.. index::
    single: Caching; ExpressionLanguage

Caching Expressions Using Parser Caches
=======================================

The ExpressionLanguage component already provides a
:method:`Symfony\\Component\\ExpressionLanguage\\ExpressionLanguage::compile`
method to be able to cache the expressions in plain PHP. But internally, the
component also caches the parsed expressions, so duplicated expressions can be
compiled/evaluated quicker.

The Workflow
------------

Both :method:`Symfony\\Component\\ExpressionLanguage\\ExpressionLanguage::evaluate`
and ``compile()`` need to do some things before each can provide the return
values. For ``evaluate()``, this overhead is even bigger.

Both methods need to tokenize and parse the expression. This is done by the
:method:`Symfony\\Component\\ExpressionLanguage\\ExpressionLanguage::parse`
method. It  returns a :class:`Symfony\\Component\\ExpressionLanguage\\ParsedExpression`.
Now, the ``compile()`` method just returns the string conversion of this object.
The ``evaluate()`` method needs to loop through the "nodes" (pieces of an
expression saved in the ``ParsedExpression``) and evaluate them on the fly.

To save time, the ``ExpressionLanguage`` caches the ``ParsedExpression`` so
it can skip the tokenize and parse steps with duplicate expressions.
The caching is done by a
:class:`Symfony\\Component\\ExpressionLanguage\\ParserCache\\ParserCacheInterface`
instance (by default, it uses an
:class:`Symfony\\Component\\ExpressionLanguage\\ParserCache\\ArrayParserCache`).
You can customize this by creating a custom ``ParserCache`` and injecting this
in the object using the constructor::

    use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
    use Acme\ExpressionLanguage\ParserCache\MyDatabaseParserCache;

    $cache = new MyDatabaseParserCache(...);
    $language = new ExpressionLanguage($cache);

.. note::

    The `DoctrineBridge`_ provides a Parser Cache implementation using the
    `doctrine cache library`_, which gives you caching for all sorts of cache
    strategies, like Apc, Filesystem and Memcached.

Using Parsed and Serialized Expressions
---------------------------------------

Both ``evaluate()`` and ``compile()`` can handle ``ParsedExpression`` and
``SerializedParsedExpression``::

    use Symfony\Component\ExpressionLanguage\ParsedExpression;
    // ...

    $expression = new ParsedExpression($language->parse('1 + 4'));

    echo $language->evaluate($expression); // prints 5

.. code-block:: php

    use Symfony\Component\ExpressionLanguage\SerializedParsedExpression;
    // ...

    $expression = new SerializedParsedExpression(
        serialize($language->parse('1 + 4'))
    );

    echo $language->evaluate($expression); // prints 5

.. _DoctrineBridge: https://github.com/symfony/DoctrineBridge
.. _`doctrine cache library`: http://docs.doctrine-project.org/projects/doctrine-common/en/latest/reference/caching.html