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Added an article about ExpressionLanguage AST #7831

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8a54678
Added an article about ExpressionLanguage AST
javiereguiluz Apr 24, 2017
bc8057f
Finished the first draft of the article
javiereguiluz Apr 24, 2017
bfb54e2
Fixes and rewords
javiereguiluz May 3, 2017
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6 changes: 6 additions & 0 deletions components/expression_language.rst
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Expand Up @@ -112,6 +112,12 @@ Caching
The component provides some different caching strategies, read more about them
in :doc:`/components/expression_language/caching`.

AST Dumping and Editing
-----------------------

The AST (*Abstract Syntax Tree*) of expressions can be dumped and manipulated
as explained in :doc:`/components/expression_language/ast`.

Learn More
----------

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33 changes: 33 additions & 0 deletions components/expression_language/ast.rst
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@@ -0,0 +1,33 @@
.. index::
single: AST; ExpressionLanguage
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I think we should at least add "Abstract Syntax Tree" to the index.


Dumping and Manipulating the AST of Expressions
===============================================

In computer science, `AST`_ (*Abstract Syntax Trees*) are a tree representation
of the structure of source code written in a programming language. The
expressions created with the ExpressionLanguage component are strings, which
make them difficult to manipulate or inspect.

A better approach is to dump the AST of those expressions using the ``dump()``
method. This turns the original string expression into a set of PHP classes
describing the operations of that expression::

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;

$language = new ExpressionLanguage();
$ast = $language->dump('1 + 2');
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Hi;

Actually, this method does not exist. You can get the AST like this:

<?php

require __DIR__.'/vendor/autoload.php';

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;

$ast = (new ExpressionLanguage())
        ->parse('var.a.b + 12', ['var'])
        ->getNodes()
;

dump($ast);

And then you can dump the AST (as a string) dump($ast->dump());

Finally, Instead of getting a string, it's possible to get an array in order to manipulate it: dump($ast->toArray());

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Thanks for your help and for the examples! I've updated the article contents.

// $ast = new BinaryNode('+', new ConstantNode(1), new ConstantNode(2));

$ast = $language->dump('"a" not in ["a", "b"]');
// $ast = new BinaryNode('not in', new ConstantNode('a'), new ArrayNode(new ConstantNode('a'), new ConstantNode('b')));

$ast = $language->dump('foo[0]');
// $ast = new GetAttrNode(new NameNode('foo'), new ConstantNode(0));

Manipulating the AST
--------------------

.. TODO: https://github.com/symfony/symfony/pull/19060

.. _`AST`: https://en.wikipedia.org/wiki/Abstract_syntax_tree