FILTERS pragma

The {{% FILTERS }} pragma enables a handy pipe-notation for filtering values prior to interpolation:

{{ greeting | case.lower }}

More specifically:

1. Pipe notation is analogous to dot notation — it can be thought of as syntactic sugar for nested sections.
2. Filters are just variables in the normal context stack.
3. Values are not coerced into strings (or escaped) until they come out the other end of the pipe.
4. Unlike nested sections, the first value in the pipe is fetched prior to passing it to the next lambda.
5. If any filter is not found, or is non-callable, an UnexpectedValueException is thrown.
6. Filters are only available for interpolation ({{ foo }}) and section({{# foo }} or {{^ foo }}) tags. You can't use 'em in partial tags.
7. Filters are not intended to replace a proper View or ViewModel. While they can be (ab)used to add logic to your templates, please resist the temptation and keep your logic in code.

In practice, they look something like this:

<?php

$mustache = new Mustache_Engine;
$mustache->addHelper('case', [
    'lower' => function($value) { return strtolower((string) $value); },
    'upper' => function($value) { return strtoupper((string) $value); },
]);
$mustache->addHelper('!!', function($value) { return $value . '!!'; });

$tpl = <<<TPL
{{%FILTERS}}
{{ greeting | case.lower }}, {{ planet | case.upper | !! }}
TPL;

echo $mustache->render($tpl, [
    'greeting' => 'Hello',
    'planet'   => 'world',
]);

// "hello, WORLD!!"

See mustache/spec#41 for the discussion on including this pragma in the Mustache spec.