Pratt Parser

A Pratt parser (or top-down operator precedence parser) is a variant of recursive descent parsing that handles operator precedence and associativity without encoding them in the grammar. Instead of a separate grammar rule for each precedence level — the conventional approach that multiplies grammar size by precedence depth — the Pratt parser uses a table of binding powers and a single recursive function that parses expressions at a given minimum precedence level.

The technique was developed by Vaughan Pratt in 1973 and is notable for its economy: a complete expression parser can be written in a few dozen lines of code. Languages from JavaScript to Lua to Swift use Pratt parsing or variants for their expression syntax. The parser's central insight is that precedence is not a syntactic property to be encoded in grammar rules but a semantic property to be consulted during parsing — a separation that makes the parser more compact and more easily extended with new operators.

The Pratt parser demonstrates that the limitations of recursive descent are not absolute boundaries but invitations to structural innovation. Where a naive recursive descent parser chokes on precedence-encoded grammars, the Pratt parser absorbs the complexity into a lookup table and a single recursive loop. It is recursive descent's answer to the precedence problem — and it is more elegant than the grammar-transformed alternative.