path: root/Master/texmf-dist/source/latex/l3kernel/expl3.dtx

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%
%<*driver|package>
\RequirePackage{l3names}[2011/12/30]
%<package>\@ifpackagelater{l3names}{2011/12/30}
%<package>  {}
%<package>  {
%<package>    \PackageError{expl3}{Mismatched LaTeX3 packages detected.}
%<package>      {
%<package>        Please install an up to date~version of l3kernel
%<package>        using your TeX package manager or from CTAN.\\ \\
%<package>        Loading expl3 will abort!
%<package>      }
%<package>    \endinput
%<package>  }
\def\ExplFileName{expl3}
\def\ExplFileDescription{L3 Experimental code bundle wrapper}
\def\ExplFileDate{2012/01/31}
\def\ExplFileVersion{3287}
%</driver|package>
%<*driver>
\documentclass[full]{l3doc}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \providecommand\acro[1]{\textsc{\MakeLowercase{#1}}}
% \newenvironment{arg-description}{%
%   \begin{itemize}\def\makelabel##1{\hss\llap{\bfseries##1}}}{\end{itemize}}
%
% \title{^^A
%   The \textsf{expl3} package and \LaTeX3 programming^^A
%   \thanks{This file describes v\ExplFileVersion,
%     last revised \ExplFileDate.}^^A
% }
%
% \author{^^A
%  The \LaTeX3 Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released \ExplFileDate}
%
% \maketitle
%
% \begin{documentation}
%
% \begin{abstract}
%
% This document gives an introduction to a new set of programming
% conventions that have been designed to meet the requirements of
% implementing large scale \TeX{} macro programming projects such as
% \LaTeX{}. These programming conventions are the base layer of \LaTeX3.
%
% The main features of the system described are:
% \begin{itemize}
%   \item classification of the macros (or, in \LaTeX{} terminology,
%     commands) into \LaTeX{} functions and \LaTeX{} parameters, and also
%     into modules containing related commands;
%   \item  a systematic naming scheme based on these classifications;
%   \item  a simple mechanism for controlling the expansion of a function's
%     arguments.
% \end{itemize}
% This system is being used as the basis for \TeX{} programming within
% the \LaTeX3 project. Note that the language is not intended for either
% document mark-up or style specification. Instead, it is intended that
% such features will be built on top of the conventions described here.
%
% This document is an introduction to the ideas behind the \pkg{expl3}
% programming interface. For the complete documentation of the programming
% layer provided by the \LaTeX3 Project, see the accompanying
% \texttt{interface3} document.
%
% \end{abstract}
%
% \section{Introduction}
%
% The first step to develop a \LaTeX{} kernel beyond \LaTeXe{} is to
% address how the underlying system is programmed.  Rather than the
% current mix of \LaTeX{} and \TeX{} macros, the \LaTeX3 system provides
% its own consistent interface to all of the functions needed to
% control \TeX{}.  A key part of this work is to ensure that everything
% is documented, so that \LaTeX{} programmers and users can work
% efficiently without needing to be familiar with the internal nature
% of the kernel or with plain \TeX{}.
%
% The \pkg{expl3} bundle provides this new programming interface for
% \LaTeX{}. To make programming systematic, \LaTeX3 uses some very
% different conventions to \LaTeXe{} or plain \TeX{}. As a result,
% programmers starting with \LaTeX3 will need to become familiar with
% the syntax of the new language.
%
% The next section shows where this language fits into a complete
% \TeX{}-based document processing system.  We then describe the major
% features of the syntactic structure of command names, including the
% argument specification syntax used in function names.
%
% The practical ideas behind this argument syntax will be explained,
% together with the expansion control mechanism and the interface
% used to define variant forms of functions.
%
% As we shall demonstrate, the use of a structured naming scheme and of
% variant forms for functions greatly improves the readability of the
% code and hence also its reliability.  Moreover, experience has shown
% that the longer command names which result from the new syntax do not
% make the process of \emph{writing} code significantly harder.
%
% \section{Languages and interfaces}
%
% It is possible to identify several distinct languages related to the
% various interfaces that are needed in a \TeX{}-based document processing
% system.  This section looks at those we consider most important for
% the \LaTeX3 system.
%
% \begin{description}
%   \item[Document mark-up] This comprises those commands (often called
%     tags) that are to embedded in the document (the |.tex| file).
%
%     It is generally accepted that such mark-up should be essentially
%     \emph{declarative}. It may be traditional \TeX{}-based mark-up such
%      as \LaTeXe{}, as described in~\cite{A-W:LLa94} and~\cite{A-W:GMS94},
%     or a mark-up language defined via \acro{HTML} or \acro{XML}.
%
%     One problem with more traditional \TeX{} coding conventions (as
%     described in~\cite{A-W:K-TB}) is that the names and syntax of \TeX{}'s
%     primitive formatting commands are ingeniously designed to be
%     \enquote{natural} when used directly by the author as document mark-up or in
%     macros.  Ironically, the ubiquity (and widely recognised
%     superiority) of logical mark-up has meant that such explicit
%     formatting commands are almost never needed in documents or in
%     author-defined macros.  Thus they are used almost exclusively by
%     \TeX{} programmers to define higher-level commands, and their
%     idiosyncratic syntax is not at all popular with this community.
%     Moreover, many of them have names that could be very useful as
%     document mark-up tags were they not pre-empted as primitives
%     (\emph{e.g.}~\tn{box} or \tn{special}).
%
%   \item[Designer interface] This relates a (human) typographic
%     designer's specification for a document to a program that
%     \enquote{formats
%     the document}.  It should ideally use a declarative language that
%     facilitates expression of the relationship and spacing rules
%     specified for the layout of the various document elements.
%
%     This language is not embedded in document text and it will be very
%     different in form to the document mark-up language.  For \LaTeX{},
%     this level was almost completely missing  from \LaTeX{}2.09; \LaTeXe{}
%     made some improvements in this area but it is still the case that
%     implementing a design specification in  \LaTeX{} requires far more
%     \enquote{low-level} coding than is acceptable.
