* 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.
ASTs are needed because of the inherent nature of programming languages and their documentation. 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.
