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Glossary

Backus-Naur Form (BNF)
Formal method of specifying context-free grammars. BNF was first used in the ALGOL-60 report, 1963. See Languages and Context-Free Grammars.
Context-free grammars
Grammars specified as rules that can be applied regardless of context. Thus, if there is a rule which says that an integer can be used as an expression, integers are allowed anywhere an expression is permitted. See Languages and Context-Free Grammars.
Dynamic allocation
Allocation of memory that occurs during execution, rather than at compile time or on entry to a function.
Empty string
Analogous to the empty set in set theory, the empty string is a character string of length zero.
Finite-state stack machine
A "machine" that has discrete states in which it is said to exist at each instant in time. As input to the machine is processed, the machine moves from state to state as specified by the logic of the machine. In the case of the parser, the input is the language being parsed, and the states correspond to various stages in the grammar rules. See The Bison Parser Algorithm.
Grouping
A language construct that is (in general) grammatically divisible; for example, `expression' or `declaration' in C. See Languages and Context-Free Grammars.
Infix operator
An arithmetic operator that is placed between the operands on which it performs some operation.
Input stream
A continuous flow of data between devices or programs.
Language construct
One of the typical usage schemas of the language. For example, one of the constructs of the C language is the if statement. See Languages and Context-Free Grammars.
Left associativity
Operators having left associativity are analyzed from left to right: a+b+c first computes a+b and then combines with c. See Operator Precedence.
Left recursion
A rule whose result symbol is also its first component symbol; for example, expseq1 : expseq1 ',' exp;. See Recursive Rules.
Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token from left to right. See The Bison Parser Algorithm.
Lexical analyzer (scanner)
A function that reads an input stream and returns tokens one by one. See The Lexical Analyzer Function yylex.
Lexical tie-in
A flag, set by actions in the grammar rules, which alters the way tokens are parsed. See Lexical Tie-ins.
Literal string token
A token which consists of two or more fixed characters. See Symbols.
Look-ahead token
A token already read but not yet shifted. See Look-Ahead Tokens.
LALR(1)
The class of context-free grammars that Bison (like most other parser generators) can handle; a subset of LR(1). See Mysterious Reduce/Reduce Conflicts.
LR(1)
The class of context-free grammars in which at most one token of look-ahead is needed to disambiguate the parsing of any piece of input.
Nonterminal symbol
A grammar symbol standing for a grammatical construct that can be expressed through rules in terms of smaller constructs; in other words, a construct that is not a token. See Symbols.
Parse error
An error encountered during parsing of an input stream due to invalid syntax. See Error Recovery.
Parser
A function that recognizes valid sentences of a language by analyzing the syntax structure of a set of tokens passed to it from a lexical analyzer.
Postfix operator
An arithmetic operator that is placed after the operands upon which it performs some operation.
Reduction
Replacing a string of nonterminals and/or terminals with a single nonterminal, according to a grammar rule. See The Bison Parser Algorithm.
Reentrant
A reentrant subprogram is a subprogram which can be in invoked any number of times in parallel, without interference between the various invocations. See A Pure (Reentrant) Parser.
Reverse polish notation
A language in which all operators are postfix operators.
Right recursion
A rule whose result symbol is also its last component symbol; for example, expseq1: exp ',' expseq1;. See Recursive Rules.
Semantics
In computer languages, the semantics are specified by the actions taken for each instance of the language, i.e., the meaning of each statement. See Defining Language Semantics.
Shift
A parser is said to shift when it makes the choice of analyzing further input from the stream rather than reducing immediately some already-recognized rule. See The Bison Parser Algorithm.
Single-character literal
A single character that is recognized and interpreted as is. See From Formal Rules to Bison Input.
Start symbol
The nonterminal symbol that stands for a complete valid utterance in the language being parsed. The start symbol is usually listed as the first nonterminal symbol in a language specification. See The Start-Symbol.
Symbol table
A data structure where symbol names and associated data are stored during parsing to allow for recognition and use of existing information in repeated uses of a symbol. See Multi-function Calc.
Token
A basic, grammatically indivisible unit of a language. The symbol that describes a token in the grammar is a terminal symbol. The input of the Bison parser is a stream of tokens which comes from the lexical analyzer. See Symbols.
Terminal symbol
A grammar symbol that has no rules in the grammar and therefore is grammatically indivisible. The piece of text it represents is a token. See Languages and Context-Free Grammars.