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In computer science, LR parsers are a type of bottom-up parser that analyse deterministic context-free languages in linear time. [1] There are several variants of LR parsers: SLR parsers, LALR parsers, canonical LR(1) parsers, minimal LR(1) parsers, and generalized LR parsers (GLR parsers).
Context-free languages are a category of languages (sometimes termed Chomsky Type 2) which can be matched by a sequence of replacement rules, each of which essentially maps each non-terminal element to a sequence of terminal elements and/or other nonterminal elements.
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 ...
A lookahead LR parser (LALR) generator is a software tool that reads a context-free grammar (CFG) and creates an LALR parser which is capable of parsing files written in the context-free language defined by the CFG. LALR parsers are desirable because they are very fast and small in comparison to other types of parsers.
These parsers require much less memory than Canonical LR(1) parsers, but have slightly less language-recognition power. [5] LALR(1) parsers have been the most common implementations of the LR Parser. However, a new type of LR(1) parser, some people call a "Minimal LR(1) parser" was introduced in 1977 by David Pager [ 6 ] who showed that LR(1 ...
In computer science, a Simple LR or SLR parser is a type of LR parser with small parse tables and a relatively simple parser generator algorithm. As with other types of LR(1) parser, an SLR parser is quite efficient at finding the single correct bottom-up parse in a single left-to-right scan over the input stream, without guesswork or backtracking.
So bottom-up parsers in practice handle a somewhat larger range of computer language grammars than deterministic top-down parsers do. Bottom-up parsing is sometimes done by backtracking. But much more commonly, bottom-up parsing is done by a shift-reduce parser such as a LALR parser.
At least one parser generator has implemented LR(*) parsing (i.e. LRSTAR). The challenge is to make LR(*) efficient so that it does not impact the performance of the parser. As long as k (the number of look-aheads) is small, LR(*) parsing is efficient. — Preceding unsigned comment added by Paulbmann (talk • contribs) 10:28, May 23, 2019 (UTC)