NAME
ExtUtils::PerlPP - A Perl Preprocessor
SYNOPSIS
use ExtUtils::PerlPP;
my $config = { 'version' => $VERSION,
'driver' => $DRIVER };
# The long and winding road ...
my $self = ExtUtils::PerlPP->new();
$self->{'in_fh'} = IO::File->new('file.PL', 'r');
$self->{'out_fh'} = IO::File->new('file', 'w');
$self->{'config'} =
$self->parse();
# And now a short cut for the same:
ppp('file.PL', 'file', $config);
DESCRIPTION
Perl's installation suite, ExtUtils::MakeMaker, contains a mechanism for
installing preparsed files, so-called *PL* files: If the MakeMaker
utility detects files with the extension `.PL' then these files are
executed by *make*, usually creating a file of the same name, except the
`.PL' extension.
Writing these PL files is usually always the same, for example a typical
`.PL' file might look like this:
my $script = <<'SCRIPT';
... # True file following here
SCRIPT
# Modify variable $script, depending on configuration, local
# site or whatever
...
if (!open(FILE, ">file") || !(print FILE $script) ||
!close(FILE)) {
die "Cannot write file: $!";
}
But in essence, what else is this than a Perl preprocessor?
Traditionally you have to write such a Perl preprocessor for yourself
all the time, although I have found that they always do the same, for
example:
- Fix defaults, for example installation paths.
- Including or excluding code sections. It is a matter of taste
whether one likes to see
if ($] < 5.003) {
# Thirty lines of code following here
...
} else {
# A single line of code
...
}
when already using Perl 5.005. I don't.
This module is dedicated to simplify such tasks. In short, you can use
it like this:
Create a new preprocessor
You start with creating an instance of *ExtUtils::PerlPP* by calling the
*new* constructor:
my $ppp = ExtUtils::PerlPP->new(%attr);
The constructor accepts a list of attributes, including the following:
in_fh The input file, any kind of IO object, for example an instance of
IO::File or IO::Scalar. More general: It can be any object that
offers a *getline* method.
A scalar value (to be distinguished from an IO::Scalar
instance!) will be interpreted as a file name that the method
opens for you.
out_fh The output file; another IO object or any other object that offers a
*print* method. A scalar value is accepted as output file name.
config A hash ref of preprocessor variables. In other words
$ppp->{'config'}->{'var'} = 1;
is what `-Dvar=val' is for the C preprocessor. Similarly you can
compare
delete $ppp->{'config'};
with `-Uvar'. See the section on "Macro replacements" below.
Unlike C, variables may be arbitrarily complex, in particular
you can use hash or array refs as values.
Surprisingly you may pass a scalar value again: In that case the
file of the same name evaluated and the result is used as a
configuration hash. In other words
$ppp->{'config'} = "myapp.cfg";
is similar to
$ppp->{'config'} = do "myapp.cfg";
Such config files can easily be created using the *Data::Dumper*
module. the Data::Dumper(3) manpage.
no_config_default
If a variable name is used, but no such attribute is present in
the *config* hash, then by default the variable is looked up in
the `$Config' from the *Config* module. This behaviour is
suppressed, if you set *no_config_default* to a TRUE value. the
Config(3) manpage.
no_makedirs
By default directories are created silently if required. For
example, if you pass a value of `/usr/local/foo/bar' as output
file and only `/usr/local' exists, then the subdirectory `foo'
will be created. The option *no_makedirs* suppresses this
behaviour.
Running the preprocessor
This is done by executing
$ppp->parse();
A Perl exception will be thrown in case of errors, thus the complete use
might look like this:
eval { $ppp->parse(); };
if ($@) { print "An error occurred: $@\n" }
Using the frontend
Most applications won't call the *new* or *parse* methods directly, but
rather do a
use ExtUtils::PerlPP;
ppp('infile', 'outfile', 'configfile');
This is equivalent to
my $parser = ExtUtils::PerlPP->new('in_fh' => 'infile',
'out_fh' => 'outfile',
'config' => 'configfile');
$parser->parse();
In order to be easily used within Makefiles, the ppp frontend can read
from @ARGV. That is, you can use the module like this:
perl -MExtUtils::PerlPP -e ppp <infile> <outfile> <configfile>
from the commandline.
