Jochen Wiedmann

NAME

    HTML::EP - a system for embedding Perl into HTML

SYNOPSIS

    <html><head><title>CGI-Env</title></head>
    <ep-comment>
        This is an HTML document. You see. Perhaps you wonder about
        the unknown HTML tags like ep-comment above? They are part
        of the EP system. For example, this comment section will
        be removed and you won't see it in your browser.
    </ep-comment>
    <ep-perl>
        # This is an example of embedding Perl into the page.
        # We create a variable called time, containing the current
        # time. This variable will be used below.
        my $self = $_;
        $self->{'time'} = localtime(time());
        ''; # Return an empty string; result becomes embedded into the
            # HTML page
    </ep-perl>
    <body><h1>The current time</h1>
        Your HTML::EP system is up and running: The current time is $time$.
    </body>
    </html>

WARNING

THIS IS ALPHA SOFTWARE. It is *only* 'Alpha' because the interface (API) is not finalised. The Alpha status does not reflect code quality or stability.

DESCRIPTION

Have you ever written a CGI binary? Easy thing, isn't it? Was just fun!

Have you written two CGI binaries? Even easier, but not so much fun.

How about the third, fourth or fifth tool? Sometimes you notice that you are always doing the same:

  • Reading and parsing variables

  • Formatting output, in particular building tables

  • Sending mail out from the page

  • Building a database connection, passing CGI input to the database and vice versa

  • Talking to HTML designers about realizing their wishes

You see, it's soon to become a pain. Of course there are little helpers around, for example the CGI module, the mod_perl suite and lots of it more. Using them make live a lot easier, but not so much as you like. CGI(3). mod_perl(3).

On the other hand, there are tools like PHP/FI or WebHTML. Incredibly easy to use, but not as powerfull as Perl. Why not get the best from both worlds? This is what EP wants to give you, similar to ePerl or HTML::EmbPerl. I personally believe that EP is simpler and better extendible than the latter two. ePerl(1). HTML::EmbPerl(3).

In short, it's a single, but extensible program, that scans an HTML document for certain special HTML tags. These tags are replaced by appropriate output generated by the EP. What remains is passed to the browser. Its just like writing HTML for an enhanced browser!

Prerequisites

As far as I know EP depends on no system dependent features. However, it relies on some other Perl modules:

CGI

The CGI module should be a part of your Perl core's installation. If not, you should definitely upgrade to Perl 5.004. :-) My thanks to Lincoln D. Stein <lstein@genome.wi.mit.edu>.

HTML::Parser

This module is used for parsing the HTML templates. My thanks to Gisle Aas <aas@sn.no>.

libwww

The LWP library contains a lot of utility functions, for example HTML and URL encoding and decoding. Again, my thanks to Gisle Aas <aas@sn.no>. :-)

Perl itself and the above modules are available from any CPAN mirror, for example

       ftp://ftp.funet.fi/pub/languages/perl/CPAN/modules/by-module

Note that you don't need to install these manually, the CPAN module will help you. See the "Automatic Installation" below.

Automatic Installation

Installing this module (and the prerequisites from above) is quite simple, if you have the CPAN module available and network access, you might try an automatic installation:

        perl -MCPAN -e 'CPAN->install("Bundle::HTML::EP")'

Manual Installation

If automatic installation doesn't work, you just fetch the archive, extract it with

    gzip -cd HTML-EP-0.1000.tar.gz | tar xf -

(this is for Unix users, Windows users would prefer WinZip or something similar) and then enter the following:

    cd HTML-EP-0.1000
    perl Makefile.PL
    make
    make test

If any tests fail, let me know. Otherwise go on with

    make install

This will put the required Perl modules into a destination where Perl finds it by default. Additionally it will install a single CGI binary, called ep.cgi.

The docs are available online with

    perldoc HTML::EP

If you prefer an HTML version of the docs, try

    pod2html lib/HTML/EP.pm

in the source directory.

Using the CGI binary

You have different options for integrating EP into your WWW server, depending on which server you are using and the permissions you have. The simplest possibility is running an external CGI binary. Another option is to use mod_perl with Apache, see "Using mod_perl" below.

