Text::Xslate::Manual::Debugging - Debugging techniques for Xslate templates


This document describes techniques for debugging templates.

Setting verbose => 2

Try verbose => 2 in the first step. This option enables full warnings, especially warnings related to undef.

File names and line numbers

Xslate messages include file names, line numbers, and, if possible, source code lines which seems problems.

You can also access the file name and the line number in templates by __FILE__ and __LINE__ tokens just like as Perl.

If you want reports files and lines from your registered functions, Text::Xslate->current_file and Text::Xslate->current_line in callbacks are the same as __FILE__ and __LINE__ in templates respectively.

    sub my_sqrt {
        my($n) = @_;

        if($n < 1) {
            # return a message instead of warnings
            return sprintf "!!! Can't take sqrt of $n at %s line %d !!!",
                Text::Xslate->current_file, Text::Xslate->current_line;
        return sqrt($n);

    my $tx = Text::Xslate->new(
        function => { sqrt => \&my_sqrt },

To dump values

You can use any dumping modules via the function option, but Xslate has a builtin dump filter to dump template values.

    <: $value | dump # Dump $value with Data::Dumper :>

Detection of missing variables (or typos or variable names)

Xslate itself has warning system for use of uninitialized values, but sometimes it is not enough.

If you want fill in some string, e.g. FILL ME, for missing variables, you can use the hash_with_default() utility. For example:

    use Text::Xslate::Util qw(hash_with_default);
    $tx->render($name, hash_with_default(\%vars, sub { "FILL ME '@_' " }) );

Note that this is really slow because it is a tied-hash wrapper.

Customization of error messages

You can customize error handlers by warn_handler and die_handler. In these handlers, you can call Text::Xslate->print() method in order to add your custom messages to the output buffer, which makes debugging easier.

    #!perl -w
    use strict;
    use Text::Xslate;
    my %vpath = (
        hello => 'Hello, <: $lang :> world!' . "\n",
    my $tx = Text::Xslate->new(
        path         => \%vpath,
        verbose      => 2,
        warn_handler => sub { Text::Xslate->print('[[', @_, ']]') },

    print $tx->render('hello', { });
    # => Hello, [[use nil to print at ...]] world!