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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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49 changes: 49 additions & 0 deletions components/expression_language/ast.rst
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@@ -0,0 +1,49 @@
.. index::
single: AST; ExpressionLanguage
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I think we should at least add "Abstract Syntax Tree" to the index.

single: AST; Abstract Syntax Tree

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

Manipulating or inspecting the expressions created with the ExpressionLanguage
component is difficult because they are plain strings. A better approach is to
turn those expressions into an AST. In computer science, `AST`_ (*Abstract
Syntax Tree*) is *"a tree representation of the structure of source code written
in a programming language"*. In Symfony, a ExpressionLanguage AST is a set of
nodes that contain PHP classes representing the given expression.

Dumping the AST
---------------

Call the :method:`Symfony\\Component\\ExpressionLanguage\\ExpressionLanguage::getNodes`
method after parsing any expression to get its AST::

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;

$ast = (new ExpressionLanguage())
->parse('1 + 2')
->getNodes()
;

// dump the AST nodes for inspection
var_dump($ast);

// dump the AST nodes as a string representation
$astAsString = $ast->dump();

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

The nodes of the AST can also be dumped into a PHP array of nodes to allow
manipulating them. Call the :method:`Symfony\\Component\\ExpressionLanguage\\ExpressionLanguage::toArray`
method to turn the AST into an array::

// ...

$astAsArray = (new ExpressionLanguage())
->parse('1 + 2')
->getNodes()
->toArray()
;

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