Chomsky hierarchy

Within the field of computer science, specifically in the area of formal languages, the Chomsky hierarchy (occasionally referred to as Chomsky–Schützenberger hierarchy) is a containment hierarchy of classes of formal grammars.

This hierarchy of grammars was described by Noam Chomsky in 1956^[1]. It is also named after Marcel-Paul Schützenberger who played a crucial role in the development of the theory of formal languages.

1 Formal grammars
2 The hierarchy
3 See also
4 References
5 External links

Formal grammars

A formal grammar of this type consists of:

a finite set of terminal symbols
a finite set of nonterminal symbols
a finite set of production rules with a left and a right-hand side consisting of a sequence of these symbols
a start symbol

A formal grammar defines (or generates) a formal language, which is a (usually infinite) set of finite-length sequences of symbols (i.e. strings) that may be constructed by applying production rules to another sequence of symbols which initially contains just the start symbol. A rule may be applied to a sequence of symbols by replacing an occurrence of the symbols on the left-hand side of the rule with those that appear on the right-hand side. A sequence of rule applications is called a derivation. Such a grammar defines the formal language: all words consisting solely of terminal symbols which can be reached by a derivation from the start symbol.

Nonterminals are usually represented by uppercase letters, terminals by lowercase letters, and the start symbol by $S$ . For example, the grammar with terminals $\{a, b\}$ , nonterminals $\{S, A, B\}$ , production rules

$S$ $\rightarrow \,$ $ABS$

$S$ $\rightarrow \,$ ε (where ε is the empty string)

$BA$ $\rightarrow \,$ $AB$

$BS$ $\rightarrow \,$ $b$

$Bb$ $\rightarrow \,$ $bb$

$Ab$ $\rightarrow \,$ $ab$

$Aa$ $\rightarrow \,$ $aa$

and start symbol $S$ , defines the language of all words of the form $a^n b^n$ (i.e. $n$ copies of $a$ followed by $n$ copies of $b$ ). The following is a simpler grammar that defines the same language: Terminals $\{a, b\}$ , Nonterminals $\{S\}$ , Start symbol $S$ , Production rules

$S$ $\rightarrow \,$ $aSb$

$S$ $\rightarrow \,$ ε

The hierarchy

Set inclusions described by the Chomsky hierarchy.

The Chomsky hierarchy consists of the following levels:

Type-0 grammars (unrestricted grammars) include all formal grammars. They generate exactly all languages that can be recognized by a Turing machine. These languages are also known as the recursively enumerable languages. Note that this is different from the recursive languages which can be decided by an always-halting Turing machine.
Type-1 grammars (context-sensitive grammars) generate the context-sensitive languages. These grammars have rules of the form $\alpha A\beta \rightarrow \alpha\gamma\beta$ with $A$ a nonterminal and $\alpha$ , $\beta$ and $\gamma$ strings of terminals and nonterminals. The strings $\alpha$ and $\beta$ may be empty, but $\gamma$ must be nonempty. The rule $S \rightarrow \epsilon$ is allowed if $S$ does not appear on the right side of any rule. The languages described by these grammars are exactly all languages that can be recognized by a linear bounded automaton (a nondeterministic Turing machine whose tape is bounded by a constant times the length of the input.)
Type-2 grammars (context-free grammars) generate the context-free languages. These are defined by rules of the form $A \rightarrow \gamma$ with $A$ a nonterminal and $\gamma$ a string of terminals and nonterminals. These languages are exactly all languages that can be recognized by a non-deterministic pushdown automaton. Context-free languages are the theoretical basis for the syntax of most programming languages.
Type-3 grammars (regular grammars) generate the regular languages. Such a grammar restricts its rules to a single nonterminal on the left-hand side and a right-hand side consisting of a single terminal, possibly followed (or preceded, but not both in the same grammar) by a single nonterminal. The rule $S \rightarrow \epsilon$ is also allowed here if $S$ does not appear on the right side of any rule. These languages are exactly all languages that can be decided by a finite state automaton. Additionally, this family of formal languages can be obtained by regular expressions. Regular languages are commonly used to define search patterns and the lexical structure of programming languages.

Note that the set of grammars corresponding to recursive languages is not a member of this hierarchy.

Every regular language is context-free, every context-free language, not containing the empty string, is context-sensitive and every context-sensitive language is recursive and every recursive language is recursively enumerable. These are all proper inclusions, meaning that there exist recursively enumerable languages which are not context-sensitive, context-sensitive languages which are not context-free and context-free languages which are not regular.

