PackratParsers is a component that extends the parser combinators provided by Parsers with a memoization facility (``Packrat Parsing'').

Packrat Parsing is a technique for implementing backtracking, recursive-descent parsers, with the advantage that it guarantees unlimited lookahead and a linear parse time. Using this technique, left recursive grammars can also be accepted.

Using PackratParsers is very similar to using Parsers:

• any class/trait that extends Parsers (directly or through a subclass) can mix in PackratParsers. Example: object MyGrammar extends StandardTokenParsers with PackratParsers
• each grammar production previously declared as a def without formal parameters becomes a lazy val, and its type is changed from Parser[Elem] to PackratParser[Elem]. So, for example, def production: Parser[Int] = {...} becomes lazy val production: PackratParser[Int] = {...}
• Important: using PackratParsers is not an ``all or nothing'' decision. They can be free mixed with regular Parsers in a single grammar.

Cached parse results are attached to the input, not the grammar. Therefore, PackratsParsers require a PackratReader as input, which adds memoization to an underlying Reader. Programmers can create PackratReader objects either manually, as in production(new PackratReader(new lexical.Scanner("input"))), but the common way should be to rely on the combinator phrase to wrap a given input with a PackratReader if the input is not one itself.