I suggest that you choose an extension and configure your WWW server for feeding files with this extension into ep.cgi. For example, with Apache, you can add the following lines to your srm.conf:

    ScriptAlias /cgi-bin/ep.cgi /usr/bin/ep.cgi
    AddHandler x-ep-script .ep
    Action x-ep-script /cgi-bin/ep.cgi

This tells Apache that files with extension ep.cgi are handled by the CGI binary /usr/bin/ep.cgi. Make sure, that the ScriptAlias line is entered *before* any other ScriptAlias instruction!

From now on your server will never return files with extension .ep directly! Verify your installation by creating the following file:

    <html><head><title>Local time</title></head>
    <body>
    The current time is:
    <ep-perl>scalar(localtime(time))</ep-perl>
    </body>

(Note that this is a much shorter version of the example in the synopsis.) Store it as /test.ep on your web server and retrieve the file via your Web server. If you see the time displayed, it are up and running.

Using mod_perl

The EP package can be integrated into mod_perl, for example by using the following commands in srm.conf:

    <Files *.ep>
      SetHandler perl-script
      PerlHandler Apache::EP->handler
      Options ExecCGI
    </Files>

Keep in mind, that mod_perl differs in many details from programming CGI binaries. In particular you might need to restart Apache for loading changes in modules.

Available methods

All EP tags are starting with the prefix ep-. Some available tags are:

ep-comment

This is a multi-line tag for embedding comments into your HTML page. But why use this tag, instead of the usual HTML comment, <!--? The difference is, that the user will never see the former.

Example:

    <html>
        <!-- This is a comment. I like comments. --!>
        <ep-comment>
            This is another comment, but you won't see it
            in your browser. The HTML editor will show it
            to you, however!
        </ep-comment>
    </html>

Do not try to embed EP instructions into the comment section! They won't produce output, but they might be executed anyways.

ep-perl

This is for embedding Perl into your script. There are two versions of it: A multiline version is for embedding the Perl code immediately into your script. Example:

    <html>
        <head><title>The Date</title></head>
        <body>
            <h1>The Date</h1>
            <p>Hello, today its the</p>
            <p align=center>
            <ep-perl>
                # This little piece of Perl code will be executed
                # while scanning the page.
                #
                # Let's calculate the date!
                #
                my($sec,$min,$hour,$mday,$mon,$year)
                    = localtime(time);
                # Leave a string with the date as result. Will be
                # inserted into the HTML stream:
                sprintf("%02d.%02d.%04d", $mday, $mon+1, $year+1900);
            </ep-perl> 
            </p>
        </body>
    </html>

If you don't like to embed Perl code, you may store it into a different file. That's what the single-line version of ep-perl is for:

    <html>
        <head><title>The Date</title></head>
        <body>
            <h1>The Date</h1>
            <p>Hello, today its the</p>
            <p align=center>
            <ep-perl src="date.pl">
            </p>
        </body>
    </html>

You have noticed, that the little script's result was inserted into the HTML page, did you? It did return a date, in other words a string consisting of letters, digits and dots. There's no problem with inserting such a string into an HTML stream.

But that's not always the case! Say you have a string like

    Use </html> for terminating the HTML page.

This cannot be inserted as a raw string, for obvious reasons. Thus the ep-perl command has an attribute output. Use it like this:

    <ep-perl output=html>
        'Use </html> for terminating the HTML page.';
    </ep-perl>

Possible values of the output attribute are raw (default), html (HTML encoded) and url (URL encoded).

It's a common mistake, to use the Perl command return in embedded Perl. Never do that! If you need return (there are of course situations where returning can help), do it like this:

    <ep-perl>
        sub eval_me {
            if ($this) {
                return 'foo';
            } elsif ($that) {
                return 'bar';
            }
            '';
        }
        eval_me();
    </ep-perl>

See "Variables" below for interactions between Perl variables and EP variables.

For security reasons, you might set an attribute safe, as in

    <ep-perl safe=1>...</ep-perl>

This will create a Safe compartment for you and run the embedded script in the compartment. Using this attribute is highly recommended!

ep-mail

This command will send an e-mail. The attributes will be used for creating the email header, in particular the subject, from and to attribute should be used. Example:

    <ep-mail subject="Howdy!" from="joe@ispsoft.de"
             reply-to="joe@ispsoft.de" to="bill@whitehouse.gov">
        Hello, Bill, old chap. How are you?

