Below is a simple grammar, defined using the notation of regular expressions andExtended BackusNaur form. It describes the syntax ofS-expressions, a data syntax of the programming languageLisp, which defines productions for the syntactic categoriesexpression,atom,number,symbol, andlist:
. PWS Publishing.ISBN0-534-94728-X.
Usingnatural languageas an example, it may not be possible to assign a meaning to a grammatically correct sentence or the sentence may be false:
The Common Lisp Cookbook – Macros and Backquote. . 2007-01-16
The following discussions give examples:
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John is a married bachelor. is grammatically well formed but expresses a meaning that cannot be true.
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Syntax the form is contrasted withsemantics the meaning. In processing computer languages, semantic processing generally comes after syntactic processing, but in some cases semantic processing is necessary for complete syntactic analysis, and these are done together orconcurrently. In acompiler, the syntactic analysis comprises thefrontend, while semantic analysis comprises thebackend(and middle end, if this phase is distinguished).
Note that the lexer is unable to identify the first error all it knows is that, after producing the token LEFT_PAREN, ( the remainder of the program is invalid, since no word rule begins with _. The second error is detected at the parsing stage: The parser has identified the list production rule due to the ( token (as the only match), and thus can give an error message; in general it may be ambiguous.
Context determining what objects or variables names refer to, if types are valid, etc.
The levels generally correspond to levels in theChomsky hierarchy. Words are in aregular language, specified in thelexical grammar, which is a Type-3 grammar, generally given asregular expressions. Phrases are in acontext-free language(CFL), generally adeterministic context-free language(DCFL), specified in aphrase structure grammar, which is a Type-2 grammar, generally given asproduction rulesinBackusNaur form(BNF). Phrase grammars are often specified in much more constrained grammars than fullcontext-free grammars, in order to make them easier to parse; while theLR parsercan parse any DCFL in linear time, the simpleLALR parserand even simplerLL parserare more efficient, but can only parse grammars whose production rules are constrained. In principle, contextual structure can be described by acontext-sensitive grammar, and automatically analyzed by means such asattribute grammars, though in general this step is done manually, vianame resolutionrules andtype checking, and implemented via asymbol tablewhich stores names and types for each scope.
Distinguishing in this way yields modularity, allowing each level to be described and processed separately, and often independently. First a lexer turns the linear sequence of characters into a linear sequence oftokens; this is known aslexical analysisor lexing. Second the parser turns the linear sequence of tokens into a hierarchical syntax tree; this is known asparsingnarrowly speaking. Thirdly the contextual analysis resolves names and checks types. This modularity is sometimes possible, but in many real-world languages an earlier step depends on a later step for example,the lexer hackin C is because tokenization depends on context. Even in these cases, syntactical analysis is often seen as approximating this ideal model.
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The syntax of a language describes the form of a valid program, but does not provide any information about the meaning of the program or the results of executing that program. The meaning given to a combination of symbols is handled by semantics (eitherformalor hard-coded in areference implementation). Not all syntactically correct programs are semantically correct. Many syntactically correct programs are nonetheless ill-formed, per the languages rules; and may (depending on the language specification and the soundness of the implementation) result in an error on translation or execution. In some cases, such programs may exhibitundefined behavior. Even when a program is well-defined within a language, it may still have a meaning that is not intended by the person who wrote it.
The following are examples of well-formed token sequences in this grammar:12345,(),(a b c232 (1))
is a matched pair of parentheses, with zero or more
Various syntactic constructs used incomputer programming languages
A language can have different equivalent grammars, such as equivalent regular expressions (at the lexical levels), or different phrase rules which generate the same language. Using a broader category of grammars, such as LR grammars, can allow shorter or simpler grammars compared with more restricted categories, such as LL grammar, which may require longer grammars with more rules. Different but equivalent phrase grammars yield different parse trees, though the underlying language (set of valid documents) is the same.
An Introduction to Common Lisp Macros. Apl.jhu.edu. 1996-02-08
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Section 2.2: Pushdown Automata, pp.101114.
Introduction to the Theory of Computation
Computer language syntax is generally distinguished into three levels:
The following C language fragment is syntactically correct, but performs an operation that is not semantically defined (becausepis anull pointer, the operationsp-realandp-imhave no meaning):
Here the decimal digits, upper- and lower-case characters, and parentheses are terminal symbols.
Compilers: Principles, Techniques, and Tools
Section 4.1.3: Syntax Error Handling, pp.194195.
Colorless green ideas sleep furiously. is grammatically well formed but has no generally accepted meaning.
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chromatics example of Perl code that gives a syntax error depending on the value of random variable
is syntactically valid at the phrase level, but the correctness of the types of a and b can only be determined at runtime, as variables do not have types in Python, only values do. Whereas there is disagreement about whether a type error detected by the compiler should be called a syntax error (rather than astatic semanticerror), type errors which can only be detected at program execution time are always regarded as semantic rather than syntax errors.
The syntax of textual programming languages is usually defined using a combination ofregular expressions(forlexicalstructure) andBackusNaur form(forgrammaticalstructure) to inductively specifysyntactic categories(nonterminals) andterminalsymbols. Syntactic categories are defined by rules calledproductions, which specify the values that belong to a particular syntactic category.Terminal symbols are the concrete characters or strings of characters (for examplekeywordssuch asdefine,if,let, orvoid) from which syntactically valid programs are constructed.