%
%   \item[Programmer interface]
%     This language is the implementation language within which the
%     basic typesetting functionality is implemented, building upon the
%     primitives of \TeX{} (or a  successor program).  It may also be used
%     to implement the previous two languages \enquote{within} \TeX{}, as in
%     the
%     current \LaTeX{} system.
%
% \end{description}
%
% The last layer is covered by the conventions described in this
% document, which describes a system aimed at providing a suitable
% basis for coding \LaTeX3. Its main distinguishing features are
% summarised here:
% \begin{itemize}
%   \item A consistent naming scheme for all commands, including \TeX{}
%     primitives.
%   \item The classification of commands as \LaTeX{} functions or \LaTeX{}
%     parameters, and also their division into modules according to their
%     functionality.
%   \item A simple mechanism for controlling argument expansion.
%   \item Provision of a set of core \LaTeX{} functions that is sufficient
%     for handling programming constructs such as queues, sets, stacks,
%     property lists.
%   \item A \TeX{} programming environment in which, for example, all
%     white space is ignored.
% \end{itemize}
%
% \section{The naming scheme}
%
% \LaTeX3 does not use |@| as a \enquote{letter} for defining
% internal macros.  Instead, the symbols |_| and |:|
% are used in internal macro names to provide structure. In
% contrast to the plain \TeX{} format and the \LaTeXe{} kernel, these
% extra letters are used only between parts of a macro name (no
% strange vowel replacement).
%
% While \TeX{} is actually a macro processor, by
% convention for the \pkg{expl3} programming language we distinguish between
% \emph{functions} and \emph{variables}. Functions can have arguments and they
% are either expanded or executed.  Variables can be assigned values and they
% are used in arguments to functions; they are not used directly but are
% manipulated by functions (including getting and setting functions).
% Functions and variables with a related functionality (for example accessing
% counters, or manipulating token lists, \emph{etc.})\ are collected together
% into a
% \emph{module}.
%
% \subsection{Examples}
%
% Before giving the details of the naming scheme, here are a few typical
% examples to indicate the flavour of the scheme; first some variable
% names.
% \begin{quote}
%   \cs{l_tmpa_box} is a local variable (hence the~|l_| prefix)
%     corresponding to a box register.\\
%   \cs{g_tmpa_int} is a global variable (hence the~|g_| prefix)
%     corresponding to an integer register (i.e.~a \TeX{} count
%     register).\\
%   \cs{c_empty_tl} is the constant~(|c_|) token list variable
%     that is always empty.
% \end{quote}
%
% Now here is an example of a typical function name.
%
% \cs{seq_push:Nn} is the function which puts the token list specified
% by its second argument onto the stack specified by its first argument.
% The different natures of the two arguments are indicated by the~|:Nn|
% suffix. The first argument must be a single token which \enquote{names}
% the stack parameter: such single-token arguments are denoted~|N|.
% The second argument is a normal \TeX{} \enquote{undelimited argument},
% which
% may either be a single token or a balanced, brace-delimited token
% list (which we shall here call a \textit{braced token list}): the~|n|
% denotes such a \enquote{normal} argument form. The name of the function
% indicates it belongs to the |seq| module.
%
% \subsection{Formal naming syntax}
%
% We shall now look in more detail at the syntax of these names. A
% function name in \LaTeX3 will have a name consisting of three parts:
% \begin{quote}
%   |\|\meta{module}|_|\meta{description}|:|\meta{arg-spec}
% \end{quote}
% while a variable will have (up to) four distinct parts to its name:
% \begin{quote}
%   |\|\meta{scope}|_|\meta{module}|_|\meta{description}|_|\meta{type}
% \end{quote}
%
% The syntax of all names contains
% \begin{quote}
%   \meta{module} and \meta{description}
% \end{quote}
% these both give information about the command.
%
% A \emph{module} is a collection of closely related functions and
% variables. Typical module names include~|int| for integer parameters
% and related functions,~|seq| for sequences and~|box| for boxes.
%
% Packages providing new programming functionality will add new modules
% as needed; the programmer can choose any unused name, consisting
% of letters only, for a module. In general, the module name and module
% prefix should be related: for example, the kernel module containing
% \texttt{box} functions is called \texttt{l3box}.
%
% The \emph{description} gives more detailed information about the
% function or parameter, and provides a unique name for it.  It should
% consist of letters and, possibly,~|_|~characters. In general, the
% description should use |_| to divide up \enquote{words} or other easy to
% follow parts of the name.  For example, the \LaTeX3 kernel provides
% \cs{if_cs_exist:N} which, as might be expected, tests if a command
% name exists.
%
% Where functions for variable manipulation can perform assignments
% either locally or globally, the latter case is indicated by the inclusion of
% a |g| in the second part of the function name. Thus \cs{tl_set:Nn} is a local
% function but \cs{tl_gset:Nn} acts globally. Functions of this type are
% always documented together, and the scope of action may therefore be
% inferred from the presence or absence of a |g|. See the next subsection for
% more detail on variable scope.
%
% \subsubsection{Variables: scope and type}
%
% The \meta{scope} part of the name describes how the variable can be
% accessed.  Variables are classified as local, global or constant.
% This \emph{scope} type appears as a code at the beginning of the name;
% the codes used are:
% \begin{arg-description}
%   \item[c] constants (global variables whose value should not be
%     changed);
%   \item[g] variables whose value should only be set globally;
%   \item[l] variables whose value should only be set locally.
% \end{arg-description}
%
% Separate functions are provided to assign data to local and global
% variables; for example, |\tl_set:Nn| and |\tl_gset:Nn| respectively
% set the value of a local or global \enquote{token list} variable.
% Note that it is a poor \TeX{} practise to intermix local and global
% assignments to a variable; otherwise you risk exhausting the save stack.
% \unskip\footnote{See \emph{The \TeX{}book}, p.\,301, for further
% information.}
%
% The \meta{type} will be in the list of available
% \emph{data-types};\footnote{Of course, if a totally new data type is
% needed then this will not be the case. However, it is hoped that only
% the kernel team will need to create new data types.} these include the
% primitive \TeX{} data-types, such as the various registers, but to
% these will be added data-types built within the \LaTeX{} programming
% system.
