Text::Xslate::Manual::FAQ(3) Frequently asked questions and answers

DESCRIPTION

This manual page lists FAQs, which we've heard for now.

QUESTIONS

General

How do you pronounce 'Xslate'?

We read it "/eks-leit/".

What 'Xslate' stands for?

It stands for XS template, a template engine written in XS, although pure Perl implementations are also provided.

What are 'Kolon', 'Metakolon', and 'TTerse' ?

Xslate supports multiple template syntaxes. Kolon is the default syntax, Metakolon is suitable to output Kolon templates, and TTerse is compatible with Template-Toolkit 2. You can specify the template syntax by passing "syntax" option to the Text::Xslate constructor. 

    my $tx = Text::Xslate->new(
        syntax => 'TTerse', # by moniker
    );
    my $tx = Text::Xslate->new(
        syntax => 'Text::Xslate::Syntax::TTerse', # by fully qualified name
    );

What version of perl does Xslate require?

Xslate is tested on perl v5.8.1. No special settings should be required.

How can I install the pure-Perl version of Xslate?

Pass "PUREPERL_ONLY=1" to Makefile.PL, which requests the Xslate build system not to make XS parts.

Note that "cpanm 1.7" supports "--pp" option to install pure-Perl alternatives, so you can type "cpanm --pp Text::Xslate".

What optimizations does Xslate employs?

Here are some optimizations worth noting that makes Text::Xslate run so fast, in no particular order:

Pre-compiled templates
Text::Xslate is among the template engines that pre-compile the templates. This is similar to, say, Template::Toolkit, but Text::Xslate compiles the templates to C structures and stores them as binary data.
Built on top of a virtual machine
Text::Xslate is built on top of virtual machine that executes bytecode, and this virtual machine is fine-tuned specifically for template processing.

The virtual machine also employs optimizations such as direct-threading style coding to shave off any extra milliseconds that the engine might take otherwise

Custom byte codes for oft-used operations
Some operations which are used very often are optimized into its own byte code. For example (as described elsewhere) Text::Xslate automatically escapes HTML unless you tell it not to. Text::Xslate implements this process which involves escaping the string while appending the result to the output buffer in C, as a custom byte code. This lets you avoid the penalties usually involved in such operations.
Pre-allocation of output buffers
One of the main things to consider to reduce performance degradation while processing a template is to avoid the number of calls to "malloc()". One of the tricks that Text::Xslate employs to reduce the number of calls to "malloc()" is to pre-allocate the output buffer in an intelligent manner: For example, Text::Xslate assumes that most templates will be rendered to be about the same as the previous run, so when a template is rendered it uses the size allocated for the previous rendering as an approximation of how much space the current rendering will require. This allows you to greatly reduce the number of "malloc()" calls required to render a template.

How can I throw errors in functions and/or methods?

Handle warnings by "warn_handler" and raises exceptions if needed.

That's because Xslate catches exceptions in templates and emits them as warnings.

Configuration

When I create the Xslate instance?

Xslate instances are reusable and creating the instance costs somewhat so you're recommended to reuse them as much as possible. That is, you should make the instance global.

Consider a PSGI application: 

    # create Xslate here, not in psgi_app();
    my $xslate = Text::Xslate->new(...);
    sub psgi_app {
        my($env) = @_;
        # use $xslate and create $response
        return $response;
    }
    return \&psgi_app; # as a PSGI app

Don't create the instance in each request. It's less efficient.

How can I change instance attributes dynamically?

Instance attributes, e.g. "include_path", "function", or "syntax", are immutable, so you cannot change them dynamically.

Instead, you can create multiple instances by different options. instance in order to avoid conflicts with cache directories.

For example: 

    my %common_config = ( cache_dir => $dir, module => \@module );
    my %xslate = (
        ja => Text::Xslate->new( path => [ $template_ja ], %common_config ),
        en => Text::Xslate->new( path => [ $template_en ], %common_config ),
        ro => Text::Xslate->new( path => [ $template_ro ], %common_config ),
    );
    $xslate{$lang}->render(...);

Templates

How can I changes template tags?

Use "start_tag", "end_tag", and "line_start" options to "new" method, which can be joined together with "syntax" option: 

    my $tx = Text::Xslate->new(
        syntax     => 'TTerse',
        tag_start  => '{{',
        tag_end    => '}}',
        line_start => undef,
    );
    print $tx->render_string('Hello, {{lang}} world!', { lang => 'Xslate' });

Note that you'd better to avoid symbols which can be used for operators.

How can I iterate over HASH references?

Convert HASH references into ARRAY references because "for" methods can deal with just ARRAY references. 