Macro replacements
The primary use of preprocessor variables (aka attributes of `$ppp-
'{'config'}>) is replacing patterns in the stream written to the output
file. With `$c = $ppp-'{'config'}> in mind the typical patterns and
their replacements are:
~~a~~ $c->{'a'}
~~b~~ $c->{'b'}
~~a->b~~ $c->{'a'}->{'b'}
~~a->e~~ $c->{'a'}->{'e'}
~~a->1~~ $c->{'a'}->[1]
~~a->1->b~~ $c->{'a'}->[1]->{'b'}
I hope the idea is obvious. Real world examples might be:
my $config_file = "~~etc_dir~~/configuration";
my $VERSION = "~~version~~";
Preprocessor variables need not be scalar values: If a variable contains
a code ref, then the module will execute
&$var($ppp, $text);
and replace the pattern with the result. `$text' is the pattern being
replaced, for example, if `$ppp-'{'config'}->{'bar'}> has the value
`\&foo', then `~~bar~~' will be replaced with the result of
foo($ppp, "bar");
Arguments are not yet supported.
Creating macros
When talking about code refs, we need a possibility to create them. The
best possibility is creating them within the input file, as in
~&foo&~ my($self, $text) = @_; $text x 2; ~&&~
This example is mainly equivalent to
$ppp->{'config'}->{'foo'} = sub {
my($self, $text) = @_; $text x 2;
};
The `~&var&~' definition must start at the beginning of a line, much
like the C preprocessor. The end pattern ~&&~ may appear at any point,
but the remaining line will be ignored.
Conditional output
The next application of a preprocessor is conditional output, as in an
#ifdef var
...
#endif
segment. This can be done with
~#if#~ <expression>
...
~#elsif#~ <expression>
...
~#else#~
...
~#endif#~
`<expression>' is handled as follows: First it is subject to the usual
pattern replacements and then it is evaluated as a Perl expression
returning a TRUE or FALSE value. Examples:
~#if#~ "~~a~~"
is TRUE, if and only if $ppp->{'config'}->{'a'} is TRUE.
Currently conditionals must start at the beginning of a line and
expressions must not exceed a single line. Nesting conditions is
possible.
Embedding into MakeMaker
For using the preprocessor from within MakeMaker, I propose the
following: First of all you create a config file from within
Makefile.PL. For example the *libnet* suite creates a file `libnet.cfg'
and the *SNMP::Monitor* and *Cisco::Conf* modules create a file
`configuration'. The *Data::Dumper* module will aid you in that task.
the Data::Dumper(3) manpage.
Then you add the following to your Makefile.PL, I assume the name
`myapp.cnf' for the config file:
package MY;
sub processPL {
my($self) = shift;
return "" unless $self->{PL_FILES};
my(@m, $from, $to);
foreach $from (sort keys %{$self->{PL_FILES}}) {
$to = $self->{PL_FILES}->{$from};
push @m, "
all :: $self->{PL_FILES}->{$plfile}
$self->{NOECHO}\$(NOOP)
$self->{PL_FILES}->{$plfile} :: $plfile
\$(PERL) -I\$(INST_ARCHLIB) -I\$(INST_LIB) \
-I\$(PERL_ARCHLIB) -I\$(PERL_LIB) \
-MExtUtils::PerlPP -e 'ppp($from, $to, \"myapp.cnf\")'
";
}
join "", @m;
}
Next you create your template files under their usual names, but add an
extension `.PL'. The MakeMaker utility will automatically detect these
files for you and add appropriate rules to the Makefile it generates.
AUTHOR AND COPYRIGHT
This module is Copyright (C) 1998 by
Jochen Wiedmann
Am Eisteich 9
72555 Metzingen
Germany
Email: joe@ispsoft.de
Phone: +49 7123 14887
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
the ExtUtils::MakeMaker(3) manpage, the Data::Dumper(3) manpage