%
% The data types in \LaTeX3 are:
% \begin{description}
%   \item[bool]   either true or false (the \LaTeX3 implementation does
%                 not use \tn{iftrue} or \tn{iffalse});
%   \item[box]    box register;
%   \item[clist]  comma separated list;
%   \item[coffin] a \enquote{box with handles} --- a higher-level data
%                 type for carrying out |box| alignment operations;
%   \item[dim]    \enquote{rigid} lengths;
%   \item[fp]     floating-point values;
%   \item[ior]    an input stream (for reading from a file);
%   \item[iow]    an output stream (for writing from a file);
%   \item[int]    integer-valued count register;
%   \item[prop]   property list;
%   \item[seq]    sequence: a data-type used to implement lists (with
%                 access at both ends) and stacks;
%   \item[skip]   \enquote{rubber} lengths;
%   \item[stream] an input or output stream (for reading from or writing
%                 to, respectively);
%   \item[tl]     \enquote{token list variables}: placeholders for token lists.
% \end{description}
% When the \meta{type} and \meta{module} are identical (as often happens in
% the more basic modules) the \meta{module} part is often omitted for
% aesthetic reasons.
%
% The name \enquote{token list} may cause confusion, and so some background is
% useful.  \TeX{} works with tokens and lists of tokens, rather than
% characters. It provides two ways to store these token lists: within
% macros and as token registers (|toks|). The implementation in \LaTeX3 means
% that
% |toks| are not required, and that all operations for storing tokens can us
% the |tl| variable type.
%
% Experienced \TeX{} programmers will notice that some of the variable
% types listed are native \TeX{} registers whilst others are not. In
% general, the underlying \TeX{} implementation for a data structure may
% vary but the \emph{documented interface} will be stable. For example,
% the |prop| data type was originally implemented as a |toks|, but
% is currently built on top of the |tl| data structure.
%
% \subsubsection{Variables: guidance}
%
% Both comma lists and sequences both have similar characteristics.
% They both use special delimiters to mark out one entry from the
% next, and are both accessible at both ends. In general, it is
% easier to create comma lists `by hand' as they can be typed
% in directly. User input often takes the form of a comma separated
% list and so there are many cases where this is the obvious
% data type to use. On the other hand, sequences use special internal
% tokens to separate entries. This means that they can be used to
% contain material that comma lists cannot (such as items that may
% themselves contain commas!). In general, comma lists should be
% preferred for creating fixed lists inside programs and for
% handling user input where commas will not occur. On the other
% hand, sequences should be used to store arbitrary lists of
% data.
%
% \pkg{expl3} implements stacks using the sequence data structure.
% Thus creating stacks involves first creating a sequence, and
% then using the sequence functions which work in a stack manner
% (\cs{seq_push:Nn}, \emph{etc}.).
%
%  Due to the nature of the underlying \TeX{} implementation, it is
%  possible to assign values to token list variables and comma lists
%  without first declaring them. However, this is \emph{not supported
%  behaviour}. The \LaTeX3 coding convention is that all variables must
%  be declared before use.
%
%  The \pkg{expl3} package can be loaded with the |check-declarations| option
%  to verify that all variables are declared before use. This has a
%  performance implication and is therefore intended for testing during
%  development and not for use in production documents.
%
% \subsubsection{Functions: argument specifications}
%
% Function names end with an \meta{arg-spec} after a colon.  This
% gives an indication of the types of argument that a function takes,
% and provides a convenient method of naming similar functions that
% differ only in their argument forms (see the next section for
% examples).
%
% The \meta{arg-spec} consists of a (possibly empty) list of letters,
% each denoting one argument of the function. The letter, including
% its case, conveys information about the type of argument required.
%
% All functions have a base form with arguments using one of the
% following argument specifiers:
% \begin{arg-description}
%   \item[n]  Unexpanded token or braced token list.\\
%     This is a standard \TeX{} undelimited macro argument.
%   \item[N]  Single token (unlike~|n|, the argument must \emph{not} be
%     surrounded by braces).\\
%     A typical example of a command taking an~|N|
%     argument is~|\cs_set|, in which the command being defined must be
%     unbraced.
%   \item[p]  Primitive \TeX{} parameter specification.\\
%     This can be something simple like~|#1#2#3|, but may use arbitrary
%     delimited argument syntax such as: |#1,#2\q_stop#3|. This is used
%     when defining functions.
%   \item[T,F]
%     These are special cases of~|n| arguments, used for the
%     true and false code in conditional commands.
% \end{arg-description}
% There are two other specifiers with more general meanings:
% \begin{arg-description}
%  \item[D] This means: \textbf{Do not use}. This special case is used
%     for \TeX{} primitives and other commands that are provided for use
%     only while bootstrapping the \LaTeX{} kernel.  Programmers outside
%     the kernel team should not use these functions!
%   \item[w] This means that the argument syntax is \enquote{weird} in that it
%     does not follow any standard rule.  It is used for functions with
%     arguments that take non standard forms: examples are \TeX-level
%     delimited arguments and the boolean tests needed after certain
%     primitive |\if|\ldots{} commands.
% \end{arg-description}
%
% In case of |n| arguments that consist of a single token the
% surrounding braces can be omitted in nearly all
% situations---functions that force the use of braces even for single
% token arguments are explicitly mentioned. However, programmers are
% encouraged to always use braces around \texttt{n} arguments, as this
% makes the relationship between function and argument clearer.
%
% Further argument specifiers are available as part of the expansion
% control system.  These are discussed in the next section.
%
% \section{Expansion control}
%
% Let's take a look at some typical operations one might want to
% perform. Suppose we maintain a stack of open files and we use the
% stack |\g_ior_file_name_seq| to keep track of them (|io| is the
% file reading and writing module: it uses the module prefix
% \texttt{ior} for reading functions and variables). The basic
% operation here is to push a name onto this stack which could be
% done by the operation
% \begin{quote}
%   |\seq_gpush:Nn \g_ior_file_name_seq {#1}|
% \end{quote}
% where |#1| is the filename. In other words, this operation would
% push the file name as is onto the stack.