    : # in Kolon
    : # iterate $hash by keys
    : for $hash.keys() -> $key {
        <: $key :>
    : }
    : # by values
    : for $hash.values() -> $value {
        <: $value :>
    : }
    : # by key-value pairs
    : for $hash.kv() -> $pair {
        <: $pair.key :>=<: $pair.value :>
    : }

Note that the above methods return ARRAY references sorted by the keys.

How can I use Template-Toolkit virtual methods and filters?

Xslate itself does not support these methods and filters, but there are modules on CPAN that implement them.

Text::Xslate::Bridge::TT2 provides almost all the TT methods and filters, but it requires Template-Toolkit installed.

Text::Xslate::Bridge::TT2Like provides the same features as "T::X::Bridge::TT2", and it does not require the Template-Toolkit runtime.

These bridge modules are useful not only for TTerse users, but also for Kolon users.

How can I (write|get) plugins?

It is unlikely to need to write plugins for Xslate, because Xslate allows you to export any functions to templates. Any function-based modules are available by the "module" option.

Xslate also allows you to call methods for object instances, so you can use any object-oriented modules, except for classes which only provide class methods (they need wrappers).

If you want to add methods to builtin data types (nil, scalars, arrays and hashes), you can write bridge modules. See Text::Xslate::Bridge for details.

How to limit while-loop like Template-Toolkit?

While Template-Toolkit has a loop counter to prevent runaway "WHILE" loop, Xslate has no arbitrary limitation.

Instead, you can use "alarm()" to limit any runaway code: 

    eval {
        local $SIG{ALRM} = sub { die @_ };
        alarm(1); # set timeout
        $tx->render('<: while true { } :>', \%vars);
    };
    if($@ =~ /\b ALRM \b/xms) {
        # timeout!
    }

Does Xslate process text strings, or binary strings?

(The meaning of text string and binary string is that of Perl, see perlunifaq.)

Xslate assumes template files to be encoded in "UTF-8" by default, so the output is a text string and template parameters, including values which registered functions return, must be text strings.

However, if you want to process binary strings, you can do so by passing ":bytes" to "input_layer", although it's not recommended.

Why doesn't I cannot access $object.attr like TT2?

Template-Toolkit allows objects (i.e. blessed references) to access its element if the object has no accessor methods, i.e. "[% object.attr %]" might mean "$object->{attr}". This behavior breaks encapsulation and hides typos, so Xslate doesn't allow such fallbacks.

If you want to access object attributes, define the accessors of them, or prepare values as a non-object before calling "render()".

Can I load macros in other template files?

Not yet. Currently Xslate doesn't support external macros.

Functions, filters and macros

Where are the list of builtin functions?

See Text::Xslate::Manual::Builtin.

How can I use macros as a callback to high-level functions?

Macros are objects that overload "&{}", the CODE dereference operator, so all you have to do is to call them simply, but don't check their types because they are not a real CODE reference. 

    my $tx = Text::Xslate->new(
        function => {
            count => sub {
                my($a, $cb) = @_;
                # Don't check the type of $cb!
                return scalar grep { $cb->($_) } @{$a};
            },
        },
    );
    print $tx->render_string('<: count($a, -> $x { $x >= 50 }) :>',
        { a => [ 0 .. 100 ] },
    ); # => 50

Web Application Frameworks

How can I use Xslate in $my_favorite_WAF?

There are bridges that integrate Xslate into WAFs:

  • Catalyst::View::Xslate for Catalyst
  • MojoX::Renderer::Xslate for Mojolicious
  • Tiffany for general usage

There are WAFs which adopt Xslate as the default template engine:

  • Amon2
  • Pickles

Where are examples which use Xslate in Catalyst?

There is a real-world project that uses Xslate with Catalyst.

<https://github.com/duckduckgo/community-platform>

Initializing Xslate: <https://github.com/duckduckgo/community-platform/blob/master/lib/DDGC.pm#L268>

Working on: <https://dukgo.com/>

Enjoy!

Development and support

How can I colorize Xslate templates?

For "vim" user, there is xslate.vim for Kolon:

<https://github.com/motemen/xslate-vim>

For "emacs" user, there are plugins:

<https://github.com/samvtran/kolon-mode>

<https://github.com/yoshiki/tx-mode>

Where can I ask questions?

The mailing list is recommended to ask questions.

<http://groups.google.com/group/xslate>

If you find a bug or have a request, creating github issues is better because those tickets are less likely to disappear than the ports in the mailing list.

<https://github.com/xslate/p5-Text-Xslate/issues>

I found a bug! What can I do for you?

Please make a minimal test case to show the problem clearly. The code is the common language both I and you speak fluently ;)

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