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An abstract syntax tree (AST) is a way of representing the syntax of a programming language as a hierarchical tree-like structure. This structure is used for generating symbol tables for compilers and later code generation. The tree represents all of the constructs in the language and their subsequent rules.
An abstract syntax tree represents all of the syntactical elements of a programming language, similar to syntax trees that linguists use for human languages. The tree focuses on the rules rather than elements like braces or semicolons that terminate statements in some languages. The tree is hierarchical, with the elements of programming statements broken down into their parts. For example, a tree for a conditional statement has the rules for variables hanging down from the required operator.
ASTs are widely used in compilers to check code for accuracy. If the generated tree contains errors, the compiler prints an error message. ASTs are used because some constructs cannot be represented in a context-free grammar, such as implicit typing. ASTs are highly specific to programming languages, but research is underway on universal syntax trees.