The following table summarizes each of Chomsky's four types of grammars, the class of language it generates, the type of automaton that recognizes it, and the form its rules must have.

Grammar	Languages	Automaton	Production rules (constraints)
Type-0	Recursively enumerable	Turing machine	$\alpha \rightarrow \beta$ (no restrictions)
Type-1	Context-sensitive	Linear-bounded non-deterministic Turing machine	$\alpha A\beta \rightarrow \alpha\gamma\beta$
Type-2	Context-free	Non-deterministic pushdown automaton	$A \rightarrow \gamma$
Type-3	Regular	Finite state automaton	$A \rightarrow a$ and $A \rightarrow aB$

However, there are further categories of formal languages, some of which are given in the following table:

Automata theory: formal languages and formal grammars

Chomsky hierarchy

Type-0

—

Type-1

—

—

Type-2

—

—

Type-3

—

Grammars

Unrestricted

(no common name)

Context-sensitive

Indexed

Tree-adjoining etc.

Context-free

Deterministic context-free

Visibly pushdown

Regular

—

Languages

Recursively enumerable

Recursive

Context-sensitive

Indexed

Mildly context-sensitive

Context-free

Deterministic context-free

Visibly pushdown

Regular

Star-free

Minimal automaton

Turing machine

Decider

Linear-bounded

Nested stack

Embedded pushdown

Nondeterministic pushdown

Deterministic pushdown

Visibly pushdown

Finite

Counter-free (with aperiodic finite monoid)

Each category of languages is a proper subset of the category directly above it. - Any automaton and any grammar in each category has an equivalent automaton or grammar in the category directly above it.

References

↑ Chomsky, Noam (1956). "Three models for the description of language". IRE Transactions on Information Theory (2): 113–124. http://www.chomsky.info/articles/195609--.pdf.

Chomsky, Noam (1959). "On certain formal properties of grammars". Information and Control 2 (2): 137–167. doi:10.1016/S0019-9958(59)90362-6. http://www.diku.dk/hjemmesider/ansatte/henglein/papers/chomsky1959.pdf.
Chomsky, Noam; Schützenberger, Marcel P. (1963). "The algebraic theory of context free languages". In Braffort, P.; Hirschberg, D.. Computer Programming and Formal Languages. Amsterdam: North Holland. pp. 118–161.
Davis, Martin E.; Sigal, Ron; Weyuker, Elaine J. (1994). Computability, complexity, and languages: Fundamentals of theoretical computer science. Boston: Academic Press, Harcourt, Brace. pp. 327. ISBN 0122063821.

External links

Noam Chomsky

Political views · Bibliography · Criticism · Chomsky hierarchy · "Colorless green ideas sleep furiously"

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bibliography

Linguistics	Syntactic Structures (1957) · Current issues in linguistic theory (1964) · Aspects of the Theory of Syntax (1965) · Cartesian Linguistics: A Chapter in the History of Rationalist Thought (1966) · The Sound Pattern of English (1968) · Conditions on Transformations (1973) · The Logical Structure of Linguistic Theory (1975) · Lectures on Government and Binding: The Pisa Lectures (1981) · Knowledge of language: its nature, origin, and use (1986) · The Minimalist Program (1995) · New Horizons in the Study of Language and Mind (2000)

Politics	The Responsibility of Intellectuals (1967) · American Power and the New Mandarins (1969) · The Soviet Union Versus Socialism (1986) · Manufacturing Consent: The Political Economy of the Mass Media * (1988) · Necessary Illusions (1989) · Deterring Democracy (1992) · Hegemony or Survival: America's Quest for Global Dominance (2003) · Objectivity and Liberal Scholarship (2003) · Failed States: The Abuse of Power and the Assault on Democracy (2006)

Interviews	Class Warfare (1996) · Imperial Ambitions (2005)

Filmography

Manufacturing Consent: Noam Chomsky and the Media (1992) · Last Party 2000 (2001) · Power and Terror: Noam Chomsky in Our Times (2002) · Distorted Morality – America's War On Terror? (2003) · Noam Chomsky: Rebel Without a Pause (2003) (TV) · Peace, Propaganda & the Promised Land (2004)

Carol Chomsky(1930-2008) · Aviva Chomsky(1957-)

* with Edward S. Herman

Chomsky hierarchy

Contents

Formal grammars

The hierarchy

See also

References

External links