read.  All comments are discarded when the
input file is read.

This command returns @code{t} if there were no errors and @code{nil}
otherwise.  When called interactively, it prompts for the file name.

If @var{load} is non-@code{nil}, this command loads the compiled file
after compiling it.  Interactively, @var{load} is the prefix argument.

@example
@group
$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el
@end group

@group
(byte-compile-file "~/emacs/push.el")
     @result{} t
@end group

@group
$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el
-rw-rw-rw- 1 lewis lewis 638 Oct  8 20:25 push.elc
@end group
@end example
@end deffn

@deffn Command byte-recompile-directory directory &optional flag force
@cindex library compilation
This command recompiles every @samp{.el} file in @var{directory} (or
its subdirectories) that needs recompilation.  A file needs
recompilation if a @samp{.elc} file exists but is older than the
@samp{.el} file.

When a @samp{.el} file has no corresponding @samp{.elc} file,
@var{flag} says what to do.  If it is @code{nil}, this command ignores
these files.  If @var{flag} is 0, it compiles them.  If it is neither
@code{nil} nor 0, it asks the user whether to compile each such file,
and asks about each subdirectory as well.

Interactively, @code{byte-recompile-directory} prompts for
@var{directory} and @var{flag} is the prefix argument.

If @var{force} is non-@code{nil}, this command recompiles every
@samp{.el} file that has a @samp{.elc} file.

The returned value is unpredictable.
@end deffn

@defun batch-byte-compile &optional noforce
This function runs @code{byte-compile-file} on files specified on the
command line.  This function must be used only in a batch execution of
Emacs, as it kills Emacs on completion.  An error in one file does not
prevent processing of subsequent files, but no output file will be
generated for it, and the Emacs process will terminate with a nonzero
status code.

If @var{noforce} is non-@code{nil}, this function does not recompile
files that have an up-to-date @samp{.elc} file.

@example
$ emacs -batch -f batch-byte-compile *.el
@end example
@end defun

@node Docs and Compilation
@section Documentation Strings and Compilation
@cindex dynamic loading of documentation

  When Emacs loads functions and variables from a byte-compiled file,
it normally does not load their documentation strings into memory.
Each documentation string is dynamically loaded from the
byte-compiled file only when needed.  This saves memory, and speeds up
loading by skipping the processing of the documentation strings.

  This feature has a drawback: if you delete, move, or alter the
compiled file (such as by compiling a new version), Emacs may no
longer be able to access the documentation string of previously-loaded
functions or variables.  Such a problem normally only occurs if you
build Emacs yourself, and happen to edit and/or recompile the Lisp
source files.  To solve it, just reload each file after recompilation.

  Dynamic loading of documentation strings from byte-compiled files is
determined, at compile time, for each byte-compiled file.  It can be
disabled via the option @code{byte-compile-dynamic-docstrings}.

@defopt byte-compile-dynamic-docstrings
If this is non-@code{nil}, the byte compiler generates compiled files
that are set up for dynamic loading of documentation strings.

To disable the dynamic loading feature for a specific file, set this
option to @code{nil} in its header line (@pxref{File Variables, ,
Local Variables in Files, emacs, The GNU Emacs Manual}), like this:

@smallexample
-*-byte-compile-dynamic-docstrings: nil;-*-
@end smallexample

This is useful mainly if you expect to change the file, and you want
Emacs sessions that have already loaded it to keep working when the
file changes.
@end defopt

@cindex @samp{#@@@var{count}}
@cindex @samp{#$}
Internally, the dynamic loading of documentation strings is
accomplished by writing compiled files with a special Lisp reader
construct, @samp{#@@@var{count}}.  This construct skips the next
@var{count} characters.  It also uses the @samp{#$} construct, which
stands for the name of this file, as a string.  Do not use these
constructs in Lisp source files; they are not designed to be clear to
humans reading the file.

