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An abstract syntax tree (AST) is a data structure used in computer science to represent the structure of a program or code snippet. It is a tree representation of the abstract syntactic structure of text (often source code ) written in a formal language .
All have a common internal structure. A per-language front end parses the source code in that language and produces an abstract syntax tree ("tree" for short). These are, if necessary, converted to the middle end's input representation, called GENERIC form; the middle end then gradually transforms the program towards its final form.
A code property graph of a program is a graph representation of the program obtained by merging its abstract syntax trees (AST), control-flow graphs (CFG) and program dependence graphs (PDG) at statement and predicate nodes.
The following example shows how to use Bison and flex to write a simple calculator program (only addition and multiplication) and a program for creating an abstract syntax tree. The next two files provide definition and implementation of the syntax tree functions.
These phases themselves can be further broken down: lexing as scanning and evaluating, and parsing as building a concrete syntax tree (CST, parse tree) and then transforming it into an abstract syntax tree (AST, syntax tree). In some cases additional phases are used, notably line reconstruction and preprocessing, but these are rare.
The tree building operators were used in the grammar rules directly transforming the input into an abstract syntax tree. Unparse rules are also test functions that matched tree patterns. Unparse rules are called from a grammar rule when an abstract syntax tree is to be transformed into output code.
This is particularly used in the representation of text in computer languages, [2] which are generally stored in a tree structure as an abstract syntax tree. Abstract syntax, which only consists of the structure of data, is contrasted with concrete syntax, which also includes information about the representation. For example, concrete syntax ...
In the spectrum between interpreting and compiling, another approach is to transform the source code into an optimized abstract syntax tree (AST), then execute the program following this tree structure, or use it to generate native code just-in-time. [12] In this approach, each sentence needs to be parsed just once.