WML Macros - Writing powerful WML macros

This tutorial is a guide for writing macros in WML. It should help beginners to write their first templates, but also give useful hints to write tricky macros. To take best benefit of this document, it is highly recommended to read documentation of individual passes first.

Following examples are compiled with

  wml -q -p123 test.wml

Most of them could be passed through mp4h only, but the line below is more generic.

These definitions are those used in this document, they may differ from those of the W3C because i do not want to enter into deep details.

•

A tag is a portion of text enclosed between bracket angles, like

     <a>
     </table>
     <!-- hey this is a comment -->
     <?xml version="1.0" encoding="UTF-8"?>

•

A start tag is a tag which begins an element (see below). It consists of a left angle bracket, followed by the element name, optional attributes (see below), and a right angle bracket. All these are start tags:

     <a href="#name">
     <td>
     <meta name="generator" content="vi">

•

An end tag is a tag which ends an element (see below). It consists of a left angle bracket, a slash, the element name, and a right angle bracket, like in

     </table>
     </a>

This tag cannot contain attributes.

•

An element is an elementary unit of the document. It mainly consists of pair of start and end tags, like in

     <a href="#name">Click here</a>

•

The body of an element is the portion of text contained between the start and the end tags. In the example above, there is one element, which name is CWa, and its body is "CWClick here".

•

Attributes are parameters to make elements more flexible. They must be put in the start tag. An element may have any number of attributes, which are separated by one or more spaces, tabulations or newlines. Each element may define which attributes are mandatory and which are optional.

     <img src="logo.png" alt="Logo"
          title="Our nice and beautiful logo">

The CWimg element has 3 attributes

•

A simple tag is an element without end tag.

•

A complex tag is an element with start and end tags.

Basically all macro definitions are performed with the CW<define-tag>. Here is a trivial example:

Input:

  1| <define-tag foo>
  2| bar
  3| </define-tag>
  4| <FOO>

Output:

  1|
  2|
  3| bar
  4|

Whereas trivial this example shows some interesting points:

•

Newlines are preserved, there is the same number of lines on input and output, but we will discuss about whitespaces in detail below.

•

Tag names are case insensitive.

In HTML simple tags are an element without end tag, e.g.

    <br>

But XML specifies that simple tags must be written with one of these 2 forms:

    <br></br>
    <br/>

i.e. either as a complex tag, without body, or by adding a trailing slash to the start tag. The first one will not work with WML, and also may confuse HTML browsers, and so should be avoided. You have to choose to write this trailing slash or not, WML works with both forms.

In this document, i will now always write simple tags with this trailing slash, to conform to the new XHTML standard. This is my preferred writing of input text, but one may still continue without this trailing slash. You decide to which syntax you want to conform to.

On the other hand, HTML browsers may be confused by XHTML syntax, so output text does not contain this trailing slash. This seems contradictory, but with this approach our input files are ready to be processed by future XML tools, and we only have to run WML with adequate flags to produce XHTML compliant pages.

Each time a known element is found in input text, it is removed and its replacement text is put here. After that, this replacement text is scanned in case it contains other macros.

All user macros are defined with the CWdefine-tag element. Its first attribute is the macro name which is defined, and its body function is the replacement text which is inserted in lieu of this macro.

Let us begin with a simple example:

Input:

  1| <define-tag homepage>http://www.engelschall.com/sw/wml/</define-tag>
  2| <homepage/>

Output:

  1|
  2| http://www.engelschall.com/sw/wml/

Defining a complex tag is no more difficult, just add an CWendtag=required attribute.

Input:

  1| <define-tag foo endtag=required>bar</define-tag>
  2| <foo>baz</foo>

Output:

  1|
  2| bar

Some strings have a special meaning when found in replacement text, to allow full customization of macros: Attributes: CW%0 is the first attribute, CW%1 the second, and so on.