@node Dynamic Loading
@section Dynamic Loading of Individual Functions

@cindex dynamic loading of functions
@cindex lazy loading
  When you compile a file, you can optionally enable the @dfn{dynamic
function loading} feature (also known as @dfn{lazy loading}).  With
dynamic function loading, loading the file doesn't fully read the
function definitions in the file.  Instead, each function definition
contains a place-holder which refers to the file.  The first time each
function is called, it reads the full definition from the file, to
replace the place-holder.

  The advantage of dynamic function loading is that loading the file
becomes much faster.  This is a good thing for a file which contains
many separate user-callable functions, if using one of them does not
imply you will probably also use the rest.  A specialized mode which
provides many keyboard commands often has that usage pattern: a user may
invoke the mode, but use only a few of the commands it provides.

  The dynamic loading feature has certain disadvantages:

@itemize @bullet
@item
If you delete or move the compiled file after loading it, Emacs can no
longer load the remaining function definitions not already loaded.

@item
If you alter the compiled file (such as by compiling a new version),
then trying to load any function not already loaded will usually yield
nonsense results.
@end itemize

  These problems will never happen in normal circumstances with
installed Emacs files.  But they are quite likely to happen with Lisp
files that you are changing.  The easiest way to prevent these problems
is to reload the new compiled file immediately after each recompilation.

  The byte compiler uses the dynamic function loading feature if the
variable @code{byte-compile-dynamic} is non-@code{nil} at compilation
time.  Do not set this variable globally, since dynamic loading is
desirable only for certain files.  Instead, enable the feature for
specific source files with file-local variable bindings.  For example,
you could do it by writing this text in the source file's first line:

@example
-*-byte-compile-dynamic: t;-*-
@end example

@defvar byte-compile-dynamic
If this is non-@code{nil}, the byte compiler generates compiled files
that are set up for dynamic function loading.
@end defvar

@defun fetch-bytecode function
If @var{function} is a byte-code function object, this immediately
finishes loading the byte code of @var{function} from its
byte-compiled file, if it is not fully loaded already.  Otherwise,
it does nothing.  It always returns @var{function}.
@end defun

@node Eval During Compile
@section Evaluation During Compilation
@cindex eval during compilation

  These features permit you to write code to be evaluated during
compilation of a program.

@defspec eval-and-compile body@dots{}
This form marks @var{body} to be evaluated both when you compile the
containing code and when you run it (whether compiled or not).

You can get a similar result by putting @var{body} in a separate file
and referring to that file with @code{require}.  That method is
preferable when @var{body} is large.  Effectively @code{require} is
automatically @code{eval-and-compile}, the package is loaded both when
compiling and executing.

@code{autoload} is also effectively @code{eval-and-compile} too.  It's
recognized when compiling, so uses of such a function don't produce
``not known to be defined'' warnings.

Most uses of @code{eval-and-compile} are fairly sophisticated.

If a macro has a helper function to build its result, and that macro
is used both locally and outside the package, then
@code{eval-and-compile} should be used to get the helper both when
compiling and then later when running.

If functions are defined programmatically (with @code{fset} say), then
@code{eval-and-compile} can be used to have that done at compile-time
as well as run-time, so calls to those functions are checked (and
warnings about ``not known to be defined'' suppressed).
@end defspec

@defspec eval-when-compile body@dots{}
This form marks @var{body} to be evaluated at compile time but not when
the compiled program is loaded.  The result of evaluation by the
compiler becomes a constant which appears in the compiled program.  If
you load the source file, rather than compiling it, @var{body} is
evaluated normally.