%
% However, we might face a situation where the filename is stored in
% a variable of some sort, say |\l_ior_curr_file_tl|. In this case we
% want to retrieve the value of the variable. If we simply use
% \begin{quote}
%   |\seq_gpush:Nn \g_io_file_name_seq \l_io_curr_file_tl|
% \end{quote}
% we will not get the value of the variable pushed onto the stack,
% only the variable name itself. Instead a suitable number of
% |\exp_after:wN| would be necessary (together with extra braces) to
% change the order of expansion,\footnote{\cs{exp_after:wN} is
% the \LaTeX3 name for the \TeX{} \tn{expandafter} primitive.}, \emph{i.e.}
% \begin{quote}
%   |\exp_after:wN|                              \\
%   |   \seq_gpush:Nn|                           \\
%   |\exp_after:wN|                              \\
%   |   \g_io_file_name_seq|                   \\
%   |\exp_after:wN|                              \\
%   |   { \l_io_curr_file_tl }|
% \end{quote}
%
% The above example is probably the simplest case but already shows
% how the code changes to something difficult to understand.
% Furthermore there is an assumption in this: that the storage bin
% reveals its contents after exactly one expansion. Relying on this
% means that you cannot do proper checking plus you have to know
% exactly how a storage bin acts in order to get the correct number
% of expansions.  Therefore \LaTeX3 provides the programmer with a
% general scheme that keeps the code compact and easy to understand.
%
% To denote that some argument to a function needs special treatment one
% just uses different letters in the arg-spec part of the function to
% mark the desired behaviour. In the above example one would write
% \begin{quote}
%   |\seq_gpush:NV \g_io_file_name_seq \l_io_curr_file_tl|
% \end{quote}
% to achieve the desired effect. Here the |V| (the second argument)
% is for \enquote{retrieve the value of the variable} before passing it to
% the base function.
%
% The following letters can be used to denote special treatment of
% arguments before passing it to the base function:
% \begin{description}
%   \item[c] Character string used as a command name.\\ The argument (a
%     token or braced token list) must, when fully expanded, produce a
%     sequence of characters which is then used to construct a command
%     name (\emph{via}~|\csname| \ldots |\endcsname|). This command name is the
%     single token that is passed to the function as the argument. Hence
%     \begin{quote}
%       |\seq_gpush:cV { g_file_name_seq } \l_tmpa_tl|
%     \end{quote}
%     is equivalent to
%     \begin{quote}
%       |\seq_gpush:NV \g_file_name_seq \l_tmpa_tl"|
%     \end{quote}
%     Remember that \texttt{c} arguments are \emph{fully expanded} by
%     \TeX{} when creating csnames. This means that (a) the entire
%     argument must be expandable and (b) any variables will be
%     converted to their content. So the preceding examples are also
%     equivalent to
%     \begin{quote}
%       |\tl_new:N \g_file_seq_name_tl| \\
%       |\tl_gset:Nn \g_file_seq_name_tl { g_file_name_seq }| \\
%       |\seq_gpush:cV { \g_file_seq_name_tl } \l_tmpa_tl|.
%     \end{quote}
%     (Token list variables are expandable and do not require an
%     accessor function. Other variable types require the appropriate
%     \cs{<var>_use:N} functions to be used in this context.)
%   \item[V]  Value of a variable.\\
%     This means that the contents of the register in question is used as the
%     argument, be it an integer, a length-type register, a token list variable
%     or similar. The value is passed to the function as a braced token list.
%   \item[v] Value of a register, constructed from a character string
%     used as a command name.\\
%     This is a combination of |c| and |V| which first constructs a
%     control sequence from the argument and then passes the value of the
%     resulting register to the function.
%   \item[x]  Fully-expanded token or braced token list.\\
%     This means that the argument is expanded as in the replacement
%     text of an~|\edef|, and the expansion is passed to the function as
%     a braced token list. This means that expansion takes place until
%     only unexpandable tokens are left.
%  \item[o]  One-level-expanded token or braced token list.\\
%     This means that the argument is expanded one level, as by
%     \tn{expandafter}, and the expansion is passed to the function as a
%     braced token list.  Note that if the original argument is a braced
%     token list then only the first token in that list is expanded.
%     In general, using \texttt{V} should be preferred to using
%     \texttt{o} for simple variable retrieval.
%   \item[f] Almost the same as the |x| type except here the token list
%     is expanded fully until the first unexpandable token is found and
%     the rest is left unchanged. Note that if this function finds a
%     space at the beginning of the argument it will gobble it and not
%     expand the next argument.
% \end{description}
%
% \subsection{Simpler means better}
%
% Anyone who programs in \TeX{} is frustratingly familiar with the
% problem of arranging that arguments to functions are suitably expanded
% before the function is called.  To illustrate how expansion control
% can bring instant relief to this problem we shall consider two
% examples copied from \texttt{latex.ltx}.
%
% \begin{verbatim}
%        \global\expandafter\let
%              \csname\cf@encoding \string#1\expandafter\endcsname
%              \csname ?\string#1\endcsname
% \end{verbatim}
% This first piece of code is in essence simply a global |\let| whose
% two arguments firstly have to be constructed before |\let| is
% executed. The |#1| is a control sequence name such as
% |\textcurrency|. The token to be defined is obtained by
% concatenating the characters of the current font encoding stored in
% |\cf@encoding|, which has to be fully expanded, and the name of the
% symbol. The second token is the same except it uses the default
% encoding |?|. The result is a mess of interwoven |\expandafter|
% and |\csname| beloved of all \TeX{} programmers, and the code is
% essentially unreadable.
%
% Using the conventions and functionality outlined here, the task would
% be achieved with code such as this:
% \begin{verbatim}
%   \cs_gset_eq:cc
%     { \cf@encoding \token_to_str:N  #1 } { ? \token_to_str:N #1 }
% \end{verbatim}
% The command |\cs_gset_eq:cc| is a global~|\let| that generates
% command names out of both of its arguments before making the
% definition. This produces code that is far more readable and more
% likely to be correct first time. (\cs{token_to_str:N} is the \LaTeX3
% name for \tn{string}.)
%
% Here is the second example.
