emacs/doc/lispref/compile.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1994, 2001-2011  Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../../info/compile
@node Byte Compilation, Advising Functions, Loading, Top
@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

  @xref{Compilation Errors}, for how to investigate errors occurring in
byte compilation.

@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 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
(silly-loop 50000000)
@result{} ("Wed Mar 11 21:10:19 2009"
    "Wed Mar 11 21:10:41 2009")  ; @r{22 seconds}
@end group

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

@group
(silly-loop 50000000)
@result{} ("Wed Mar 11 21:12:26 2009"
    "Wed Mar 11 21:12:32 2009")  ; @r{6 seconds}
@end group
@end example

  In this example, the interpreted code required 22 seconds to run,
whereas the byte-compiled code required 6 seconds.  These results are
representative, but actual results will vary greatly.

@node Compilation Functions
@comment  node-name,  next,  previous,  up
@section The 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}.

  The byte compiler produces error messages and warnings about each file
in a buffer called @samp{*Compile-Log*}.  These report things in your
program that suggest a problem but are not necessarily erroneous.

@cindex macro compilation
  Be careful when writing macro calls in files that you may someday
byte-compile.  Macro calls are expanded when they are compiled, so the
macros must already be defined for proper compilation.  For more
details, see @ref{Compiling Macros}.  If a program does not work the
same way when compiled as it does when interpreted, erroneous macro
definitions are one likely cause (@pxref{Problems with 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.

  Normally, compiling a file does not evaluate the file's contents or
load the file.  But it does execute any @code{require} calls at top
level in the file.  One way to ensure that necessary macro definitions
are available during compilation is to require the file that defines
them (@pxref{Named Features}).  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}).

@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;
i.e., the compiler does not follow indirection to another symbol.
@code{byte-compile} returns the new, compiled definition of
@var{symbol}.

  If @var{symbol}'s definition is a byte-code function object,
@code{byte-compile} does nothing and returns @code{nil}.  Lisp records
only one function definition for any symbol, and if that is already
compiled, non-compiled code is not available anywhere.  So there is no
way to ``compile the same definition again.''

@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

@noindent
The result is a byte-code function object.  The string it contains is
the actual byte-code; each character in it is an instruction or an
operand of an instruction.  The vector contains all the constants,
variable names and function names used by the function, except for
certain primitives that are coded as special instructions.

If the argument to @code{byte-compile} is a @code{lambda} expression,
it 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     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     791 Oct  5 20:31 push.el
-rw-rw-rw-  1 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

@defun byte-code code-string data-vector max-stack
@cindex byte-code interpreter
This function actually interprets byte-code.  A byte-compiled function
is actually defined with a body that calls @code{byte-code}.  Don't call
this function yourself---only the byte compiler knows how to generate
valid calls to this function.

In Emacs version 18, byte-code was always executed by way of a call to
the function @code{byte-code}.  Nowadays, byte-code is usually executed
as part of a byte-code function object, and only rarely through an
explicit call to @code{byte-code}.
@end defun

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

  Functions and variables loaded from a byte-compiled file access their
documentation strings dynamically from the file whenever needed.  This
saves space within Emacs, and makes loading faster because the
documentation strings themselves need not be processed while loading the
file.  Actual access to the documentation strings becomes slower as a
result, but this normally is not enough to bother users.

  Dynamic access to documentation strings does have drawbacks:

@itemize @bullet
@item
If you delete or move the compiled file after loading it, Emacs can no
longer access the documentation strings for the functions and variables
in the file.

@item
If you alter the compiled file (such as by compiling a new version),
then further access to documentation strings in this file will
probably give nonsense results.
@end itemize

  If your site installs Emacs following the usual procedures, these
problems will never normally occur.  Installing a new version uses a new
directory with a different name; as long as the old version remains
installed, its files will remain unmodified in the places where they are
expected to be.

  However, if you have built Emacs yourself and use it from the
directory where you built it, you will experience this problem
occasionally if you edit and recompile Lisp files.  When it happens, you
can cure the problem by reloading the file after recompiling it.