@cindex compile-time constant
If you have a constant that needs some calculation to produce,
@code{eval-when-compile} can do that at compile-time.  For example,

@lisp
(defvar my-regexp
  (eval-when-compile (regexp-opt '("aaa" "aba" "abb"))))
@end lisp

@cindex macros, at compile time
If you're using another package, but only need macros from it (the
byte compiler will expand those), then @code{eval-when-compile} can be
used to load it for compiling, but not executing.  For example,

@lisp
(eval-when-compile
  (require 'my-macro-package))
@end lisp

The same sort of thing goes for macros and @code{defsubst} functions
defined locally and only for use within the file.  They are needed for
compiling the file, but in most cases they are not needed for
execution of the compiled file.  For example,

@lisp
(eval-when-compile
  (unless (fboundp 'some-new-thing)
    (defmacro 'some-new-thing ()
      (compatibility code))))
@end lisp

@noindent
This is often good for code that's only a fallback for compatibility
with other versions of Emacs.

@strong{Common Lisp Note:} At top level, @code{eval-when-compile} is analogous to the Common
Lisp idiom @code{(eval-when (compile eval) @dots{})}.  Elsewhere, the
Common Lisp @samp{#.} reader macro (but not when interpreting) is closer
to what @code{eval-when-compile} does.
@end defspec

@node Compiler Errors
@section Compiler Errors
@cindex compiler errors

  Error and warning messages from byte compilation are printed in a
buffer named @file{*Compile-Log*}.  These messages include file names
and line numbers identifying the location of the problem.  The usual
Emacs commands for operating on compiler output can be used on these
messages.

  When an error is due to invalid syntax in the program, the byte
compiler might get confused about the errors' exact location.  One way
to investigate is to switch to the buffer @w{@file{ *Compiler
Input*}}.  (This buffer name starts with a space, so it does not show
up in the Buffer Menu.)  This buffer contains the program being
compiled, and point shows how far the byte compiler was able to read;
the cause of the error might be nearby.  @xref{Syntax Errors}, for
some tips for locating syntax errors.

  A common type of warning issued by the byte compiler is for
functions and variables that were used but not defined.  Such warnings
report the line number for the end of the file, not the locations
where the missing functions or variables were used; to find these, you
must search the file manually.

  If you are sure that a warning message about a missing function or
variable is unjustified, there are several ways to suppress it:

@itemize @bullet
@item
You can suppress the warning for a specific call to a function
@var{func} by conditionalizing it on an @code{fboundp} test, like
this:

@example
(if (fboundp '@var{func}) ...(@var{func} ...)...)
@end example

@noindent
The call to @var{func} must be in the @var{then-form} of the
@code{if}, and @var{func} must appear quoted in the call to
@code{fboundp}.  (This feature operates for @code{cond} as well.)

@item
Likewise, you can suppress the warning for a specific use of a
variable @var{variable} by conditionalizing it on a @code{boundp}
test:

@example
(if (boundp '@var{variable}) ...@var{variable}...)
@end example

@noindent
The reference to @var{variable} must be in the @var{then-form} of the
@code{if}, and @var{variable} must appear quoted in the call to
@code{boundp}.

@item
You can tell the compiler that a function is defined using
@code{declare-function}. @xref{Declaring Functions}.

@item
Likewise, you can tell the compiler that a variable is defined using
@code{defvar} with no initial value.  (Note that this marks the
variable as special.)  @xref{Defining Variables}.
@end itemize

  You can also suppress any and all compiler warnings within a certain
expression using the construct @code{with-no-warnings}:

@c This is implemented with a defun, but conceptually it is
@c a special form.

@defspec with-no-warnings body@dots{}
In execution, this is equivalent to @code{(progn @var{body}...)},
but the compiler does not issue warnings for anything that occurs
inside @var{body}.

We recommend that you use this construct around the smallest
possible piece of code, to avoid missing possible warnings other than
one you intend to suppress.
@end defspec

  Byte compiler warnings can be controlled more precisely by setting
the variable @code{byte-compile-warnings}.  See its documentation
string for details.

@node Byte-Code Objects
@section Byte-Code Function Objects
@cindex compiled function
@cindex byte-code function
@cindex byte-code object

  Byte-compiled functions have a special data type: they are
@dfn{byte-code function objects}.  Whenever such an object appears as
a function to be called, Emacs uses the byte-code interpreter to
execute the byte-code.