        Yours sincerely,
        Jochen
    </ep-mail>

You can still use EP variables in the E-mail body, for example the following works:

    <ep-mail subject="Test" reply-to="$cgi->email$" to="joe@ispsoft.de"
             from="webmaster@www.mydomain.com">
        Hello, Joe,
        this e-mail was sent to you by $@cgi->name$.
    </ep-mail>

But note that we suppress conversion into HTML format in the mail body! See "Variables" below for details.

Recent sendmail versions are quite picky about validity of email addresses. Thus it is highly recommended that you use valid addresses for the to and from fields. If you want to customize the sender, you'd better still fix the from field and modify the reply-to field only, as in the example above.

ep-errhandler

This command advices EP, what to do in case of errors. See "Error handling" below. Example:

    <ep-comment>
        Set the template being used for system errors.
    </ep-comment>
    <ep-errhandler type=system src=/templates/syserr.html>
    <ep-comment>
        Likewise, set the template for user errors.
    </ep-comment>
    <ep-errhandler type=user src=/templates/usererr.html>

If an error occurs, the given scripts are loaded and used as templates instead of the current one. You don't need external files! Instead you can use

    <ep-errhandler type=user>
        <HTML><HEAD><TITLE>User error</TITLE></HEAD>
        <BODY><H1>User error</H1>
        <P>Replace user and continue. :-)</P>
        <P>To be serious, the following problem happened:</P>
        <PRE>$errmsg$</PRE>
        <P>Please return to the calling page, fix the problem
        and retry.</P>
        </BODY></HTML>
    </ep-errhandler>

However, you might prefer to use a single error template and of course it's faster to use external error templates than parsing builtin templates. (At least, if no error occurs. :-)

ep-error

This command forces an error message. See "Error handling" below. You can trigger user or system errors by setting the type attribute to the values system (default) or user. The msg attribute is for setting the error message.

Example:

    <ep-comment>
        If no email address was entered, force a user error.
    </ep-comment>
    <ep-if eval="$cgi->email$" type=user>
        <ep-error msg="Missing email address">
    </ep-if>
ep-database

This command connects to a database. Its attributes are dsn, user and password corresponding to the same attributes of the DBI connect method. See DBI(3) for details on DBI.

Example:

    <ep-database dsn="DBI:mysql:test" user="joe"
                 password="Authorized?Me?">

You can use different database connections by using the dbh attribute:

    <ep-database dbh="dbh2" dsn="DBI:mSQL:test">

The dbh attribute advices EP to store the DBI handle in the given variable. (Default: dbh) See "Variables" below.

ep-query

This command executes an SQL statement. The query attribute will be used for passing the SQL statement. Of course a multiline version is available, thus

    <ep-query statement="INSERT INTO foo VALUES (1, 'bar')">

is the same as

    <ep-query>
        INSERT INTO foo VALUES (1, 'bar')
    </ep-query>

If your query retrieves a result, use the result attribute to store it in a variable, for example like this:

    <ep-query statement="SELECT * FROM employees" result="employees"
              resulttype="hash" startat=0 limit=-1>

This will create a variable employees, an array ref of hash refs. You can use the ep-list command for displaying the output. See "Variables" below.

When using multiple database connections, use the dbh attribute for choosing the connection. (See the ep-database method above.)

If you have big result tables, you might prefer DBI's fetchrow_arrayref method over creating hash refs, because arrays are created faster than hash refs. This is achieved by setting the attribute resulttype to array. The default is hash. Other resulttypes are single_hash and single_array: If your query will return at most a single element, then the result variable will contain the first row (or an undefined value) and not an array.

Sometimes you don't want to retrieve the complete result table. In that case you can use the attributes startat and limit. For example, to retrieve rows 0-19, use startat=0 and limit=20. Likewise you would use startat=20 and limit=20 for rows 20-39.

When using the MySQL engine, the startat and limit attributes are directly mapped to MySQL's LIMIT clause.

ep-list

This command is used to display an array of refs. Lets assume, that the variable employees contains a an array ref of refs with the attributes name and department. Then you could create a table of employees as follows:

    <table><tr><th>Nr.</th><th>Name</th><th>Department</th>
    <ep-list items="employees" item="e">
           <tr><td>$i$</td><td>$e->name$</td><td>$e->department$</td>
    </ep-list>
    </table>

This will be processed as follows: For any item in the array, retrieved from the variable employees, create a variable e and display the text between ep-list and /ep-list for it by replacing the patterns $e->name$ and $e->department$ with the corresponding values.

