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Text of the page (random words):
mputer languages are based on sequences of characters while visual programming languages are based on the spatial layout and connections between symbols which may be textual or graphical documents that are syntactically invalid are said to have a syntax error when designing the syntax of a language a designer might start by writing down examples of both legal and illegal strings before trying to figure out the general rules from these examples 2 syntax therefore refers to the form of the code and is contrasted with semantics the meaning in processing computer languages semantic processing generally comes after syntactic processing however in some cases semantic processing is necessary for complete syntactic analysis and these are done together or concurrently in a compiler the syntactic analysis comprises the frontend while the semantic analysis comprises the backend and middle end if this phase is distinguished contents 1 levels of syntax 1 1 examples of errors 2 syntax definition 2 1 example lisp s expressions 2 2 complex grammars 3 syntax versus semantics 4 see also 5 references 6 external links levels of syntax edit computer language syntax is generally distinguished into three levels words the lexical level determining how characters form tokens phrases the grammar level narrowly speaking determining how tokens form phrases context determining what objects or variables names refer to if types are valid etc 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 of tokens this is known as lexical analysis or lexing second the parser turns the linear sequence of tokens into a hierarchical syntax tree this is known as parsing narrowly 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 hack in c is because tokenization depends on context even in these cases syntactical analysis is often seen as approximating this ideal model the parsing stage itself can be divided into two parts the parse tree or concrete syntax tree which is determined by the grammar but is generally far too detailed for practical use and the abstract 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 the levels generally correspond to levels in the chomsky hierarchy words are in a regular language specified in the lexical grammar which is a type 3 grammar generally given as regular expressions phrases are in a context free language cfl generally a deterministic context free language dcfl specified in a phrase structure grammar which is a type 2 grammar generally given as production rules in backus naur form bnf phrase grammars are often specified in much more constrained grammars than full context free grammars in order to make them easier to parse while the lr parser can parse any dcfl in linear time the simple lalr parser and even simpler ll parser are more efficient but can only parse grammars whose production rules are constrained in principle contextual structure can be described by a context sensitive grammar and automatically analyzed by means such as attribute grammars though in general this step is done manually via name resolution rules and type checking and implemented via a symbol table which stores names and types for each scope 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 use declarative programming rather than need to have procedural or functional programming a notable example is the lex yacc pair these automatically produce a concrete syntax tree the parser writer must then manually write code describing how this is converted to an abstract syntax 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 examples of errors edit main article syntax error 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 _ 1 1 lexical error _ is not valid add 1 1 parsing error missing closing 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 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 as semantic errors instead 3 4 5 as an example the python code a 1 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 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 a b 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 a static semantic error type errors which can only be detected at program execution time are always regarded as semantic rather than syntax errors syntax definition edit parse tree of python code with inset tokenization the syntax of textual programming languages is usually defined using a combination of regular expressions for lexical structure and backus naur form for grammatical structure to inductively specify syntactic categories nonterminals and terminal symbols syntactic categories are defined by rules called productions which specify the values that belong to a particular syntactic category 1 terminal symbols are the concrete characters or strings of characters for example keywords such as define if let or void from which syntactically valid programs are constructed 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 example lisp s expressions edit below is a simple grammar defined using the notation of regular expressions and extended backus naur form it describes the syntax of s expressions a data syntax of the programming language lisp which defines productions for the syntactic categories expression atom number symbol and list expression atom list atom number symbol number 0 9 symbol a z a z 0 9 list expression this grammar specifies the following an expression is either an atom or a list an atom is either a number or a symbol a number is an unbroken sequence of one or more decimal digits optionally preceded by a plus or minus sign a symbol is a letter followed by zero or more of any characters excluding whitespace and a list is a matched pair of parentheses with zero or more expressions inside it here the decimal digits upper and lower case characters and parentheses are terminal symbols the following are examples of well formed token sequences in this grammar 12345 a b c232 1 complex grammars edit the grammar needed to specify a programming language can be classified by its position in the chomsky hierarchy the phrase grammar of most programming languages can be specified using a type 2 grammar i e they are context free grammars 6 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 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 an undecidable problem in these languages meaning that the parsing phase may not finish for example in perl it is possible to execute code during parsing using a begin statement and perl function prototypes may alter the syntactic interpretation and possibly even the syntactic validity of the remaining code 7 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 lisp macros introduced by the defmacro syntax 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 8 9 syntax versus semantics edit 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 either formal or hard coded in a reference implementation not all syntactically correct programs are semantically correct many syntactically correct programs are nonetheless ill formed per the language s 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 exhibit undefined 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 using natural language as an example it may not be possible to assign a meaning to a grammatically correct sentence or the sentence may be false colorless green ideas sleep furiously is grammatically well formed but has no generally accepted meaning john is a married bachelor is grammatically well formed but expresses a meaning that cannot be true the following c language fragment is syntactically correct but performs an operation that is not semantically defined because p is a null pointer the operations p real and p im have no meaning complex p null complex abs_p sqrt p real p real p im p im as a simpler example int x printf d x is syntactically valid but not semantically defined as it uses an uninitialized 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 as semantic errors rather than syntax errors 5 10 see also edit naming convention programming comparison of programming languages syntax to quickly compare syntax of various programming languages take a look at the list of hello world program examples prolog syntax and semantics perl syntax php syntax and semantics c syntax c syntax java syntax javascript syntax python syntax and semantics lua syntax haskell syntax references edit a b friedman daniel p mitchell wand christopher t haynes 1992 essentials of programming languages 1st ed the mit press isbn 0 262 06145 7 smith dennis 1999 designing maintainable software springer science business media aho alfred v monica s lam ravi sethi jeffrey d ullman 2007 compilers principles techniques and tools 2nd ed addison wesley isbn 0 321 48681 1 section 4 1 3 syntax error handling pp 194 195 louden kenneth c 1997 compiler construction principles and practice brooks cole isbn 981 243 694 4 exercise 1 3 pp 27 28 a b semantic errors in java michael sipser 1997 introduction to the theory of computation pws publishing isbn 0 534 94728 x section 2 2 pushdown automata pp 101 114 the following discussions give examples perl and undecidability ltu comment clarifying that the undecidable problem is membership in the class of perl programs chromatic s example of perl code that gives a syntax error depending on the value of random variable an introduction to common lisp macros apl jhu edu 1996 02 08 archived from the original on 2013 08 06 retrieved 2013 08 17 the common lisp cookbook macros and backquote cl cookbook sourceforge net 2007 01 16 retrieved 2013 08 17 issue of syntax or semantics external links edit various syntactic constructs used in computer programming languages python error importerror no module named why how command line solved2021 retrieved from https en wikipedia org w index php title syntax_ programming_languages oldid 1115820870 categories programming language syntax programming language topics source code hidden categories articles with short description short description is different from wikidata articles needing additional references from august 2013 all articles needing additional references navigation menu personal tools not logged in talk contributions create account log in namespaces article talk english views read edit view history more navigation main page contents current events random article about wikipedia contact us donate contribute help learn to edit community portal recent changes upload file tools what links here related changes upload file special pages permanent link page information cite this page 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