  Internally, a byte-code function object is much like a vector; its
elements can be accessed using @code{aref}.  Its printed
representation is like that for a vector, with an additional @samp{#}
before the opening @samp{[}.  It must have at least four elements;
there is no maximum number, but only the first six elements have any
normal use.  They are:

@table @var
@item argdesc
The descriptor of the arguments.  This can either be a list of
arguments, as described in @ref{Argument List}, or an integer encoding
the required number of arguments.  In the latter case, the value of
the descriptor specifies the minimum number of arguments in the bits
zero to 6, and the maximum number of arguments in bits 8 to 14.  If
the argument list uses @code{&rest}, then bit 7 is set; otherwise it's
cleared.

If @var{argdesc} is a list, the arguments will be dynamically bound
before executing the byte code.  If @var{argdesc} is an integer, the
arguments will be instead pushed onto the stack of the byte-code
interpreter, before executing the code.

@item byte-code
The string containing the byte-code instructions.

@item constants
The vector of Lisp objects referenced by the byte code.  These include
symbols used as function names and variable names.

@item stacksize
The maximum stack size this function needs.

@item docstring
The documentation string (if any); otherwise, @code{nil}.  The value may
be a number or a list, in case the documentation string is stored in a
file.  Use the function @code{documentation} to get the real
documentation string (@pxref{Accessing Documentation}).

@item interactive
The interactive spec (if any).  This can be a string or a Lisp
expression.  It is @code{nil} for a function that isn't interactive.
@end table

Here's an example of a byte-code function object, in printed
representation.  It is the definition of the command
@code{backward-sexp}.

@example
#[256
  "\211\204^G^@@\300\262^A\301^A[!\207"
  [1 forward-sexp]
  3
  1793299
  "^p"]
@end example

  The primitive way to create a byte-code object is with
@code{make-byte-code}:

@defun make-byte-code &rest elements
This function constructs and returns a byte-code function object
with @var{elements} as its elements.
@end defun

  You should not try to come up with the elements for a byte-code
function yourself, because if they are inconsistent, Emacs may crash
when you call the function.  Always leave it to the byte compiler to
create these objects; it makes the elements consistent (we hope).

@node Disassembly
@section Disassembled Byte-Code
@cindex disassembled byte-code

  People do not write byte-code; that job is left to the byte
compiler.  But we provide a disassembler to satisfy a cat-like
curiosity.  The disassembler converts the byte-compiled code into
human-readable form.

  The byte-code interpreter is implemented as a simple stack machine.
It pushes values onto a stack of its own, then pops them off to use them
in calculations whose results are themselves pushed back on the stack.
When a byte-code function returns, it pops a value off the stack and
returns it as the value of the function.

  In addition to the stack, byte-code functions can use, bind, and set
ordinary Lisp variables, by transferring values between variables and
the stack.

@deffn Command disassemble object &optional buffer-or-name
This command displays the disassembled code for @var{object}.  In
interactive use, or if @var{buffer-or-name} is @code{nil} or omitted,
the output goes in a buffer named @file{*Disassemble*}.  If
@var{buffer-or-name} is non-@code{nil}, it must be a buffer or the
name of an existing buffer.  Then the output goes there, at point, and
point is left before the output.

The argument @var{object} can be a function name, a lambda expression
(@pxref{Lambda Expressions}), or a byte-code object (@pxref{Byte-Code
Objects}).  If it is a lambda expression, @code{disassemble} compiles
it and disassembles the resulting compiled code.
@end deffn

  Here are two examples of using the @code{disassemble} function.  We
have added explanatory comments to help you relate the byte-code to the
Lisp source; these do not appear in the output of @code{disassemble}.