In a dynamically typed language, where type can only be determined at runtime, many type errors can only be detected at runtime. For example, the Python code
Type errors and undeclared variable errors are sometimes considered to be syntax errors when they are detected at compile-time (which is usually the case when compiling strongly-typed languages), though it is common to classify these kinds of error assemanticerrors instead.
Aho, Alfred V.; Monica S. Lam; Ravi Sethi; Jeffrey D. Ullman (2007).
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As an example,(add 1 1)is a syntactically valid Lisp program (assuming the add function exists, else name resolution fails), adding 1 and 1. However, the following are invalid:
Compiler Construction: Principles and Practice
is syntactically valid, but not semantically defined, as it uses anuninitialized variable. Even though compilers for some programming languages (e.g., Java and C) would detect uninitialized variable errors of this kind, they should be regarded assemanticerrors rather than syntax errors.
The parsing stage itself can be divided into two parts: theparse treeor concrete syntax tree which is determined by the grammar, but is generally far too detailed for practical use, and theabstract syntax tree(AST), which simplifies this into a usable form. The AST and contextual analysis steps can be considered a form of semantic analysis, as they are adding meaning and interpretation to the syntax, or alternatively as informal, manual implementations of syntactical rules that would be difficult or awkward to describe or implement formally.
In some languages like Perl and Lisp the specification (or implementation) of the language allows constructs that execute during the parsing phase. Furthermore, these languages have constructs that allow the programmer to alter the behavior of the parser. This combination effectively blurs the distinction between parsing and execution, and makes syntax analysis anundecidable problemin these languages, meaning that the parsing phase may not finish. For example, in Perl it is possible to execute code during parsing using aBEGINstatement, and Perl function prototypes may alter the syntactic interpretation, and possibly even the syntactic validity of the remaining code.Colloquially this is referred to as only Perl can parse Perl (because code must be executed during parsing, and can modify the grammar), or more strongly even Perl cannot parse Perl (because it is undecidable). Similarly, Lispmacrosintroduced by thedefmacrosyntax also execute during parsing, meaning that a Lisp compiler must have an entire Lisp run-time system present. In contrast, C macros are merely string replacements, and do not require code execution.
Essentials of Programming Languages
The grammar needed to specify a programming language can be classified by its position in theChomsky hierarchy. The phrase grammar of most programming languages can be specified using a Type-2 grammar, i.e., they arecontext-free grammars,though the overall syntax is context-sensitive (due to variable declarations and nested scopes), hence Type-1. However, there are exceptions, and for some languages the phrase grammar is Type-0 (Turing-complete).
LtU comment clarifying that the undecidable problem is membership in the class of Perl programs
Words the lexical level, determining how characters form tokens;
Phrases the grammar level, narrowly speaking, determining how tokens form phrases;
To quickly compare syntax of various programming languages, take a look at the list ofHello, World! programexamples:
Incomputer science, thesyntaxof acomputer languageis the set of rules that defines the combinations of symbols that are considered to be a correctly structured document or fragment in that language. This applies both toprogramming languages, where the document representssource code, andmarkup languages, where the document represents data. The syntax of a language defines its surface form.Text-based computer languages are based on sequences of characters, whilevisual programming languagesare based on the spatial layout and connections between symbols (which may be textual or graphical). Documents that are syntactically invalid are said to have asyntax error.
is a letter followed by zero or more of any characters (excluding whitespace); and
is an unbroken sequence of one or more decimal digits, optionally preceded by a plus or minus sign;
contains a type error because it adds a string literal to an integer literal. Type errors of this kind can be detected at compile-time: They can be detected during parsing (phrase analysis) if the compiler uses separate rules that allow integerLiteral + integerLiteral but not stringLiteral + integerLiteral, though it is more likely that the compiler will use a parsing rule that allows all expressions of the form LiteralOrIdentifier + LiteralOrIdentifier and then the error will be detected during contextual analysis (when type checking occurs). In some cases this validation is not done by the compiler, and these errors are only detected at runtime.
Friedman, Daniel P.; Mitchell Wand; Christopher T. Haynes (1992).
Syntax highlightingandindent styleare often used to aid programmers in recognizing elements of source code. Color coded highlighting is used in this piece of code written inPython.
This grammar specifies the following:
(1st ed.). The MIT Press.ISBN0-262-06145-7.
Tools have been written that automatically generate a lexer from a lexical specification written in regular expressions and a parser from the phrase grammar written in BNF: this allows one to usedeclarative programming, rather than need to have procedural or functional programming. A notable example is thelexyaccpair. These automatically produce aconcretesyntax tree; the parser writer must then manually write code describing how this is converted to anabstractsyntax tree. Contextual analysis is also generally implemented manually. Despite the existence of these automatic tools, parsing is often implemented manually, for various reasons perhaps the phrase structure is not context-free, or an alternative implementation improves performance or error-reporting, or allows the grammar to be changed more easily. Parsers are often written in functional languages, such as Haskell, or in scripting languages, such as Python or Perl, or in C or C++.