.. index:: Auxiliary modules; Tree Decoration

Tree Decoration
---------------    

When traversing the tree for type checking, we may need to store information about a node
in the tree that will be used later, either in type cheking, or in code generation.

Associating information to certaing nodes in the parse tree is called *decorating* the tree.

For example, an aritmethic expression (e.g. a sum) will have a type that will depend on the
type of its operands (that may be other expressions, and in turn depend on the type of their
operands...). Since expressions are not symbols, their type is not stored in the Symbol Table.
Thus, when we climb the tree, we need to compute the type for each subexpression to allow
nodes higher in the tree perform the checks they may need.
  
Class ``TreeDecoration`` stores information associated to tree nodes, and offers methods
to retrieve it.
Information that our compiler will need to keep about nodes is: 

 - The type of the node (e.g. for nodes corresponding to expressions).
 - Whether the node is an l-value (e.g. it is at the left-hand side of an assignment). 
 - The scope to which the node belongs.

(note than not all nodes will be decorated with all this information. Some node will not
be decorated at all)

Nodes are represented as ``antlr4::ParserRuleContext *``, that is, pointers to nodes in the ParseTree
built by ANTLR using our grammar rules.
   
.. literalinclude:: ../../../practica/common/TreeDecoration.h
   :language: c++
   :linenos:
   :lines: 41-77,83