@example
@group
(defun factorial (integer)
  "Compute factorial of an integer."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
     @result{} factorial
@end group

@group
(factorial 4)
     @result{} 24
@end group

@group
(disassemble 'factorial)
     @print{} byte-code for factorial:
 doc: Compute factorial of an integer.
 args: (integer)
@end group

@group
0   varref   integer      ; @r{Get the value of @code{integer} and}
                          ;   @r{push it onto the stack.}
1   constant 1            ; @r{Push 1 onto stack.}
@end group
@group
2   eqlsign               ; @r{Pop top two values off stack, compare}
                          ;   @r{them, and push result onto stack.}
@end group
@group
3   goto-if-nil 1         ; @r{Pop and test top of stack;}
                          ;   @r{if @code{nil}, go to 1, else continue.}
6   constant 1            ; @r{Push 1 onto top of stack.}
7   return                ; @r{Return the top element of the stack.}
@end group
@group
8:1 varref   integer      ; @r{Push value of @code{integer} onto stack.}
9   constant factorial    ; @r{Push @code{factorial} onto stack.}
10  varref   integer      ; @r{Push value of @code{integer} onto stack.}
11  sub1                  ; @r{Pop @code{integer}, decrement value,}
                          ;   @r{push new value onto stack.}
12  call     1            ; @r{Call function @code{factorial} using first}
                          ;   @r{(i.e., top) stack element as argument;}
                          ;   @r{push returned value onto stack.}
@end group
@group
13 mult                   ; @r{Pop top two values off stack, multiply}
                          ;   @r{them, and push result onto stack.}
14 return                 ; @r{Return the top element of the stack.}
@end group
@end example

The @code{silly-loop} function is somewhat more complex:

@example
@group
(defun silly-loop (n)
  "Return time before and after N iterations of a loop."
  (let ((t1 (current-time-string)))
    (while (> (setq n (1- n))
              0))
    (list t1 (current-time-string))))
     @result{} silly-loop
@end group

@group
(disassemble 'silly-loop)
     @print{} byte-code for silly-loop:
 doc: Return time before and after N iterations of a loop.
 args: (n)
@end group

@group
0   constant current-time-string  ; @r{Push @code{current-time-string}}
                                  ;   @r{onto top of stack.}
@end group
@group
1   call     0            ; @r{Call @code{current-time-string} with no}
                          ;   @r{argument, push result onto stack.}
@end group
@group
2   varbind  t1           ; @r{Pop stack and bind @code{t1} to popped value.}
@end group
@group
3:1 varref   n            ; @r{Get value of @code{n} from the environment}
                          ;   @r{and push the value on the stack.}
4   sub1                  ; @r{Subtract 1 from top of stack.}
@end group
@group
5   dup                   ; @r{Duplicate top of stack; i.e., copy the top}
                          ;   @r{of the stack and push copy onto stack.}
6   varset   n            ; @r{Pop the top of the stack,}
                          ;   @r{and bind @code{n} to the value.}

;; @r{(In effect, the sequence @code{dup varset} copies the top of the stack}
;; @r{into the value of @code{n} without popping it.)}
@end group

@group
7   constant 0            ; @r{Push 0 onto stack.}
8   gtr                   ; @r{Pop top two values off stack,}
                          ;   @r{test if @var{n} is greater than 0}
                          ;   @r{and push result onto stack.}
@end group
@group
9   goto-if-not-nil 1     ; @r{Goto 1 if @code{n} > 0}
                          ;   @r{(this continues the while loop)}
                          ;   @r{else continue.}
@end group
@group
12  varref   t1           ; @r{Push value of @code{t1} onto stack.}
13  constant current-time-string  ; @r{Push @code{current-time-string}}
                                  ;   @r{onto the top of the stack.}
14  call     0            ; @r{Call @code{current-time-string} again.}
@end group
@group
15  unbind   1            ; @r{Unbind @code{t1} in local environment.}
16  list2                 ; @r{Pop top two elements off stack, create a}
                          ;   @r{list of them, and push it onto stack.}
17  return                ; @r{Return value of the top of stack.}
@end group
@end example