%name

Macro name

%attributes

Space-separated list of all attributes

%body

Macro body (for complex tags only)

%#

Number of arguments

%%

A percent sign

Input:

  1| <define-tag foo endtag=required>
  2| Macro name:          %name
  3| Number of arguments: %#
  4| First argument:      %0
  5| Second argument:     %1
  6| All arguments:       %attributes
  7| Body macro:          %body
  8| </define-tag>
  9| <foo Here are attributes>
 10| And the body
 11| goes here.
 12| </foo>

Output:

  1| 
  2| 
  3| Macro name:          foo
  4| Number of arguments: 3
  5| First argument:      Here
  6| Second argument:     are
  7| All arguments:       Here are attributes
  8| Body macro:          
  9| And the body
 10| goes here.
 11| 
 12|

These special strings may also be altered by modifiers, which are a set of letters (one or more) put after the percent sign. These modifiers, and their actions, are:

U (Unexpanded)

Text is replaced, but not expanded (see section about expansion for details).

A (Array)

Lists are separated by newlines instead of spaces. This modifier makes sense with CW%attributes only. Input:

  1| <define-tag foo endtag=required>
  2| First argument:      %A0
  3| All arguments:       %Aattributes
  4| Body macro:          %Abody
  5| </define-tag>
  6| <foo Here are attributes>
  7| And the body
  8| goes here.
  9| </foo>

Output:

  1| 
  2| 
  3| First argument:      Here
  4| All arguments:       Here
  5| are
  6| attributes
  7| Body macro:          
  8| And the body
  9| goes here.
 10| 
 11|

Note that these sequences are replaced when macro is read, after what replacement text is scanned again. This is very important, because you should never write constructs like

   <if <get-var foo /> %body />

Indded, CW%body is replaced before CW<if> element is scanned, which may cause unpredictable results. A better solution is

   <if <get-var foo /> "%body" />

but it will cause trouble when CW%body contains double quotes. For this reason, you should never use CW<if> (and derivatives) tests when one of its arguments is a special sequence. Use instead

   <when <get-var foo />>
   %body
   </when>

Previous examples show that expansion prints lots of unused newlines. There are some techniques to remove them. The first one is with pass 1, by putting a backslash at end of line, which will discard this end of line.

Input:

  1| <define-tag foo>\
  2| bar\
  3| </define-tag>\
  4| <FOO/>

Output:

  1| bar

Another solution is to specify CWwhitespace=delete when defining macros, e.g.

  1| <define-tag foo whitespace=delete>
  2| bar
  3| </define-tag>
  4| <FOO/>

Output:

  1|
  1| bar

The first line is caused by newline after CW</define-tag> which is not discarded.

When this attribute is used, all trailing and leading whitespaces are removed, and also newlines outside of angle brackets.

One nice feature of WML is its ability to deal with arbitrary attributes. There are many ways to define macros accepting attributes, we will discuss here the one used in all WML modules, and is so the standard way.

Attributes are stored in variables, because HTML syntax CWattribute=value is very closed to assignment to variables. In order to keep variables local, a mechanism of push/pop is used. Here is an example

Input:

  1| <define-tag href whitespace=delete>
  2| <preserve url />
  3| <preserve name />
  4| <set-var %attributes />
  5| <if <get-var name /> ""
  6|   <set-var name="<tt><get-var url /></tt>" /> />
  7| <a href="<get-var url />"><get-var name /></a>
  8| <restore name />
  9| <restore url />
 10| </define-tag>
 11| <href url="http://www.w3.org/" />

Output:

  1| 
  2| <a href="http://www.w3.org/"><tt>http://www.w3.org/</tt></a>

The CW<preserve> tag pushes the variable passed in argument in top of a stack and clears this variable. So this variable is non-null only when it has been assigned via CW<set-var %attributes>. The CW<resstore<gt> tag pops the value at top of the stack and sets the variable passed in argument to this value.

In HTML some attributes are valid without value. This attribute may be detected with

Input:

  1| #use wml::std::info
  2| <define-tag head whitespace=delete>
  3| <preserve title>
  4| <preserve info>
  5| <set-var info=*>
  6| <set-var %attributes>
  7| <head*>
  8| <ifeq "<get-var info>" "" <info style=meta>>
  9| <if "<get-var title>" "<title*><get-var title></title*>">
 10| </head*>
 11| <restore info>
 12| <restore title>
 13| </define-tag>
 14| <head title="Test page 1">
 15| <head info title="Test page 2">

Output: (only non-blank lines are printed)

     <head><title>Test page 1</title></head>
     <head>
     <nostrip><meta name="Author"    content="Denis Barbier, barbier@localhost">
     <meta name="Generator" content="WML 2.0.2 (21-Jun-2000)">
     <meta name="Modified"  content="2000-05-09 23:57:31">
     </nostrip>
     <title>Test page 2</title></head>

In HTML it is possible to specify attributes containing several words, by quoting them with single or double quotes. WML knows only double quotes.

