Copyright © 2006 The McGraw-Hill Companies, Inc. Programming Languages 2nd edition Tucker and Noonan Chapter 2 Syntax A language that is simple to parse for the compiler is also simple to parse for the human programmer. N. Wirth

Copyright © 2006 The McGraw-Hill Companies, Inc. Contents 2.1 Grammars Backus-Naur Form Derivations Parse Trees Associativity and Precedence Ambiguous Grammars 2.2 Extended BNF 2.3 Syntax of a Small Language: Clite Lexical Syntax Concrete Syntax 2.4 Compilers and Interpreters 2.5 Linking Syntax and Semantics Abstract Syntax Abstract Syntax Trees Abstract Syntax of Clite

Copyright © 2006 The McGraw-Hill Companies, Inc. Translation/Execution – Compiler review

Copyright © 2006 The McGraw-Hill Companies, Inc. Thinking about Syntax The syntax of a programming language is a precise description of all its grammatically correct programs. Grammar rules are a common technique for describing language syntax precisely. Precise syntax was first used with Algol 60. Three levels: –Lexical syntax –Concrete syntax –Abstract syntax

Copyright © 2006 The McGraw-Hill Companies, Inc. Levels of Syntax Lexical syntax: describes the basic symbols of the language (names, values, operators, etc.) Concrete syntax: rules for writing expressions, statements and programs. Abstract syntax: describes an internal representation of the program, emphasizes content over form. The authors define Clite, a mini-language, to use as a teaching tool in the study syntax and semantics.

Copyright © 2006 The McGraw-Hill Companies, Inc. 2.1 Grammars A metalanguage is a language used to define other languages. cf metaknowledge A grammar is a set of rules, written in a metalanguage, and used to define the syntax of a language.

Copyright © 2006 The McGraw-Hill Companies, Inc Backus-Naur Form (BNF) Notation for describing a context-free grammar (see Chomsky hierarchy) Sometimes called Backus Normal Form First used to define syntax of Algol 60 Now used to define syntax of most major languages

Copyright © 2006 The McGraw-Hill Companies, Inc. Elements of a Context-Free Grammar Set of productions: P terminal symbols: T nonterminal symbols: N start symbol: A production has the form and ω is a string from N and T.

Copyright © 2006 The McGraw-Hill Companies, Inc. Example: Binary Digits Consider the grammar: binaryDigit  0 binaryDigit  1 or equivalently: binaryDigit  0 | 1 Here, | and  are metacharacters (metasymbols)

Copyright © 2006 The McGraw-Hill Companies, Inc Derivations Consider the grammar: Integer  Digit | Integer Digit Digit  0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 We can derive any unsigned integer, like 352, from this grammar. (Derivations can (1) produce all legal integers or (2) show that a particular integer is correctly formed)

Copyright © 2006 The McGraw-Hill Companies, Inc. Derivation of 352 as an Integer A 6-step process, begins with the start symbol Step 1: Integer  Integer Digit Replace a nonterminal by a RHS of one of its rules: Step 1: Integer  Integer Digit Step 2:  Integer Digit Digit Step 3:  Digit Digit Digit Step 4:  3 Digit Digit Step 5:  3 5 Digit Step 6:  Finished when there are only terminals on the RHS

Copyright © 2006 The McGraw-Hill Companies, Inc. A Different Derivation of 352 Integer  Integer Digit  Integer 2  Integer Digit 2  Integer 5 2  Digit 5 2  This is called a rightmost derivation, since at each step the rightmost nonterminal is replaced. (The first one was a leftmost derivation.)

Copyright © 2006 The McGraw-Hill Companies, Inc. Notation for Derivations Integer  * 352 Means that 352 can be derived in a finite number of steps using the grammar for Integer. 352  L(G) Means that 352 is a member of the language defined by grammar G. Definition: the language L defined by a BNF grammar G is the set of all terminal strings that can be derived from the start symbol.

