LALR Parsers

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LALR - Look Ahead Left-to-right parse, Rightmost-derivation.

LALR parsers are a specialized form of LR parsers that can deal with more context-free grammars than Simple LR parsers [INSERIR LINKS] but less than LR(1) parsers can.

LALR parsers were introduced as practical parsers for deterministic languages. Rather than building an exponential number of LR(k) states, LALR(k) parsers add lookahead sets to the actions of the small LR(0) parser. While SLR uses follow sets to construct reduce actions, LALR uses lookahead sets, which are more specific because they take more of the parsing context into account.

In LR(1) parsing, an item A ::= a (s1) is different from A ::= a (s2) if s1 is different from s2. This results to a large number of states since the combinations of expected lookahead symbols can be very large. To reduce the number of states, when we have two items like those two, instead of creating two states (one for each item), we combine the two states into one by creating an item A := a (s3) where s3 is the union of s1 and s2. Since we make the expected lookahead sets larger, there is a danger that some conflicts will have worse chances to be resolved. But the number of states we get is the same as that for LR(0). This grammar is called LALR(1).

LALR parser generators

  1. AnaGram
  3. CUP
  4. ELI
  5. Gold
  6. Lemon
  7. LISA
  8. Ulm's Modula-2 System
  9. YACC
  10. YaYacc
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