Added an article about ExpressionLanguage AST

Author: javiereguiluz

components/expression_language.rst

@@ -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
 ----------
 

components/expression_language/ast.rst

@@ -0,0 +1,49 @@
+.. index::
+    single: AST; ExpressionLanguage
+    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