Copyright © 2006 The McGraw-Hill Companies, Inc Parse Trees A parse tree is a graphical representation of a derivation. The root node of the tree is the start symbol. Each internal node of the tree corresponds to a non-terminal The child(ren) of a node represent a right-hand side of a production for which the node is the left-hand side. Each leaf node represents a terminal symbol of the derived string, reading from left to right.

Copyright © 2006 The McGraw-Hill Companies, Inc. E.g., The step Integer  Integer Digit appears in the parse tree as: Integer Digit

Copyright © 2006 The McGraw-Hill Companies, Inc. Parse Tree for 352 as an Integer Figure 2.1

Copyright © 2006 The McGraw-Hill Companies, Inc. Arithmetic Expression Grammar The following grammar defines the language of arithmetic expressions with 1-digit integers, addition, and subtraction. Expr  Expr + Term | Expr – Term | Term Term  0 |... | 9 | ( Expr )

Copyright © 2006 The McGraw-Hill Companies, Inc. Parse of the String Figure 2.2

Copyright © 2006 The McGraw-Hill Companies, Inc Associativity and Precedence Grammars define associativity and precedence among the operators in an expression. Precedence: which operator is evaluated first; e.g., in the expression “a + b / c” Associativity: evaluation order for equal precedence (adjacent) operators; e.g., in the expression “a + b + c”

Copyright © 2006 The McGraw-Hill Companies, Inc. Grammar G 1: Consider the more interesting grammar G 1 : Expr  Expr + Term | Expr – Term | Term Term  Term * Factor | Term / Factor | Term % Factor | Factor Factor  Primary ** Factor | Primary Primary  0 |... | 9 | ( Expr )

Copyright © 2006 The McGraw-Hill Companies, Inc. Parse of 4**2**3+5*6+7 for Grammar G 1 Figure 2.3 Expr  Expr + Term |Expr – Term | Term Term  Term * Factor | Term / Factor | Term % Factor | Factor Factor  Primary ** Factor | Primary Primary  0 |... | 9 | ( Expr )

Copyright © 2006 The McGraw-Hill Companies, Inc. PrecedenceAssociativityOperators 3right ** 2left * / % 1left + - The structure of the parse tree shows operator precedence & associativity: Operators lower in the tree are evaluated first. An operation can’t be performed until its operands are evaluated Associativity and Precedence for Grammar G 1 Table 2.1

Copyright © 2006 The McGraw-Hill Companies, Inc. Precedence & Associativity in Grammars An operator’s precedence is determined by the length of the shortest derivation from the start symbol to the operator (see Figure 2.3) Left- or right- associativity is determined by left- or right- recursion. compare the operators ** and + in Figure 2.3

Copyright © 2006 The McGraw-Hill Companies, Inc Ambiguous Grammars A grammar is ambiguous if one of its strings has two or more different parse trees. C, C++, and Java have a large number of –operators and –precedence levels Instead of using a large grammar, we can: –Write a smaller ambiguous grammar, and –Give separate precedence and associativity rules (e.g., Table 2.1)

Copyright © 2006 The McGraw-Hill Companies, Inc. An Ambiguous Expression Grammar G 2 Expr -> Expr Op Expr | ( Expr ) | Integer Op -> + | - | * | / | % | ** Notes: –G 2 is equivalent to G 1. i.e., its language is the same. –G 2 has fewer productions and nonterminals than G 1. –However, G 2 is ambiguous.

Copyright © 2006 The McGraw-Hill Companies, Inc. Ambiguous Parse of Using Grammar G 2 Figure 2.4

Copyright © 2006 The McGraw-Hill Companies, Inc. The Dangling Else IfStatement → if ( Expression ) Statement | if ( Expression ) Statement else Statement where Statement → Assignment | IfStatement | Block Suppose one of the statements was another If?

Copyright © 2006 The McGraw-Hill Companies, Inc. Example With which ‘if’ does the following ‘else’ associate? if (x < 0) if (y < 0) y = y - 1; else y = 0; Answer: either one!