The variable i is initially set to 0 and incremented by one with any element.

You must not create a variable for the items attribute: When using a range attribute instead of items then a variable list will be created for you. The range attribute can be either start..stop in which case a list of the numbers start..stop will be created. Otherwise the attribute must be a comma separated list of values. See the ep-select command below for an example of the range attribute.

ep-select

This is similar to ep-list, but it is specifically designed for creating SELECT boxes and similar things. We explain it by example:

    <ep-select name="year" range="1991..1995" item="y"
               selected="$cgi->year$">
        <OPTION $selected$>$y$</ep-select>

If you supply a selected attribute, then a variable selected will be created for any item. The value will be either an empty string or the word SELECTED (configurable via the attribute selected-text), depending on whether the item matches the selected value or not.

ep-input

This is usefull for reading an objects data out of CGI variables. Say you have a form with input fields describing an address, the field names being address_t_name, address_t_street, address_n_zip and address_t_city. By using the command

    <ep-input prefix="address_" dest="address">

the EP program will create a variable "address" for you which is an hash ref as follows:

    $cgi = $_->{cgi};
    $_->{address} = {
        name =>   { col => 'name',
                    val => $cgi->param("address_name"),
                    type => 't',
                  },
        street => { col => 'street',
                    val => $cgi->param("address_street"),
                    type => 't',
                  },
        zip =>    { col => 'zip',
                    val => $cgi->param("address_zip"),
                    type => 'n',
                  },
        city =>   { col => 'city',
                    val => $cgi->param("address_city"),
                    type => 't'
                    }
    };

In general column names beginning with address will be splitted into prefix_type_suffix, the type being one of

t

Text columns (CHAR, VARCHAR, BINARY, or whatever), that will be quoted using the $self->{'dbh'}->quote() method.

n

Numeric columns (INTEGER, REAL, ...) that will be left untouched

dd
dm
dy

Day, month and year of a date. The combined fields will be quoted as 'yyyy-mm-dd'.

s

Set columns as created using MultiSelect boxes; the selected values will be used for creating a comma separated string.

The idea is generating SQL queries automatically out of the address variable. This task is supported by the sqlquery attribute:

    <ep-database dsn="DBI:mysql:test">
    <ep-input prefix="address_" dest="a" sqlquery=1>
    <ep-comment>Create a new record, if no ID is given</ep-comment>
    <ep-if eval="$cgi->id$">
    <ep-query
     statement="INSERT INTO addresses ($@a->names$) VALUES ($@a->values$)">
    <ep-else>
    <ep-query
     statement="UPDATE addresses SET $@a->update$ WHERE id = $@id$">

The sqlquery creates attributes names, values and update for you, that may be used in INSERT or UPDATE queries. Note that the ep-input must be preceeded by an ep-database call, because it is using DBI's quote method. DBI(3).

There are situations where you want to fetch not only a single object, but a list of objects. Suggest an order form of articles. Then you might have input fields art_0_t_name, art_0_n_count, art_0_n_price, art_1_t_name, ...

In that case you can give the ep-input command an attribute list, like this:

    <ep-input prefix="art_" dest="art" list=1>

The module will read an array ref of objects to the variable dest. Any object will have an additional scalar variable i referring to the items number, beginning with 0. In other words, you can process the order form as follows:

    <ep-input prefix="art_" dest="art" list=1>
    <ep-perl>
      my $self = $_;
      my $sum = 0.0;
      for (my $i = 0;  defined($self->{cgi}->param("art_$i_n_count"));
           $i++) {
          $sum += $self->{cgi}->param("art_$i_n_count") *
                  $self->{cgi}->param("art_$i_n_price")
      }
      ''
    </ep-perl>
    <ep-mail from="$cgi->email$" to="order@mydomain"
             subject="Order form">
      The following items have been ordered:

        Nr.    Price   Article

      <ep-list>
        $art->count->val$  $art->price->val$  $art->name->val$
      </ep-list>

      Total sum: $sum$
    </ep-mail>
ep-include

Sometimes you want to source external files. This can be done by using

    <ep-include file="myfile">

If a file with the given name doesn't exist, the file name is treated as being relative to your WWW servers DOCUMENT_ROOT directory.

ep-exit

This directive terminates processing of the current HTML page.