% \begin{verbatim}
%   \expandafter
%     \in@
%   \csname sym#3%
%     \expandafter
%       \endcsname
%     \expandafter
%       {%
%     \group@list}%
% \end{verbatim}
% This piece of code is part of the definition of another function. It
% first produces two things: a token list, by expanding |\group@list| once;
% and a token whose name comes from~`|sym#3|'.  Then the function~|\in@|
% is called and this tests if its first argument occurs in the token list
% of its second argument.
%
% Again we can improve enormously on the code.  First we shall rename
% the function~|\in@| according to our conventions.  A function such as
% this but taking two normal \enquote{\texttt{n}} arguments might reasonably be
% named |\seq_test_in:Nn|; thus the variant function we need will be
% defined with the appropriate argument types and its name will be
% |\seq_test_in:cV|.  Now this code fragment will be simply:
% \begin{verbatim}
%   \seq_test_in:cV { sym #3 } \l_group_seq
% \end{verbatim}
% Note that, in addition to the lack of |\expandafter|, the space after
% the~|}| will be silently ignored since all white space is ignored in
% this programming environment.
%
% \subsection{New functions from old}
%
% For many common functions the \LaTeX3 kernel will provide variants
% with a range of argument forms, and similarly it is expected that
% extension packages providing new functions will make them available in
% the all the commonly needed forms.
%
% However, there will be occasions where it is necessary to construct a
% new such variant form; therefore the expansion module provides a
% straightforward mechanism for the creation of functions with any
% required argument type, starting from a function that takes \enquote{normal}
% \TeX{} undelimited arguments.
%
% To illustrate this let us suppose you have a \enquote{base function}
% |\demo_cmd:Nnn| that takes three normal arguments, and that you need
% to construct the variant |\demo_cmd:cnx|, for which the first argument
% is used to construct the \emph{name} of a command, whilst the third
% argument must be fully expanded before being passed to
% |\demo_cmd:Nnn|.
% To produce the variant form from the base form, simply use this:
% \begin{verbatim}
%   \cs_generate_variant:Nn \demo_cmd:Nnn { cnx }
% \end{verbatim}
% This defines the variant form so that you can then write, for example:
% \begin{verbatim}
%   \demo_cmd:cnx { abc } { pq } { \rst \xyz }
% \end{verbatim}
% rather than \ldots\ well, something like this!
% \begin{verbatim}
%   \def \tempa {{pq}}%
%   \edef \tempb {\rst \xyz}%
%   \expandafter
%     \demo@cmd:nnn
%   \csname abc%
%     \expandafter
%       \expandafter
%     \expandafter
%         \endcsname
%     \expandafter
%       \tempa
%     \expandafter
%       {%
%     \tempb
%       }%
% \end{verbatim}
%
% Another example: you may wish to declare a function
% |\demo_cmd_b:xcxcx|, a variant of an existing function
% |\demo_cmd_b:nnnnn|, that fully
% expands arguments 1,~3 and~5, and produces commands to pass as
% arguments 2 and~4 using~|\csname|.
% The definition you need is simply
% \begin{verbatim}
%   \cs_generate_variant:Nn \demo_cmd_b:nnnnn { xcxcx }
% \end{verbatim}
%
% This extension mechanism is written so that if the same new form of
% some existing command is implemented by two extension packages then the
% two definitions will be identical and thus no conflict will occur.
%
% \section{The distribution}
%
% At present, the \pkg{expl3} modules are designed to be loaded on top
% of \LaTeXe{}. In time, a \LaTeX3 format will be produced based on this
% code. This allows the code to be used in \LaTeXe{} packages \emph{now}
% while a stand-alone \LaTeX3 is developed.
%
% \begin{bfseries}
%   While \pkg{expl3} is still experimental, the bundle is now regarded
%   as broadly stable. The syntax conventions and functions provided
%   are now ready for wider use. There may still be changes to some
%   functions, but these will be minor when compared to the scope of
%   \pkg{expl3}.
% \end{bfseries}
%
%  New modules will be added to the distributed version of \pkg{expl3}
%  as they reach maturity. At present, the \pkg{expl3} bundle consists
%  of a number of modules, most of which are loaded by including the
%  line:
% \begin{verbatim}
%   \RequirePackage{expl3}
% \end{verbatim}
% in a \LaTeXe{} package, class or other file. The \pkg{expl3} modules
% regarded as stable, and therefore suitable for basing real code on,
% are as follows:
%
% \begin{description}
%   \providecommand\explpkg[2]{\item[#1]#2}
%   \explpkg{l3basics}{
%     This contains the basic definition modules used
%     by the other packages.
%   }
%   \explpkg{l3box}{
%     Primitives for dealing with boxes.
%   }
%   \explpkg{l3clist}{
%     Methods for manipulating comma-separated token lists.
%   }
%   \explpkg{l3coffins}{
%     Augmented box constructs for alignment operations.
%   }
%   \explpkg{l3expan}{
%     This is the argument expansion module discussed earlier in this
%     document.
%   }
%   \explpkg{l3int}{
%     This implements the integer data-type \texttt{int}.
%   }
%   \explpkg{l3io}{
%     A module providing low level input and output functions.
%   }
%   \explpkg{l3keys}{
%     For processing lists of the form
%     \texttt{\{ key1=val1 , key2=val2 \}}, intended to work
%     as a \LaTeX3 version of \pkg{xkeyval}/\pkg{kvoptions}, although
%     with input syntax more like that of \pkg{pgfkeys}.
%   }
%   \explpkg{l3msg}{
%     Communicating with the user: includes low-level hooks to allow
%     messages to be filtered (higher-level interface for filtering
%     to be written!).
%   }
%   \explpkg{l3names}{
%     This sets up the basic naming scheme and renames all
%     the \TeX{} primitives.
%   }
%   \explpkg{l3prg}{
%     Program control structures such as boolean data type |bool|, generic
%     do-while loops, case-switches, sorting routines and stepwise loops.
%   }
%   \explpkg{l3prop}{
%     This implements the data-type for \enquote{property lists} that are used, in
%     particular, for storing key/value pairs.
%   }
%   \explpkg{l3quark}{
%     A \enquote{quark} is a command that is defined to expand to
%     itself!  Therefore they must never be expanded as this will generate
%     infinite recursion; they do however have many uses, \emph{e.g.}~as
%     special markers and delimiters within code.