  1| <define-tag foo>\
  2| Number of arguments: %#
  3| First argument:      %0
  4| </define-tag>\
  5| <foo Here are attributes />\
  6| <foo "Here are" attributes />\

Output:

  1| Number of arguments: 3
  2| First argument:      Here
  3| Number of arguments: 2
  4| First argument:      Here are

In this section, all examples are processed with the command line

   wml -W2,-dat -q -p123

and all output lines beginning with CWtrace are generated by these debug flags.

This section is harder to understand, but one can work with WML without understanding it, because these notions are required in rare cases (mostly when writing macros for WML tutorials).

By default, macros are expanded when tags are scanned.

Input:

  1| <define-tag foo>%attributes</define-tag>\
  2| <define-tag bar>baz</define-tag>\
  3| <foo name="<bar/>" />

Output:

  1| trace: -1- <define-tag foo>
  2| trace: -1- <define-tag bar>
  3| trace: -2- <bar>
  4| trace: -1- <foo name=baz>
  5| name=baz

We see that the CW<bar> macro is processed first (digit between hyphens represent enesting level), and then CW<foo>. Indeed WML finds the CWfoo name. As this is a macro name, attributes are searched for. When scanning attributes, it finds the CW<bar>. As this macro has no attribute, it is now replaced by its replacement text, after that scanning of CW<foo> attributes is finished.

Consider now

Input:

  1| <define-tag foo attributes=verbatim>%attributes</define-tag>\
  2| <define-tag bar>baz</define-tag>\
  3| <foo name="<bar/>" />

Output:

  1| trace: -1- <define-tag foo>
  2| trace: -1- <define-tag bar>
  3| trace: -2- <bar>
  4| trace: -1- <foo name=<bar>>
  5| trace: -1- <bar>
  6| name=baz

The CWattributes=verbatim attribute tells WML that when scanning this macro attributes, no expansion is performed. So the four first lines are now easy to understand. But after CW<foo> is expanded into

   name=<bar>

this text is scanned again and CW<bar> is expanded in turn.

The solution to forbid this expansion is to use the CWU modifier, explained in section Special Text.

Input:

  1| <define-tag foo attributes=verbatim>%Uattributes</define-tag>\
  2| <define-tag bar>baz</define-tag>\
  3| <foo name="<bar/>" />

Output:

  1| trace: -1- <define-tag foo>
  2| trace: -1- <define-tag bar>
  3| trace: -2- <bar>
  4| trace: -1- <foo name=<bar>>
  5| name=<bar>

After these preliminaries it is time to see how to mix mp4h and ePerl. The following section is a bit tricky, you may skip to section How to use these macros to quickly learn which changes are needed.

Consider this macro:

   <define-tag show-attr><: print "attrs:%attributes"; :></define-tag>

At first look, it behaves like

   <define-tag show-attr-ok>attrs:%attributes</define-tag>

But what happens when these macros are nested?

Input:

  1| <show-attr-ok <show-attr-ok 0 /> />

Output:

  1| attrs:attrs:0

It works fine! On the other hand,

Input:

  1| <show-attr <show-attr 0 /> />

Output:

  1| ePerl:Error: Perl parsing error (interpreter rc=255)
  2| 
  3| ---- Contents of STDERR channel: ---------
  4| Backslash found where operator expected at /tmp/wml.1183.tmp1.wml line
  5| 10, near ""attrs:<: print attrs:0; print "\"
  6|         (Missing operator before \?)
  7| syntax error at /tmp/wml.1183.tmp1.wml line 10, near ""attrs:<: print
  8| attrs:0; print "\"
  9| Execution of /tmp/wml.1151.tmp1.wml aborted due to compilation errors.
 10| ------------------------------------------
 11| ** WML:Break: Error in Pass 3 (rc=74).

Huh, looks like something went wrong. Output after pass 2 is

  1| <: print "attrs:<: print attrs:0; :>"; :>

And because ePerl commands cannot be nested, an error is reported (if you do not understand why we have this text after pass 2, reread previous section).