  You can turn off this feature at compile time by setting
@code{byte-compile-dynamic-docstrings} to @code{nil}; this is useful
mainly if you expect to change the file, and you want Emacs processes
that have already loaded it to keep working when the file changes.
You can do this globally, or for one source file by specifying a
file-local binding for the variable.  One way to do that is by adding
this string to the file's first line:

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

@defvar 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.
@end defvar

@cindex @samp{#@@@var{count}}
@cindex @samp{#$}
  The dynamic documentation string feature writes compiled files that
use a special Lisp reader construct, @samp{#@@@var{count}}.  This
construct skips the next @var{count} characters.  It also uses the
@samp{#$} construct, which stands for ``the name of this file, as a
string.''  It is usually best not to use these constructs in Lisp source
files, since 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

  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

  Byte compilation outputs all errors and warnings into the buffer
@samp{*Compile-Log*}.  The messages include file names and line
numbers that identify the location of the problem.  The usual Emacs
commands for operating on compiler diagnostics work properly on
these messages.

  However, the warnings about functions that were used but not
defined are always ``located'' at the end of the file, so these
commands won't find the places they are really used.  To do that,
you must search for the function names.

  You can suppress the compiler warning for calling an undefined
function @var{func} by conditionalizing the function call 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.)

  You can tell the compiler that a function is defined using
@code{declare-function} (@pxref{Declaring Functions}).  Likewise, you
can tell the compiler that a variable is defined using @code{defvar}
with no initial value.

  You can suppress the compiler warning for a specific use of an
undefined variable @var{variable} by conditionalizing its use on a
@code{boundp} test, like this:

@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}.

  You can 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
one you intend to suppress.
@end defspec

  More precise control of warnings is possible by setting the variable
@code{byte-compile-warnings}.

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

  Byte-compiled functions have a special data type: they are
@dfn{byte-code function objects}.

  Internally, a byte-code function object is much like a vector;
however, the evaluator handles this data type specially when it appears
as a function to be called.  The printed representation for a byte-code
function object is like that for a vector, with an additional @samp{#}
before the opening @samp{[}.

  A byte-code function object 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 arglist
The list of argument symbols.

@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
#[(&optional arg)
  "^H\204^F^@@\301^P\302^H[!\207"
  [arg 1 forward-sexp]
  2
  254435
  "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).

  You can access the elements of a byte-code object using @code{aref};
you can also use @code{vconcat} to create a vector with the same
elements.

@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 @samp{*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
or a byte-code object.  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}}
                            ;   @r{and 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,}
                            ;   @r{else continue.}
6   constant 1              ; @r{Push 1 onto top of stack.}
7   return                  ; @r{Return the top element}
                            ;   @r{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}
                            ;   @r{the first (i.e., the top) element}
                            ;   @r{of the stack as the 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 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)

0   constant current-time-string  ; @r{Push}
                                  ;   @r{@code{current-time-string}}
                                  ;   @r{onto top of stack.}
@end group

@group
1   call     0              ; @r{Call @code{current-time-string}}
                            ;   @r{with no argument,}
                            ;   @r{pushing result onto stack.}
@end group

@group
2   varbind  t1             ; @r{Pop stack and bind @code{t1}}
                            ;   @r{to popped value.}
@end group

@group
3:1 varref   n              ; @r{Get value of @code{n} from}
                            ;   @r{the environment and push}
                            ;   @r{the value onto the stack.}
4   sub1                    ; @r{Subtract 1 from top of stack.}
@end group

@group
5   dup                     ; @r{Duplicate the top of the stack;}
                            ;   @r{i.e., copy the top of}
                            ;   @r{the stack and push the}
                            ;   @r{copy onto the 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}}
                            ;   @r{copies the top of the stack}
                            ;   @r{into the value of @code{n}}
                            ;   @r{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 top of 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,}
                            ;   @r{create a list of them,}
                            ;   @r{and push list onto stack.}
17  return                  ; @r{Return value of the top of stack.}
@end group
@end example