%   }
%   \explpkg{l3seq}{
%     This implements data-types such as queues and stacks.
%   }
%   \explpkg{l3skip}{
%     Implements the \enquote{rubber length} datatype \texttt{skip} and the
%     \enquote{rigid length} datatype |dim|.
%   }
%   \explpkg{l3tl}{
%     This implements a basic data-type, called a \textit{token-list
%     variable} (|tl var.|), used for storing named token lists: these are
%     \TeX{} macros with no arguments.
%   }
%   \explpkg{l3token}{
%     Analysing token lists and token streams, including peeking ahead to
%     see what's coming next and inspecting tokens to detect which kind
%     they are.
%   }
% \end{description}
%
% \section{Moving from \LaTeXe{} to \LaTeX3}
%
% To help programmers to use \LaTeX3 code in existing \LaTeXe{} package,
% some short notes on making the change are probably desirable.
% Suggestions for inclusion here are welcome! Some of the following
% is concerned with code, and some with coding style.
%
% \begin{itemize}
%   \item \pkg{expl3} is mainly focussed on programming. This means that
%     some areas still require the use of \LaTeXe{} internal macros.
%     For example, you may well need \tn{@ifpackageloaded}, as there
%     is currently no native \LaTeX3 package loading module.
%   \item User level macros should be generated using the mechanism
%     available in the \pkg{xparse} package, which is available from CTAN
%     or the \LaTeX3 SVN repository.
%   \item At an internal level, most functions should be generated
%     \tn{long} (using \cs{cs_new:Npn}) rather than \enquote{short} (using
%     \cs{cs_new_nopar:Npn}). However, functions which take no arguments
%     should be set \enquote{short}.
%   \item Where possible, declare all variables and functions (using
%     \cs{cs_new:Npn}, \cs{tl_new:N}, etc.) before use.
%   \item Prefer \enquote{higher-level} functions over \enquote{lower-level},
%     where possible. So for example use \cs{cs_if_exist:N(TF)} in preference
%     \cs{if_cs_exist:N}.
%   \item Use space to make code readable. In general, we recommend
%     a layout such as:
%     \begin{verbatim}
%       \cs_new:Npn \foo_bar:Nn #1#2
%         {
%           \cs_if_exist:NTF #1
%             { \foo_bar_aux_i:n {#2} }
%             { \foo_bar_aux_ii:nn {#2} { literal } }
%         }
%     \end{verbatim}
%     where spaces are used around |{| and |}| except for isolated
%     |#1|, |#2|, \emph{etc.}
%   \item Put different code items on separate lines: readability is
%     much more useful than compactness.
%   \item Use long, descriptive names for functions and variables,
%     and for auxiliary functions use the parent function name plus
%     |aux|, |aux_i|, |aux_ii| and so on.
%   \item If in doubt, ask the team via the LaTeX-L list: someone will
%     soon get back to you!
% \end{itemize}
% 
% \section{Load-time options for \pkg{expl3}}
%
% To support code authors, the \pkg{expl3} package for \LaTeXe{} includes
% a small number of load-time options. These all work in a key--value
% sense, recognising the \texttt{true} and \texttt{false} values. Giving
% the option name alone is equivalent to using the option with the
% \texttt{true} value.
% 
% \DescribeOption{check-declarations}
% All variables used in \LaTeX3 code should be declared. This is enforced
% by \TeX{} for variable types based on \TeX{} registers, but not for those
% which are constructed using macros as the underlying storage system. The
% \texttt{check-declarations} option enables checking for all variable
% assignments, issuing an error if any variables are assigned without being
% initialised.
% 
% \DescribeOption{log-functions}
% The \texttt{log-functions} option is used to enable recording of every new
% function name in the \texttt{.log} file. This is useful for debugging
% purposes, as it means that there is a complete list of all functions
% created by each module loaded (with the exceptions of a very small number
% required by he bootstrap code for \LaTeX3).
% 
% \DescribeOption{native-drivers}
% The \texttt{native-drivers} option instructs \LaTeX3 not to use the \LaTeXe{}
% graphics and colour driver code. Setting this option \texttt{true} uses the
% driver code intended for a \LaTeX3 format.
%
% \section{The \LaTeX3 Project}
%
% Development of \LaTeX3 is carried out by The \LaTeX3 Project. Over time,
% the membership of this team has naturally varied. Currently, the members
% are
% \begin{itemize}
%   \item Johannes Braams
%   \item David Carlisle
%   \item Robin Fairbairns
%   \item Morten H{\o}gholm
%   \item Bruno Le Floch
%   \item Thomas Lotze
%   \item Frank Mittelbach
%   \item Will Robertson
%   \item Chris Rowley
%   \item Rainer Sch{\"o}pf
%   \item Joseph Wright
% \end{itemize}
% while former members are
% \begin{itemize}
%   \item Michael Downes
%   \item Denys Duchier
%   \item Alan Jeffrey
%   \item Martin Schr{\"o}der
% \end{itemize}
%
% \begin{thebibliography}{1}
%
%   \bibitem{A-W:K-TB}
%     Donald E Knuth
%     \newblock \emph{The \TeX{}book}.
%     \newblock Addison-Wesley, Reading, Massachusetts, 1984.
%
%   \bibitem{A-W:GMS94}
%     Goossens, Mittelbach and Samarin.
%     \newblock \emph{ The \LaTeX{} Companion}.
%     \newblock Addison-Wesley, Reading, Massachusetts, 1994.
%
%   \bibitem{A-W:LLa94}
%     Leslie Lamport.
%     \newblock \emph{\LaTeX{}: A Document Preparation System}.
%     \newblock Addison-Wesley, Reading, Massachusetts, second edition, 1994.
%
%   \bibitem{tub:MR97-1}
%     Frank Mittelbach and Chris Rowley.
%     \newblock \enquote{The \LaTeX3 Project}.
%     \newblock \emph{TUGboat},
%     Vol.\,18, No.\,3, pp.\,195--198, 1997.