This example is simplistic, and a workaround is trivial (use CW<show-attr-ok> instead), but there are many cases where these problems are much more difficult to track. For instance if you nest macros defined in WML modules, you do not know whether they use ePerl code or not.

One problem is that ePerl commands cannot be nested, according to its documentation. So our first try is to count nested levels and print ePerl delimeters when in outer mode only.

Input:

  1| <set-var __perl:level=0 />\
  2| <define-tag perl endtag=required whitespace=delete>
  3| <increment __perl:level />
  4| <when <eq <get-var __perl:level /> 1 />>
  5| <: %body :>
  6| </when>
  7| <when <neq <get-var __perl:level /> 1 />>
  8| %body
  9| </when>
 10| <decrement __perl:level />
 11| </define-tag>\
 12| <define-tag add1 endtag=required>\
 13| <perl>$a += 1; %body</perl>\
 14| </define-tag>\
 15| <add1><add1><add1></add1></add1></add1>
 16| <:= $a :>

Output:

  1|
  2| 3

Another example (lines 1-11 are left unchanged)

Input:

 12| <define-tag remove-letter endtag=required whitespace=delete>
 13| <perl>
 14|   $string = q|%body|; $string =~ s|%0||g; print $string;
 15| </perl>
 16| </define-tag>\
 17| <remove-letter e>Hello this is a test</remove-letter>

Output:

  1| Hllo this is a tst

With previous definitions, here is what happens when nesting CW<remove-letter> tags:

Input:

 17| <remove-letter s><remove-letter e>\
 18| Hello this is a test\
 19| </remove-letter></remove-letter>

Output:

  1| ePerl:Error: Perl parsing error (interpreter rc=255)
  2| 
  3| ---- Contents of STDERR channel: ---------
  4| Bareword found where operator expected at /tmp/wml.1198.tmp1.wml
  5| line 10, near "q|$string = q|Hello"
  6| syntax error at /tmp/wml.1198.tmp1.wml line 10, near "q|$string =
  7| q|Hello this "syntax error at /tmp/wml.1198.tmp1.wml line 10, near ";|"
  8| Execution of /tmp/wml.1198.tmp1.wml aborted due to compilation errors.
  9| ------------------------------------------
 10| ** WML:Break: Error in Pass 3 (rc=74).

To understand why this error is reported, we run only the first two passes to see which input is sent to ePerl:

    prompt$ wml -q -p12 qaz.wml
    <: $string = q|$string = q|Hello this is a test|; $string =~ s|e||g;
    print $string;|; $string =~ s|s||g; print $string; :>

As expected ePerl delimiters are only put around the whole sentence, and are not nested. But we can see this is not sufficient, because the CW%body directive was replaced by ePerl code, and not a string.

In one word, there will be trouble whenever special sequences (CW%<digit>, CW%body, CW%attributes, ...) appear within ePerl delimiters, because you can not ensure that replacement text does not contain ePerl commands too.

The wml::std::tags(3) module provides a solution to deal with nested ePerl commands. Previous example may be written like this

Input:

  1| #use wml::std::tags
  2| 
  3| <define-tag remove-letter endtag=required whitespace=delete>
  4| <perl>
  5| <perl:assign $string>%body</perl:assign>
  6| <perl:assign $letter>%0</perl:assign>
  7| $string =~ s|$letter||g;
  8| <perl:print: $string />
  9| </perl>
 10| </define-tag>\ 
 11| <remove-letter s><remove-letter e>\
 12| Hello this is a test\
 13| </remove-letter></remove-letter>

Output:

      ...61 empty lines...
  62| Hllo thi i a tt
  63|
  64|

How this works is beyond the scope of this document, and we will focus on commands provided by the wml::std::tags module, and how to use them. In the list below, pseudo-perl commands show an equivalent form of these macros.

<perl:var />

This macro expands to a Perl variable, which is different in all nested levels.

    $perl_var<get-var __perl:level />

<perl:print>string</perl:print>

This complex tag prints its body.

   print qq(string);

<perl:print: string />

This simple tag prints its attributes.

   print string;

<perl:print:var />

Prints the CW<perl:var> variable

  print $perl_var<get-var __perl:level />;

Assign a Perl variable. If there is no attribute, value is assigned to CW<perl:var>.

