An LALR parser generator is a software tool that reads a BNF grammar and creates an LALR parser which is capable of parsing files written in the computer language defined by the BNF grammar. LALR parsers are desirable because they are very fast and small in comparison to other types of parsers.

There are other types of parser generators, such as SLR, LR, GLR and LL parser generators. What differentiates one from another is the type of BNF grammar which they are capable of accepting and the type of parsing algorithm which is used in the generated parser. Obviously, an LALR parser generator accepts an LALR grammar as input and generates a parser that uses an LALR parsing algorithm (which is driven by LALR parser tables).

In practice, LALR offers a good solution, because LALR(1) grammars are more powerful than SLR(1) and LL(1) grammars. LR(1) grammars are more powerful than LALR(1), however, canonical LR(1) parsers can be extremely large in size and are considered not practical. Minimal LR(1) parsers are small in size and comparable to LALR(1) parsers.


Frank DeRemer invented LALR parsers with his PhD dissertation, called "Practical LR(k) Translators", in 1969, at MIT. This was an important breakthrough, because LR(k) translators, as defined by Donald Knuth in his 1965 paper, "On the Translation of Languages from Left to Right", were much too large for implementation on computer systems in the 1960s and 70's.

An early LALR parser generator and probably the most popular one for many years was "yacc", created by Stephen Johnson in 1975 at AT&T Labs. Another, "TWS", was created by Frank DeRemer and Tom Pennello.

Today, there are many LALR parser generators available, for example GNU bison.

See also

Comparison of parser generators - For a more complete list, which also includes LL, SLR, GLR and LR parser generators.

External links

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