Conditional HTML

It is possible to blank out parts of the HTML document. See the following example:

    <html><head><title>Conditional HTML</title></head>
    <body>
      <h1>Conditional HTML</h1>
      <ep-if epperl-eval="$_->{cgi}->param('i') < 0">
        You have entered a negative number for i!
      <ep-else epperl-eval="$_->{cgi}->param('i') == 0">
        You have entered zero for i!
      <ep-else>
        <ep-if epperl-eval="$_->{cgi}->param('j') < 0">
          You have entered a negative number for j!
        <ep-else epperl-eval="$_->{cgi}->param('h') == 0">
          You have entered zero for j!
        <ep-else>
          Ok, both numbers are positive.
        </ep-else>
      </ep-else>
    </body>
    </html>

The example is of course somewhat unnatural, because you'd better use a single ep-perl in that case, but it shows that we can use arbitrary complex structures of conditional HTML.

Localization

Localization is available via the HTML::EP::Locale module. Currently it only offers methods for localizing strings.

To access the module, start your HTML page with

    <ep-package name="HTML::EP::Locale" require=1>

When the package is loaded, it tries to guess your documents language. The default language is de (german; I ought to make this configurable). You can specify another language (for example en for english or fr for french) by either supplying a CGI variable language=xy or giving your document a secondary extension like page.xy.ep. That is, if your document is called

    mypage.de.ep

then the language de will be choosen, but de will be choosen for

    mypage.en.ep

Two methods are available for localizing strings. For short strings like titles, headers or Link refs you might prefer this version:

    <ep-language de="Titel" en="Title">

Obviously this is not appropriate for longer strings and it must not contain HTML patterns. Thus another version is available:

    <ep-language language=de>
      <p>Dies ist ein Absatz.</p>
      <p>Dies ist der zweite Absatz.</p>
    </ep-language>
    <ep-language language=en>
      <p>This is one paragraph.</p>
      <p>This is another paragraph.</p>
    </ep-language>

Error handling

Error handling with EP is quite simple: All you do in case of errors is throwing a Perl exception. For example, DBI handles are created with the RaiseError attribute set to 1, so that SQL errors trigger a Perl exception. You never care for errors!

However, what happens in case of errors? In that case, EP will use the template that you have set with ep-errhandler and treat it like an ordinary EP document, by setting the variables errmsg and admin.

If you don't set an error handler, the following template will be used, which is well suited for creating an own error template:

    <html><head><title>Internal error</title></head>
    <body><h1>Internal error</h1>
    <p>An internal error occurred. The server has not been able to
    fullfill your request. The error message is:</p>
    <pre>
        $errmsg$
    </pre>
    <p>Please contact the Webmaster,
    <a href="mailto:$admin$">$admin$</a>,
    tell him the URL, the time and error message.
    </p>
    <p>We apologize for any inconvenience, please try again later!</p>
    <br><br><br><p>Yours sincerely,</p><p>The Webmaster</p>
    </body>
    </html>

Variables

It is important to understand, how EP variables work, in particular when working with ep-perl.

You always have an object $_, which is an instance of the HTML::EP class (a subclass of HTML::Parser). This object has certain attributes, in particular $_->{cgi}, a CGI object and $_->{dbh}, the DBI handle. (Of course valid after ep-database only.) If you want to set or modify a variable, you have to set $_->{varname}. If you want to retrieve the value, use the same. Note that you cannot use $_ for a long time, as it will be changed by Perl loops and the like, thus your Perl code typically starts with

    $_ = $self;

But how do you access the variable from within EP documents? You just write

        $varname$

This will be replaced automatically by the parser with the value of $_->{varname}. Even more, the value will be converted into HTML source!

If varname is a structured variable, for example a hash or array ref, you may as well use

        $varname->attrname$
or

        $varname->0$

to access $_->{varname}->{attrname} or $_->{varname}->[0], respectively. A special value of varname is cgi: This will access the CGI variable of the same name, thus the following are equivalent:

        $cgi->email$

and

        $_->{cgi}->param('email');

But what, if you don't want your variable to be HTML encoded? You may as well use

        $@varname$      (Raw)
        $#varname$      (URL encoded)
        $~varname$      (SQL encoded)

The latter uses the $_->{dbh}->quote() method. In particular this implies that you have to be connected to a database, before using this tag!