%
% \end{thebibliography}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{expl3} implementation}
%
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
%
% Load \pkg{etex} as otherwise we are likely to get into trouble
% with registers. Some inserts are reserved also as these have to
% be from the standard pool.
%    \begin{macrocode}
\RequirePackage{etex}
\reserveinserts{32}
%    \end{macrocode}
%
% Do the package identification: this will turn on code syntax.
%    \begin{macrocode}
\ProvidesExplPackage
  {\ExplFileName}{\ExplFileDate}{\ExplFileVersion}{\ExplFileDescription}
%    \end{macrocode}
%
% \begin{macro}[aux]{\expl@create@bool@option}
% \begin{variable}
%   {
%     \l@expl@check@declarations@bool ,
%     \l@expl@log@functions@bool      ,
%     \l@expl@native@drivers@bool
%   }
%   Options to be set up.
%   These have to be done by hand as there is no \pkg{expl3} yet: the logging
%   option is needed before loading \pkg{l3basics}!
%    \begin{macrocode}
\newcommand \expl@create@bool@option [ 2 ]
  {
    \DeclareOption {#1}            { \renewcommand* #2 { 1 } }
    \DeclareOption { #1 =  true }  { \renewcommand* #2 { 1 } }
    \DeclareOption { #1 =  false } { \renewcommand* #2 { 0 } }
    \newcommand* #2 { 0 }
  }
\expl@create@bool@option { check-declarations } \l@expl@check@declarations@bool
\expl@create@bool@option { log-functions }      \l@expl@log@functions@bool
\expl@create@bool@option { native-drivers }     \l@expl@native@drivers@bool
\ProcessOptions \relax
\let \expl@create@bool@option \@undefined
%    \end{macrocode}
% \end{variable}
% \end{macro}
%
%    \begin{macrocode}
\ifodd \l@expl@native@drivers@bool \relax
\else
  \RequirePackage { color , graphics }
\fi
%    \end{macrocode}
%
% We already loaded \pkg{l3names} at the beginning of the \texttt{dtx} file.
% We now load the base of \LaTeX3, stopping once token list variables are
% defined.
%    \begin{macrocode}
\RequirePackage{ l3basics, l3expan, l3tl }
%    \end{macrocode}
%
% \begin{macro}{\cs_check_exists:N}
%   When used as a package, there is an option to be picky and to
%   check definitions exist. This part of the process is done now, so that
%   variable types based on |tl| (for example |clist|, |seq| and |prop|) will
%   inherit the appropriate definitions.
%    \begin{macrocode}
\ifodd \l@expl@check@declarations@bool \relax
  \cs_new_protected:Npn \cs_check_exists:N #1
    {
      \cs_if_exist:NF #1
        {
          \msg_kernel_error:nnx { check } { non-declared-variable }
            { \token_to_str:N #1 }
        }
    }
  \cs_set_protected:Npn \tl_set:Nn #1#2
    {
      \cs_check_exists:N #1
      \cs_set_nopar:Npx #1 { \exp_not:n {#2} }
    }
  \cs_set_protected:Npn \tl_set:Nx #1#2
    {
      \cs_check_exists:N #1
      \cs_set_nopar:Npx #1 {#2}
    }
  \cs_set_protected:Npn \tl_gset:Nn #1#2
    {
      \cs_check_exists:N #1
      \cs_gset_nopar:Npx #1 { \exp_not:n {#2} }
    }
  \cs_set_protected:Npn \tl_gset:Nx #1#2
    {
      \cs_check_exists:N #1
      \cs_gset_nopar:Npx #1 {#2}
    }
  \cs_set_protected:Npn \tl_set_eq:NN #1#2
    {
      \cs_check_exists:N #1
      \cs_check_exists:N #2
      \cs_set_eq:NN #1 #2
    }
  \cs_set_protected:Npn \tl_gset_eq:NN #1#2
    {
      \cs_check_exists:N #1
      \cs_check_exists:N #2
      \cs_gset_eq:NN #1 #2
    }
\fi
%    \end{macrocode}
% \end{macro}
%
% The rest of the code can now be loaded.
%    \begin{macrocode}
\RequirePackage{
  l3seq,
  l3int,
  l3quark,
  l3prg}
%    \end{macrocode}
%
%  Booleans are not based on token lists but do need checking: another
%  block of code.
%    \begin{macrocode}
\ifodd \l@expl@check@declarations@bool \relax
  \cs_set_protected:Npn \bool_set_true:N #1
    {
      \cs_check_exists:N #1
      \cs_set_eq:NN #1 \c_true_bool
    }
  \cs_set_protected:Npn \bool_set_false:N #1
    {
      \cs_check_exists:N #1
      \cs_set_eq:NN #1 \c_false_bool
    }
  \cs_set_protected:Npn \bool_gset_true:N #1
    {
      \cs_check_exists:N #1
      \cs_gset_eq:NN #1 \c_true_bool
    }
  \cs_set_protected:Npn \bool_gset_false:N #1
    {
      \cs_check_exists:N #1
      \cs_gset_eq:NN #1 \c_false_bool
    }
  \cs_set_protected:Npn \bool_set_eq:NN  #1
    {
      \cs_check_exists:N #1
      \cs_set_eq:NN #1
    }
  \cs_undefine:N \bool_set_eq:cN
  \cs_undefine:N \bool_set_eq:Nc
  \cs_undefine:N \bool_set_eq:cc
  \cs_generate_variant:Nn \bool_set_eq:NN { c , Nc , cc }
  \cs_set_protected:Npn \bool_gset_eq:NN  #1
    {
      \cs_check_exists:N #1
      \cs_set_eq:NN #1
    }
  \cs_undefine:N \bool_gset_eq:cN
  \cs_undefine:N \bool_gset_eq:Nc
  \cs_undefine:N \bool_gset_eq:cc
  \cs_generate_variant:Nn \bool_gset_eq:NN { c , Nc , cc }
  \cs_set_protected:Npn \bool_set:Nn #1#2
    {
      \cs_check_exists:N #1
      \tex_chardef:D #1 = \bool_if_p:n {#2}
    }
  \cs_set_protected:Npn \bool_gset:Nn #1#2
    {
      \cs_check_exists:N #1
      \tex_global:D \tex_chardef:D #1 = \bool_if_p:n {#2}
    }
\fi
%    \end{macrocode}
%
%    \begin{macrocode}
\RequirePackage{
  l3clist,
  l3token,
  l3prop,
  l3msg,
  l3io,
  l3file,
  l3skip,
  l3keys,
  l3fp,
  l3box,
  l3coffins,
  l3color,
  l3luatex
}
%    \end{macrocode}
%
% The error message for variables cannot be created before now: this means that
% \pkg{expl3} itself may cause some odd errors.
