Editing LR parser (section)

=== Variants of LR parsers ===

The LR parser generator decides what should happen for each combination of parser state and lookahead symbol.  These decisions are usually turned into read-only data tables that drive a generic parser loop that is grammar- and state-independent.  But there are also other ways to turn those decisions into an active parser.

Some LR parser generators create separate tailored program code for each state, rather than a parse table.  These parsers can run several times faster than the generic parser loop in table-driven parsers.  The fastest parsers use generated assembler code.

In the [[recursive ascent parser]] variation, the explicit parse stack structure is also replaced by the implicit stack used by subroutine calls.  Reductions terminate several levels of subroutine calls, which is clumsy in most languages.  So recursive ascent parsers are generally slower, less obvious, and harder to hand-modify than [[recursive descent parser]]s.

Another variation replaces the parse table by pattern-matching rules in non-procedural languages such as [[Prolog]].

'''GLR''' [[Generalized LR parser]]s use LR bottom-up techniques to find all possible parses of input text, not just one correct parse.   This is essential for ambiguous grammar such as used for human languages.  The multiple valid parse trees are computed simultaneously, without backtracking.  GLR is sometimes helpful for computer languages that are not easily described by a conflict-free LALR(1) grammar.

'''LC''' [[Left corner parser]]s use LR bottom-up techniques for recognizing the left end of alternative grammar rules.  When the alternatives have been narrowed down to a single possible rule, the parser then switches to top-down LL(1) techniques for parsing the rest of that rule.  LC parsers have smaller parse tables than LALR parsers and better error diagnostics.  There are no widely used generators for deterministic LC parsers.  Multiple-parse LC parsers are helpful with human languages with very large grammars.