You can even use these symbols in attributes of EP commands. For example, the following will be usefull when sending a mail:

    <ep-mail subject="Howdy!" from="$@cgi->email$"
             to="bill@whitehouse.gov">

Attributes may include EP variables, just like ordinary HTML code. Even more, they may contain Perl code which is evaluated just like code between <ep-perl> and </ep-perl>. However, you need to use the variable $_ in the code, because the package otherwise doesn't detect what you want it to do.

See "Custom variable formatting" for setting up your own formats.

Custom variable formatting

Sometimes the builtin formatting methods of HTML::EP are not sufficient. A good example are currencies. Such format methods are stored in the variable _ep_custom_formats. As an example we create a method for formatting german currency values:

    <ep-perl>
        my $self = $_;
        my $formats = ($self->{'_ep_custom_formats'} ||= {});
        $formats->{'DM'} = sub {
            my($self, $val) = @_;
            sprintf("%.2f DM", $val);
            $val =~ s/\./,/;
            $val;
        };
        ''
    </ep-perl>

This can be used as follows: Suggest we have the following variables:

    a = 1
    b = 2.4
    c = 34.47

then we can use

    a = $&DM->a$                =>  1,00 DM
    b = $&DM->b$                =>  2,40 DM
    c = $&DM->c$                => 34,47 DM

In other words: Use the special marker &, followed by the custom formats method name, the dereferencing operator and finally the variable name.

The above method is already predefined by the HTML::EP::Locale module.

Modifying the CGI headers

The attribute $self->{'_ep_heaaders'} is containing arguments that you want to pass to CGI->header(). Use it like this:

    <ep-perl>
      $_->{'_ep_headers'} = { '-status' => '402 Payment required',
                              '-expires' => '+3d'
                             }
    </ep-perl>

Producing Non-HTML

Say you want a CGI binary that creates a gif and not an HTML document. (See the ifgif.ep file from the SNMP::Monitor distribution for an example.) Two problems are arising here: First of all you have to create your own headers. Next you *must* prevent that EP is emitting any output, because this might trash your images validity. Here's what to do:

    <ep-perl>
        my $self = $_;
        my $cgi = $self->{'cgi'};
        $self->_ep_database({'dsn' => 'DBI:mysql:test',
                             'user' => 'joe',
                             'password' => 'joe'});
        $self->_ep_query({'statement' => "SELECT image FROM images WHERE"
                              . " id = " . $cgi->param('id'),
                          'result' => 'im',
                          'resultype' => 'single_hash'});
        $self->print($cgi->header('-type' => 'image/gif'),
                     $self->{im}->{'image'});
        $self->Stop();
    </ep-perl>

Note the use of $self->Stop()!

NPH Scripts

The Apache server and other WWW servers have a nice feature that allows you to return HTML pages line by line: If your script is called something like nph-myscript then the server is passing the scripts output to the browser immediately. This is usefull, for example, if you display the output of a traceroute command: You'd like to see any line immediately, but it may take some time untill the next line arrives.

Unfortunately NPH scripts are not well suited for HTML::EP: One of EP's major targets is that you need not care for errors: They may occur at any point but are still catched and handled by creating an error message. However, there's no other way to guarantee this without buffering output until the script ends.

If you really need to use NPH, do it like this:

    <ep-perl>
      my $self = $_;
      local $| = 1; # make sure that Perl isn't buffering
      $self->print($self->{'cgi'}->header('-type' => 'text/html',
                                          '-nph' => 1));
      for (my $i = 0;  $i < 20;  $i++) {
        $self->print("This is line $i. Waiting one second ...\n")
        sleep 1;
      }
      $self->Stop();
    </ep-perl>

Note, that *no* more output will be produced by EP after calling $self->Stop()!

DEBUGGING

Debugging CGI applications is always a problem. The EP module does its best to support you.

Whenever you supply a CGI variable debug, then the module will enter debugging mode. For example if your document is /mypage.ep, then tell your browser to fetch /mypage.ep?debug=1. You won't see the usual HTML page, but a plain text page with lots of debugging messages and the created HTML source.