   $variable = qq(value);

Assign a Perl variable. If there is no attribute, value is assigned to CW<perl:var>.

   $variable = q(value);

Now that we know our problem has a solution, you are certainly impatient to learn how to proceed. There are two golden rules:

1

Never write special sequences (CW%<digit>, CW%body, CW%attributes, ...) inside a Perl statement.

2

Never use the Perl CWprint statement, nor its derivatives.

First rule tells to replace

  $var1 = qq|%body|;
  $var2 = q|%body|;

by

  <perl:assign $var1>%body</perl:assign>
  <perl:assign:sq $var2>%body</perl:assign:sq>

and second rule

  print $string;
  print "<img src=\"$src\" alt=\"$alt\">";

by

  <perl:print: $string>
  <perl:print><img src="$src" alt="$alt"></perl:print>

Example 1: simplified version of CWwml::des::lowsrc

Non-nestable version:

  <define-tag lowsrc>
  <:
  {
      my $src = '%0';
      my $lowsrc = $src;
      $lowsrc =~ s|\.([^.]+)$|.lowsrc.$1|;
      system("convert -monochrome $src $lowsrc");
      print "lowsrc=\"$lowsrc\"";
  }
  :>
  </define-tag>

Nestable version:

  <define-tag lowsrc>
  <perl>
  {
      my $src;
      <perl:assign:sq $src>%0</perl:assign:sq>
      my $lowsrc = $src;
      $lowsrc =~ s|\.([^.]+)$|.lowsrc.$1|;
      system("convert -monochrome $src $lowsrc");
      <perl:print> lowsrc="$lowsrc"</perl:print>
  }
  </perl>
  </define-tag>

The first change (assignment to CW$src) allows attribute to be an ePerl command, and second change (print result) allows this macro to appear inside ePerl commands. As you see, this is fairly straightforward, and you may look how WML modules are written.

In all previous examples and definitions, output was printed to standard output. But sometimes it is printed to filehandles. Here is how to proceed, with an example taken from CWwml::fmt::xtable.

Non-nestable version:

  <define-tag xtable endtag=required>
  <:
  {
      my $options = qq|%attributes|;
      my $tmpfile = "<get-var WML_TMPDIR>/wml.table.$$.tmp";
      local (*FP);
      open(FP, ">$tmpfile");
      print FP "<" . "wwwtable $options>\n";
      print FP <<'__XTABLE__EOT'
  %body
  __XTABLE__EOT
  ;
      print FP "<" . "/wwwtable>\n";
      close(FP);
      open(FP, "$WML_LOC_LIBDIR/exec/freetable -w $tmpfile|");
      local ($/) = undef;
      print <FP>;
      close(FP);
      unlink("$tmpfile");
  }
  :>
  </define-tag>

Nestable version:

  <set-var __xtable:level=0 />
  <define-tag xtable endtag=required>
  <increment __xtable:level />
  <perl filehandle="FH_XTABLE">
  {
      my $tmpfile = "<get-var WML_TMPDIR />/wml.table.$$.tmp";
      my $options;
      <perl:assign $options>%attributes</perl:assign>;
      <when <eq <get-var __xtable:level /> 1 />>
      local *FH_XTABLE;
      open(FH_XTABLE, ">$tmpfile");
      </when>
      <perl:assign>
      <wwwtable $options>
          %body
      </wwwtable>
      </perl:assign>
  </perl>
  #   we cut here to change filehandle
  <perl>
      <when <eq <get-var __xtable:level /> 1 />>
      print FH_XTABLE <perl:var/>;
      close(FH_XTABLE);
      open(FH_XTABLE_IN,
         "<get-var WML_LOC_LIBDIR />/exec/freetable -w $tmpfile |");
      local ($/) = undef;
      #  The asterisk below prevents expansion during pass 2 and is
      #  removed after this pass.
      <perl:var/> = <*FH_XTABLE_IN>;
      close(FH_XTABLE_IN);
      <perl:print:var/>
      unlink("$tmpfile");
      </when>
  }
  </perl>
  <decrement __xtable:level />
  </define-tag>

Filehandles are defined via attributes to the CWperl tag. All subsequent calls to CW<perl:print> are then printed to this filehandle.

 Denis Barbier
 barbier@engelschall.com