Parsers

A parser is a software component that analyzes a sequence of tokens to determine its grammatical structure with respect to a given formal grammar. Parsers typically produce a parse tree or an abstract syntax tree that represents the hierarchical relationships in the input, enabling subsequent semantic analysis or code generation. In a typical compiler or interpreter pipeline, a lexer first tokenizes the input, and the parser then processes the tokens according to the grammar.

Parsers are categorized by parsing strategies. Top-down parsers build trees from the root toward the leaves, as in recursive-descent and LL(k) parsers. Bottom-up parsers construct trees from the leaves upward, including LR, SLR, and LALR parsers. Many practical parsers are generated from grammar specifications using tools such as YACC, Bison, ANTLR, or JavaCC, though hand-written parsers are common for performance or special requirements.

Common use cases include compilers and interpreters for programming languages, as well as parsers for data formats like JSON, XML, and SQL, and for domain-specific languages. Grammars specify valid syntax and may include notions such as operator precedence and associativity to resolve ambiguities. Some grammars are designed to be unambiguous for a deterministic parser, while others use disambiguation strategies or multiple-pass parsing.

Key concerns in parser design include handling syntax errors gracefully, restoring state after errors, ensuring reasonable performance, and supporting incremental or streaming parsing for large inputs. The typical output is an abstract syntax tree that reflects the source structure, which is then used for semantic analysis, optimization, or translation into another form.