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Sometimes the grammar is modified so that it is unambiguous, such as by requiring an endif statement or making else mandatory. In other cases the grammar is left ambiguous, but the ambiguity is resolved by making the overall phrase grammar context-sensitive, such as by associating an else with the nearest if. In this latter case the grammar is ...
Alternatively, the grammar can be rewritten to remove the conflict, at the expense of an increase in grammar size (see below). If the parser is hand-written, the programmer may use a non-ambiguous context-free grammar. Alternatively, one may rely on a non-context-free grammar or a parsing expression grammar.
DCFGs are always unambiguous, and are an important subclass of unambiguous CFGs; there are non-deterministic unambiguous CFGs, however. DCFGs are of great practical interest, as they can be parsed in linear time and in fact a parser can be automatically generated from the grammar by a parser generator. They are thus widely used throughout ...
All conflicts that arise in applying a LALR(1) parser to an unambiguous LR(1) grammar are reduce/reduce conflicts. The SLR(1) parser performs further merging, which introduces additional conflicts. The standard example of an LR(1) grammar that cannot be parsed with the LALR(1) parser, exhibiting such a reduce/reduce conflict, is: [ 10 ] [ 11 ]
DCFLs are always unambiguous, meaning that they admit an unambiguous grammar. There are non-deterministic unambiguous CFLs, so DCFLs form a proper subset of unambiguous CFLs. DCFLs are of great practical interest, as they can be parsed in linear time, and various restricted forms of DCFGs admit simple practical parsers. They are thus widely ...
It is decidable whether a given grammar is a regular grammar, [f] as well as whether it is an LL grammar for a given k≥0. [26]: 233 If k is not given, the latter problem is undecidable. [26]: 252 Given a context-free grammar, it is not decidable whether its language is regular, [27] nor whether it is an LL(k) language for a given k.
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The grammar for an LR parser must be unambiguous itself, or must be augmented by tie-breaking precedence rules. This means there is only one correct way to apply the grammar to a given legal example of the language, resulting in a unique parse tree with just one meaning, and a unique sequence of shift/reduce actions for that example.