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But by convention, the LR name stands for the form of parsing invented by Donald Knuth, and excludes the earlier, less powerful precedence methods (for example Operator-precedence parser). [1] LR parsers can handle a larger range of languages and grammars than precedence parsers or top-down LL parsing . [ 3 ]
The LR(1) parser is a deterministic automaton and as such its operation is based on static state transition tables. These codify the grammar of the language it recognizes and are typically called "parsing tables". The parsing tables of the LR(1) parser are parameterized with a lookahead terminal.
A grammar is said to be SLR(1) if the following simple LR parser algorithm results in no ambiguity. If state s contains any item of the form A → a • Xb , where X is a terminal, and X is the next token in the input string, then the action is to shift the current input token onto the stack, and the new state to be pushed on the stack is the ...
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.
The dangling-else problem dates back to ALGOL 60, [1] and subsequent languages have resolved it in various ways. In LR parsers , the dangling else is the archetypal example of a shift-reduce conflict .
The generated parsers are portable: they do not require any specific compilers. Bison by default generates LALR(1) parsers but it can also generate canonical LR, IELR(1) and GLR parsers. [4] In POSIX mode, Bison is compatible with Yacc, but also has several extensions over this earlier program, including Generation of counterexamples for conflicts
The LALR parser and its alternatives, the SLR parser and the Canonical LR parser, have similar methods and parsing tables; their main difference is in the mathematical grammar analysis algorithm used by the parser generation tool. LALR generators accept more grammars than do SLR generators, but fewer grammars than full LR(1).
Left recursion often poses problems for parsers, either because it leads them into infinite recursion (as in the case of most top-down parsers) or because they expect rules in a normal form that forbids it (as in the case of many bottom-up parsers [clarification needed]). Therefore, a grammar is often preprocessed to eliminate the left recursion.