You may extend the debugging code with sequences like

    <ep-perl>
        my $self = $_;
        if ($self->{debug}) { $self->print("I'm here!\n"); }
    </ep-perl>

Note that you should not call the print function directly, but the print method! The former works well in CGI environments, but EP should work even in other environments as well.

But sometimes this is not sufficient: What's inserting debugging messages compared to using the Perl debugger? In that case you can emulate a CGI environment as follows:

    export DOCUMENT_ROOT=/usr/local/www/htdocs
    export PATH_TRANSLATED=$DOCUMENT_ROOT/mypage.ep
    export REQUEST_METHOD=GET
    export QUERY_STRING="var1=val1&var2=val2"
    perl -d /usr/bin/ep.cgi

This allows you single-stepping through your program, displaying variable values and the like.

However, the debug variable is obviously a security problem. For that reason the debughosts variable is present in HTML::EP::Config: You should set it to a regular expression matching the IP numbers of hosts that are allowed to enable debugging mode. The Makefile.PL should query for appropriate settings automatically while running.

EXAMPLES

The program comes with a set of examples incorporated. These are:

  - A simple Unix user administration in examples/admin.

  - A generic frontend for editing DBI tables in examples/edit.ep

Other examples are:

  - The SNMP monitor, a utility for watching, logging and displaying
    interface utilization and status of network interfaces via WWW.
    See the directory authors/id/JWIED on any CPAN mirror, for example
    ftp://ftp.funet.fi/pub/languages/perl/CPAN.

EXTENSIONS

It is quite easy to write own methods.

Single-line extensions

For example, suggest you want a method for setting EP variables:

    <ep-set var="e" value=1>
  

Once the variable is created, you can access it via

   $e$
  

This can be done like this:

    <ep-perl package="HTML::EP">
      # Write a handler for ep-env:
      sub _ep_set ($$) {
          my($self, $attr) = @_;
          my $var = $attr->{'var'};
          my $val = $attr->{'value'};
          $self->{$var} = $val;
          '';
      }
  
      # Return an empty string:
      '';
    </ep-perl>
  

Note that we used the name _ep_set: By default the EP module takes the method name (ep-env in out case) translates all hyphens to underscores and adds a leading underscore. We'll see in the next section how you can override this behaviour.

Multi-line extensions

But how to write methods, that use a <tag> .. </tag> syntax? As an example, we write a method for creating external files. The method receives two attributes, a file attribute for the files name and a contents attribute for the files contents. The method can be used in two ways:

    <ep-file file="test.dat" contents="Hi!">

or like this, in multiline mode:

    <ep-file file="test.dat">
        Hi!
    </ep-file>

Here it is:

    <ep-perl package="HTML::EP">
    # Write a handler for ep-file:
    sub _ep_file ($$) {
        my($self, $attr) = @_;
        my $contents = $attr->{contents};
        if (!defined($contents)) { # Multiline method, no "contents"
            return undef;          # attribute given; return undef
        }                          # until we are called again.
        my $file = $attr->{file};
        require Symbol;
        my $fh = Symbol::gensym();
        if (!open($fh, ">$file)  ||  !(print $fh ($contents))  ||
            !close($fh)) {
            die "Error while creating $file: $!";
        }
        '';
    }
    # Register the handler in the list of handlers
    # Note the use of the "default" attribute:
    $self->{_ep_funcs}->{'ep-file'} = { method => '_ep_file',
                                       default => 'contents' };
    # Return an empty string:
    '';
    </ep-perl>

The main difference to the previous example is that we install a handler description in $self->{'_ep_funcs'}. This handler description is a hash ref with the keys method (the HTML::EP method to execute, by default _ep_file, so we could well omit it here) and the default attribute, which is used as follows:

If EP finds the <ep-file>, it calls _ep_file for the first time. The method checks, whether a contents value is given. If so, it starts working immediately. If not, it returns an undefined value. This advices EP that the method is waiting for a </ep-fileE</gt>. Thus EP enters multi-line mode, collects the following input and then calls _ep_file again, this time by setting the handlers default attribute.

Note the use of the Symbol package when accessing files: *Never* use global handles like

    open(FILE, ...)

as this might break future multithreading code!

