Syntax and Semantics - Methods

Syntax and Semantics

Different Methods to describe syntax and semantics

Syntax

• Lexical Structure of Programming Languages

• Context-Free Grammars and BNFs

• Parse Trees and Abstract Syntax Trees

• Ambiguity, Associativity, and Precedence

• EBNFs and Syntax Diagrams

• Parsing Techniques and Tools

• Lexics versus Syntax versus Semantics

Lexical Structure of Programming Languages

• Tokens – are words which comprise a programming language

• Lexical structure - structure of words/tokens

• Scanning phase – collects sequences of characters from the input program into tokens

• Parsing phase – processes the tokens syntactic structure


• Categories of Tokens:

• Reserved words or keywords

• Literals or constants

• Special symbols

• Identifiers


• The format of a program can affect the way tokens are recognized.

• Certain tokens are separated by token delimiters or white space

• Indentation can also be used

• Free-format language – one in which format has no effect on the program structure

• Fixed format – all tokens must occur in pre-specified locations on the page


• Tokens in programming language are often described in English, but they can also be described formally by regular expressions (descriptions of patterns of characters).

• 3 basic operations: - concatenation (sequencing the items without an explicit operation)

- repetition (‘*’)

- choice or selection (‘|’)

• Parentheses are also often included to allow for the grouping of operations

• Square brackets with a hyphen indicate a range of characters

• ‘+’ indicates one or more repetitions

• ‘?’ indicates an optional item

• ‘.’ indicates any character

Context-Free Grammars and BNFs

• Context-Free Grammars – consists of a series of grammar rules:

• Rules consist of a left-hand side that is a single structure name

• Then the metasymbol “”

• Followed by the right-hand side consisting of a sequence of items that can be symbols or other structure names

• Nonterminals – names of structures, broken down into further structures

• Terminals – words or token symbols, never broken down

• Grammar rules = production – they “produce” the strings of the language using derivations


(1)sentence noun-phrase verb-phrase .

(2)noun-phrase article noun

(3)article a | the

(4)noun girl | dog

(5)verb-phrase verb noun-phrase

(6)verb sees | pets


• “” – “consists of” or “is the same as”; metasymbol which separates the left-hand side from the right-hand side of a rule

• The italics serve to distinguish the names of the structures from the actual words or tokens that may appear in the language

• “|” – also a metasymbol; “or”

• Other metasymbols: “::=“, angle brackets, double quotes


• BNF notation – ISO standard format for notation conventional conventions in describing syntax of programming languages

• Start symbol (left-hand side) and derivation (right-hand side, foregoing rules)


(1)sentence noun-phrase verb-phrase .

(2)noun-phrase article noun

(3)article a | the

(4)noun girl | dog

(5)verb-phrase verb noun-phrase

(6)verb sees | pets

Parse Trees and [Abstract] Syntax Trees

• Parse tree - describes graphically the replacement process in a derivation.

• “the girl sees a dog”

• “234”

• “3 + 4 * 5”

Parse Trees and Abstract Syntax Trees

• A parse tree is labelled by nonterminals at interior nodes and terminals at leaves.

• The structure of the parse tree is completely specified by the grammar rules of the language and a derivation of particular sequence of terminals

• Abstract syntax trees – do away with terminals that are redundant once the structure of the tree is determined.

Ambiguity, Associativity, and Precedence

• Two different derivations can lead to the same parse tree or syntax tree

• Different derivations can also lead to difference parse trees

• Ambiguity – present difficulties since no clear structure; addressed/prevented by special derivations

• Leftmost derivation – where the leftmost remaining nonterminal is singled out for replacement at each step.

• Disambiguity rule/ precedence

• Right- or left- associative

EBNFs and Syntax Diagrams

• Extended Backus-Naur Form – special notation which expresses more clearly the repetitive nature of their structures

• “{ }” – stand for zero or more repetitions

• “[ ]” – indicate optional parts of the structure

EBNFs and Syntax Diagrams

• Syntax diagrams – which indicates the sequence of terminals and nonterminals encountered in the right-hand side of the rule

• Use circles or ovals for terminals and squares or rectangles for nonterminals, connecting them with lines and arrows to indicate appropriate sequencing

Parsing Techniques and Tools

• Grammar explicitly describes the strings of tokens that are syntactically legal in a programming language

• Grammar implicitly describes the actions that a parser must take to parse a string of tokens correctly

• Recognizer - simplest form of parser; program that accepts or rejects strings, based on whether they are legal strings in the language; build parse trees

Parsing Techniques and Tools

• Bottom-up parsers – when a match occurs, the right-hand side is replaced by or reduced to the nonterminal on the left; construct derivations and parse trees from the leaves to the root; shift-reduce parsers

• Top-down parsers – nonterminals are expanded to match incoming tokens and directly construct a derivation

• Recursive-descent parsers – operates by turning the nonterminals into a group of mutually recursive procedures whose actions are based on the right-hand sides

Lexics versus Syntax versus Semantics

Next Topic: Semantics

• Axiomatic Semantics

• Denotational Semantics

• Translation Semantics

• Algebraic Semantics

• Operational Semantics

Syntax and Semantics - Methods

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