%    \begin{macrocode}
\bool_if:nT { \l@expl@check@declarations@bool }
  {
    \msg_kernel_new:nnnn { check } { non-declared-variable }
      { The~variable~#1~has~not~been~declared~\msg_line_context:. }
      {
        Checking~is~active,~and~you~have~tried~do~so~something~like: \\
        \ \ \tl_set:Nn ~ #1 ~ \iow_char:N \{ ~ ... ~ \iow_char:N \} \\
        without~first~having: \\
        \ \ \tl_new:N ~ #1  \\
        \\
        LaTeX~will~create~the~variable~and~continue.
      }
  }
%    \end{macrocode}
%
% \begin{macro}{\color}
%   If the native drivers are being used, they can be loaded now. The
%   \cs{color} macro must be defined for showing coffin poles, so
%   a no-op version is provided here.
%    \begin{macrocode}
\bool_if:nTF { \l@expl@native@drivers@bool }
  {
    \xetex_if_engine:TF
      { \file_input:n { l3xdvipdfmx.def } }
      {
        \RequirePackage { ifpdf }
        \ifpdf
          \file_input:n { l3pdfmode.def }
        \else
          \file_input:n { l3dvips.def }
        \fi
      }
    \AtBeginDocument
      {
        \cs_if_free:NT \color
          { \DeclareRobustCommand \color [2] [ ] { } }
      }
  }
%    \end{macrocode}
% \end{macro}
% \begin{macro}
%   {
%     \box_rotate:Nn,
%     \box_resize:Nnn,
%     \box_resize_to_ht_plus_dp:Nn,
%     \box_resize_to_wd:Nn,
%     \box_scale:Nnn
%   }
%     If not, alter various definitions to use the \pkg{graphicx} package
%     instead.
%    \begin{macrocode}
  {
    \cs_set_protected:Npn \box_rotate:Nn #1#2
      { \hbox_set:Nn #1 { \rotatebox {#2} { \box_use:N #1 } } }
    \cs_set_protected:Npn \box_resize:Nnn #1#2#3
      {
        \hbox_set:Nn #1
          {
            \resizebox *
              { \etex_dimexpr:D #2 \scan_stop: }
              { \etex_dimexpr:D #3 \scan_stop: }
              { \box_use:N #1 }
          }
      }
    \cs_set_protected:Npn \box_resize_to_ht_plus_dp:Nn #1#2
      {
        \hbox_set:Nn #1
          {
            \resizebox * { ! } { \etex_dimexpr:D #2 \scan_stop: }
              { \box_use:N #1 }
          }
      }
    \cs_set_protected:Npn \box_resize_to_wd:Nn #1#2
      {
        \hbox_set:Nn #1
          {
            \resizebox * { \etex_dimexpr:D #2 \scan_stop: } { ! }
              { \box_use:N #1 }
          }
      }
    \cs_set_protected:Npn \box_scale:Nnn #1#2#3
      { \hbox_set:Nn #1 { \scalebox {#2} [#3] { \box_use:N #1 } } }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\ExplSyntaxOff, \ExplSyntaxOn}
%   These are redefined here to reduce unnecessary work when switching.
%   The definition for \cs{ExplSyntaxOff} copies the code from earlier,
%   so that category codes are restored.
%    \begin{macrocode}
\cs_set_protected_nopar:Npn \ExplSyntaxOn
  {
    \bool_if:NF \l_expl_status_bool
      {
        \cs_set_protected_nopar:Npx \ExplSyntaxOff
          {
            \char_set_catcode:nn { 9 }   { \char_value_catcode:n { 9 } }
            \char_set_catcode:nn { 32 }  { \char_value_catcode:n { 32 } }
            \char_set_catcode:nn { 34 }  { \char_value_catcode:n { 34 } }
            \char_set_catcode:nn { 36 }  { \char_value_catcode:n { 36 } }
            \char_set_catcode:nn { 38 }  { \char_value_catcode:n { 38 } }
            \char_set_catcode:nn { 58 }  { \char_value_catcode:n { 58 } }
            \char_set_catcode:nn { 94 }  { \char_value_catcode:n { 94 } }
            \char_set_catcode:nn { 95 }  { \char_value_catcode:n { 95 } }
            \char_set_catcode:nn { 124 } { \char_value_catcode:n { 124 } }
            \char_set_catcode:nn { 126 } { \char_value_catcode:n { 126 } }
            \tex_endlinechar:D = \tex_the:D \tex_endlinechar:D \scan_stop:
            \bool_set_false:N \l_expl_status_bool
            \cs_set_protected_nopar:Npn \ExplSyntaxOff { }
          }
      }
    \char_set_catcode_ignore:n           { 9 }   % tab
    \char_set_catcode_ignore:n           { 32 }  % space
    \char_set_catcode_other:n            { 34 }  % double quote
    \char_set_catcode_math_toggle:n      { 36 }  % dollar
    \char_set_catcode_alignment:n        { 38 }  % ampersand
    \char_set_catcode_letter:n           { 58 }  % colon
    \char_set_catcode_math_superscript:n { 94 }  % circumflex
    \char_set_catcode_letter:n           { 95 }  % underscore
    \char_set_catcode_other:n            { 124 } % pipe
    \char_set_catcode_space:n            { 126 } % tilde
    \tex_endlinechar:D = 32 \scan_stop:
    \bool_set_true:N \l_expl_status_bool
  }
\cs_set_protected_nopar:Npx \ExplSyntaxOff
  {
    \exp_not:o \ExplSyntaxOff
    \cs_set_protected_nopar:Npn \ExplSyntaxOff { }
  }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%</package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex