LALR parser

In computer science, an LALR parser^[a] (look-ahead, left-to-right, rightmost derivation parser) is part of the compiling process where human readable text is converted into a structured representation to be read by computers. An LALR parser is a software tool to process (parse) text into a very specific internal representation that other programs, such as compilers, can work with. This process happens according to a set of production rules specified by a formal grammar for a computer language.

An LALR parser is a simplified version of a canonical LR parser.

The LALR parser was invented by Frank DeRemer in his 1969 PhD dissertation, Practical Translators for LR(k) languages,^[1] in his treatment of the practical difficulties at that time of implementing LR(1) parsers. He showed that the LALR parser has more language recognition power than the LR(0) parser, while requiring the same number of states as the LR(0) parser for a language that can be recognized by both parsers. This makes the LALR parser a memory-efficient alternative to the LR(1) parser for languages that are LALR. It was also proven that there exist LR(1) languages that are not LALR. Despite this weakness, the power of the LALR parser is sufficient for many mainstream computer languages,^[2] including Java,^[3] though the reference grammars for many languages fail to be LALR due to being ambiguous.^[2]

The original dissertation gave no algorithm for constructing such a parser given a formal grammar. The first algorithms for LALR parser generation were published in 1973.^[4] In 1982, DeRemer and Tom Pennello published an algorithm that generated highly memory-efficient LALR parsers.^[5] LALR parsers can be automatically generated from a grammar by an LALR parser generator such as Yacc or GNU Bison. The automatically generated code may be augmented by hand-written code to augment the power of the resulting parser.

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