The source code produced should be sufficiently similar to the original in appearance and identical in execution, upon recompilation.
Syntax tree in compiler design examples verification#
To support compiler verification it should be possible to unparse an AST into source code form. As a result, an AST used to represent code written in such a language has to also be flexible enough to allow for quick addition of an unknown quantity of children. However, some language constructs require an arbitrarily large number of children, such as argument lists passed to programs from the command shell. Some operations will always require two elements, such as the two terms for addition. These requirements can be used to design the data structure for the AST. Identifiers and their assigned values must be stored for assignment statements.Left and right components of binary operations must be stored and correctly identified.The order of executable statements must be explicitly represented and well defined.Variable types must be preserved, as well as the location of each declaration in source code.The design of an AST is often closely linked with the design of a compiler and its expected features.
Operator overloading is yet another case where correct usage and final function are context-dependent. Another example is duck typing, where the type of an element can change depending on context. Even if a language has a predefined set of types, enforcing proper usage usually requires some context. 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. These are details that require a context to determine their validity and behaviour. 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. In order to avoid this ambiguity, programming languages are often specified as a context-free grammar (CFG). For example, it may store the position of each element in the source code, allowing the compiler to print useful error messages.ĪSTs are needed because of the inherent nature of programming languages and their documentation. An AST usually contains extra information about the program, due to the consecutive stages of analysis by the compiler.Compared to the source code, an AST does not include inessential punctuation and delimiters (braces, semicolons, parentheses, etc.).Such editing and annotation is impossible with the source code of a program, since it would imply changing it. An AST can be edited and enhanced with information such as properties and annotations for every element it contains.It often serves as an intermediate representation of the program through several stages that the compiler requires, and has a strong impact on the final output of the compiler.Īn AST has several properties that aid the further steps of the compilation process:
An AST is usually the result of the syntax analysis phase of a compiler. Once built, additional information is added to the AST by means of subsequent processing, e.g., contextual analysis.Ībstract syntax trees are also used in program analysis and program transformation systems.Ībstract syntax trees are data structures widely used in compilers to represent the structure of program code. Parse trees are typically built by a parser during the source code translation and compiling process. This distinguishes abstract syntax trees from concrete syntax trees, traditionally designated parse trees. Likewise, a syntactic construct like an if-condition-then statement may be denoted by means of a single node with three branches. For instance, grouping parentheses are implicit in the tree structure, so these do not have to be represented as separate nodes. The syntax is "abstract" in the sense that it does not represent every detail appearing in the real syntax, but rather just the structural or content-related details. Each node of the tree denotes a construct occurring in the text. In computer science, an abstract syntax tree ( AST), or just syntax tree, is a tree representation of the abstract syntactic structure of text (often source code) written in a formal language. While b ≠ 0 : if a > b : a := a - b else : b := b - a return a