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LALR
In computer science, an LALR parser or Look-Ahead LR parser is a simplified version of a canonical LR parser, to parse a text according to a set of production rules specified by a formal grammar for a computer language. ("LR" means left-to-right, rightmost derivation.) The LALR parser was invented by Frank DeRemer in his 1969 PhD dissertation, ''Practical Translators for LR(k) languages'', in his treatment of the practical difficulties at that time of implementing LR(1) parsers. He showed that the LALR parser has more language recognition power than the LR(0) parser, while requiring the same number of states as the LR(0) parser for a language that can be recognized by both parsers. This makes the LALR parser a memory-efficient alternative to the LR(1) parser for languages that are LALR. It was also proven that there exist LR(1) languages that are not LALR. Despite this weakness, the power of the LALR parser is sufficient for many mainstream computer languages,''LR Parsing: Theory a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Comparison Of Parser Generators
This is a list of notable lexer generators and parser generators for various language classes. Regular languages Regular languages are a category of languages (sometimes termed Chomsky Type 3) which can be matched by a state machine (more specifically, by a deterministic finite automaton or a nondeterministic finite automaton) constructed from a regular expression. In particular, a regular language can match constructs like "A follows B", "Either A or B", "A, followed by zero or more instances of B", but cannot match constructs which require consistency between non-adjacent elements, such as "some instances of A followed by the same number of instances of B", and also cannot express the concept of recursive "nesting" ("every A is eventually followed by a matching B"). A classic example of a problem which a regular grammar cannot handle is the question of whether a given string contains correctly-nested parentheses. (This is typically handled by a Chomsky Type 2 grammar, also te ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
LR Parser
In computer science, LR parsers are a type of bottom-up parser that analyse deterministic context-free languages in linear time. There are several variants of LR parsers: SLR parsers, LALR parsers, Canonical LR(1) parsers, Minimal LR(1) parsers, and GLR parsers. LR parsers can be generated by a parser generator from a formal grammar defining the syntax of the language to be parsed. They are widely used for the processing of computer languages. An LR parser (Left-to-right, Rightmost derivation in reverse) reads input text from left to right without backing up (this is true for most parsers), and produces a rightmost derivation in reverse: it does a bottom-up parse – not a top-down LL parse or ad-hoc parse. The name LR is often followed by a numeric qualifier, as in LR(1) or sometimes LR(''k''). To avoid backtracking or guessing, the LR parser is allowed to peek ahead at ''k'' lookahead input symbols before deciding how to parse earlier symbols. Typically ''k'' is 1 and is n ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
LALR Parser Generator
A lookahead LR parser (LALR) 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 Simple LR parser, LR parser, GLR parser, LL parser and GLL 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. 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 can parse most practical LL(1) grammars. LR(1) grammars are more powerful than LALR(1), but canonical LR(1) parsers ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
SLR Parser
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 parser is mechanically generated from a formal grammar for the language. SLR and the more-general methods LALR parser and Canonical LR parser have identical methods and similar tables at parse time; they differ only in the mathematical grammar analysis algorithms used by the parser generator tool. SLR and LALR generators create tables of identical size and identical parser states. SLR generators accept fewer grammars than do LALR generators like yacc and Bison. Many computer languages don't readily fit the restrictions of SLR, as is. Bending the language's natural grammar into SLR grammar form requires more compromises and gra ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Canonical LR Parser
In computer science, a canonical LR parser or LR(1) parser is an LR(k) parser for ''k=1'', i.e. with a single lookahead terminal. The special attribute of this parser is that any LR(k) grammar with ''k>1'' can be transformed into an LR(1) grammar. However, back-substitutions are required to reduce k and as back-substitutions increase, the grammar can quickly become large, repetitive and hard to understand. LR(k) can handle all deterministic context-free languages. In the past this LR(k) parser has been avoided because of its huge memory requirements in favor of less powerful alternatives such as the LALR and the LL(1) parser. Recently, however, a "minimal LR(1) parser" whose space requirements are close to LALR parsers, is being offered by several parser generators. Like most parsers, the LR(1) parser is automatically generated by compiler-compilers like GNU Bison, MSTA, Menhir, HYACC, LRSTAR. History In 1965 Donald Knuth invented the LR(k) parser (Left to right, Rightmost d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Parse
Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar. The term ''parsing'' comes from Latin ''pars'' (''orationis''), meaning part (of speech). The term has slightly different meanings in different branches of linguistics and computer science. Traditional sentence parsing is often performed as a method of understanding the exact meaning of a sentence or word, sometimes with the aid of devices such as sentence diagrams. It usually emphasizes the importance of grammatical divisions such as subject and predicate. Within computational linguistics the term is used to refer to the formal analysis by a computer of a sentence or other string of words into its constituents, resulting in a parse tree showing their syntactic relation to each other, which may also contain semantic and other information (p-values). Some parsing algori ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lookahead (parsing)
Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar. The term ''parsing'' comes from Latin ''pars'' (''orationis''), meaning part (of speech). The term has slightly different meanings in different branches of linguistics and computer science. Traditional sentence parsing is often performed as a method of understanding the exact meaning of a sentence or word, sometimes with the aid of devices such as sentence diagrams. It usually emphasizes the importance of grammatical divisions such as subject and predicate. Within computational linguistics the term is used to refer to the formal analysis by a computer of a sentence or other string of words into its constituents, resulting in a parse tree showing their syntactic relation to each other, which may also contain semantic and other information (p-values). Some parsing algorit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Yacc
Yacc (Yet Another Compiler-Compiler) is a computer program for the Unix operating system developed by Stephen C. Johnson. It is a Look Ahead Left-to-Right Rightmost Derivation (LALR) parser generator, generating a LALR parser (the part of a compiler that tries to make syntactic sense of the source code) based on a formal grammar, written in a notation similar to Backus–Naur Form (BNF). Yacc is supplied as a standard utility on BSD and AT&T Unix. GNU-based Linux distributions include Bison, a forward-compatible Yacc replacement. History In the early 1970s, Stephen C. Johnson, a computer scientist at Bell Labs / AT&T, developed Yacc because he wanted to insert an exclusive or operator into a B language compiler (developed using McIlroy's TMG compiler-compiler), but it turned out to be a hard task. As a result, he was directed by his colleague at Bell Labs Al Aho to Donald Knuth's work on LR parsing, which served as the basis for Yacc. Yacc was influenced by and received ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Context-free Grammar
In formal language theory, a context-free grammar (CFG) is a formal grammar whose production rules are of the form :A\ \to\ \alpha with A a ''single'' nonterminal symbol, and \alpha a string of terminals and/or nonterminals (\alpha can be empty). A formal grammar is "context-free" if its production rules can be applied regardless of the context of a nonterminal. No matter which symbols surround it, the single nonterminal on the left hand side can always be replaced by the right hand side. This is what distinguishes it from a context-sensitive grammar. A formal grammar is essentially a set of production rules that describe all possible strings in a given formal language. Production rules are simple replacements. For example, the first rule in the picture, :\langle\text\rangle \to \langle\text\rangle = \langle\text\rangle ; replaces \langle\text\rangle with \langle\text\rangle = \langle\text\rangle ;. There can be multiple replacement rules for a given nonterminal symbol. The ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rightmost Derivation
In formal language theory, a context-free grammar (CFG) is a formal grammar whose production rules are of the form :A\ \to\ \alpha with A a ''single'' nonterminal symbol, and \alpha a string of terminals and/or nonterminals (\alpha can be empty). A formal grammar is "context-free" if its production rules can be applied regardless of the context of a nonterminal. No matter which symbols surround it, the single nonterminal on the left hand side can always be replaced by the right hand side. This is what distinguishes it from a context-sensitive grammar. A formal grammar is essentially a set of production rules that describe all possible strings in a given formal language. Production rules are simple replacements. For example, the first rule in the picture, :\langle\text\rangle \to \langle\text\rangle = \langle\text\rangle ; replaces \langle\text\rangle with \langle\text\rangle = \langle\text\rangle ;. There can be multiple replacement rules for a given nonterminal symbol. The ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bottom-up Parsing
In computer science, parsing reveals the grammatical structure of linear input text, as a first step in working out its meaning. Bottom-up parsing recognizes the text's lowest-level small details first, before its mid-level structures, and leaving the highest-level overall structure to last. Bottom-up Versus Top-down The bottom-up name comes from the concept of a parse tree, in which the most detailed parts are at the bottom of the upside-down tree, and larger structures composed from them are in successively higher layers, until at the top or "root" of the tree a single unit describes the entire input stream. A bottom-up parse discovers and processes that tree starting from the bottom left end, and incrementally works its way upwards and rightwards. Compilers: Principles, Techniques, and Tools (2nd Edition), by Alfred Aho, Monica Lam, Ravi Sethi, and Jeffrey Ullman, Prentice Hall 2006. A parser may act on the structure hierarchy's low, mid, and highest levels without ever creat ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
LL Parser
In computer science, an LL parser (Left-to-right, leftmost derivation) is a top-down parser for a restricted context-free language. It parses the input from Left to right, performing Leftmost derivation of the sentence. An LL parser is called an LL(''k'') parser if it uses ''k'' tokens of lookahead when parsing a sentence. A grammar is called an LL(''k'') grammar if an LL(''k'') parser can be constructed from it. A formal language is called an LL(''k'') language if it has an LL(''k'') grammar. The set of LL(''k'') languages is properly contained in that of LL(''k''+1) languages, for each ''k'' ≥ 0. A corollary of this is that not all context-free languages can be recognized by an LL(''k'') parser. An LL parser is called LL-regular (LLR) if it parses an LL-regular language. The class of LLR grammars contains every LL(k) grammar for every k. For every LLR grammar there exists an LLR parser that parses the grammar in linear time. Two nomenclative outlier parser typ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
