Editing Abstract syntax tree (section)

=== Motivation ===
An AST has several properties that aid the further steps of the compilation process:
* An AST can be edited and enhanced with information such as properties and annotations for every element it contains. Such editing and annotation is impossible with the source code of a program, since it would imply changing it.
* Compared to the [[source code]], an AST does not include inessential punctuation and delimiters (braces, semicolons, parentheses, etc.).
* An AST usually contains extra information about the program, due to the consecutive stages of analysis by the compiler. For example, it may store the position of each element in the source code, allowing the compiler to print useful error messages.

Languages are often [[syntactic ambiguity|ambiguous]] by nature. In order to avoid this ambiguity, programming languages are often specified as a [[context-free grammar]] (CFG). However, there are often aspects of programming languages that a CFG can't express, but are part of the language and are documented in its specification. These are details that require a context to determine their validity and behaviour. For example, if a language allows new types to be declared, a CFG cannot predict the names of such types nor the way in which they should be used. Even if a language has a predefined set of types, enforcing proper usage usually requires some context. Another example is [[duck typing]], where the type of an element can change depending on context. [[Operator overloading]] is yet another case where correct usage and final function are context-dependent.