External extensions

So far we have discussed only minor extensions that we did install as part of HTML::EP. However, this is somewhat dangerous with mod_perl: Suggest you have multiple virtual hosts. It might happen, that virtual host www.foo.com defines a method ep-file, but www.bar.net defines another. Thus we'd have the classical name clash.

To prevent such a namespace pollution, EP supports external extensions with an inheritage model that is borrowed from Perl. Suggest we write a Shop extension. Of course this extension borrows from HTML::EP::Session. Thus we have the following class design:

    Shop  ---isa--->  HTML::EP::Session  ---isa--->  HTML::EP

We start writing our Shop extension like any other Perl package:

    package Shop;
    @Shop::ISA = qw(HTML::EP::Session); # HTML::EP::Session inherits
                                        # from HTML::EP

    sub _ep_shop_session {
        my($self, $attr) = @_;
        # Initialize the session
        ...
        '';
    }

    sub init {
        my $self = shift;
        if (!$self->{'_ep_shop_initialized'}) {
            $self->SUPER::init();
            $self->{'_ep_funcs'}->{'ep-shop-session'} = {
                'method' => '_ep_shop_session'
                };
            $self->{'_ep_shop_initialized'} = 1;
        }
    }
 

In the HTML page we load this package as follows:

    <ep-package name="Shop">
    <ep-shop-session>

This will do a require Shop and bless the object $self into the class Shop and call $self-init()>. Note that we allow the init method to be called more than once, this is important for stacking packages.

By default packages are loaded from Perl's system directories. You can use the attribute lib to add private directories to the library search path. This can be relative to the servers document root, as in

    <ep-package name="Shop" lib="/perl-lib">

Selfloader methods

In the above examples the extension methods have been compiled immediately. This is not always a good idea: For example the ep-mail method is loading big external packages like Mail::Internet for sending the mail. In such cases you might wish to use HTML::EP's builtin self loader, which is quite similar to that of CGI. We choose ep-mail as an example:

    <ep-perl package="HTML::EP">
    my $self = $_;
    # Create a string that can be compiled for loading the method:
    $AUTOLOADED_SUBS{_ep_mail} = <<'end_of__ep_mail';
        require Mail::Internet;
        sub _ep_mail ($$) {
            my($self, $attr) = @_;
            ...
        }
    end_of__ep_mail

    $self->{_ep_funcs}->{ep-mail} = { method => '_ep_mail',
                                      default => 'body' };
    </ep-perl>

The advantage is that you have the method available, but the performance penalty of loading it is almost omitted, if the method is not used.

PERFORMANCE CONSIDERATIONS

The following are merely hints for general Perl programming, but apply in particular to EP programming:

Local variables

Use local variables instead of hash attributes, as in

    my $debug;
    if ($debug = $self->{'debug'}) {
        ...
    }
    if (!$debug) {
        ...
    }

This is approximately 10-15% faster than the equivalent

    if ($self->{'debug'}) {
        ...
    }
    if (!$self->{'debug'}) {
        ...
    }

Of course you win even more with any further use of $debug.

CHANGES

This section describes user visible changes against previous versions. For details and other modifications see the ChangeLog file, that is part of the distribution.

epparse
epperl

In previous versions it was not possible to include EP variables or Perl code in attributes of EP commands, unless using a prefix epparse- or epperl-, as in

    <ep-mail to=joe epparse-from="$cgi->mail$" subject=Hello>

This is no longer the case, because the package now autodetects whether you are using such constructs. (At least it should. :-) The obvious disadvantage is an incompatibility, but the new version is much better readable and surprisingly even (much!) faster, because only hash values are modified and not hash structures.

TODO

  • mod_perl support

  • Create an EP server that is accessible via a small C wrapper

AUTHOR AND COPYRIGHT

This module is

    Copyright (C) 1998    Jochen Wiedmann
                          Am Eisteich 9
                          72555 Metzingen
                          Germany

                          Phone: +49 7123 14887
                          Email: joe@ispsoft.de

All rights reserved.

You may distribute this module under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.

SEE ALSO

DBI(3), CGI(3), HTML::Parser(3)

1 POD Error

The following errors were encountered while parsing the POD:

Around line 1107:

Unknown E content in E</gt>