[PDF] Syntax and Semantics

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5. Operator Precedence• Operators that appear lower in the parse tree are evaluated first, so we want to force our grammarsto push higher precedent operators lower down in the parse tree.• Example: increasing operator precedence(a) +, -(b) * /• Resulting grammar:6. Associativity ::= ::= ::= ::= ::= ’(’ + + ’)’ ::= num | id ::= ’+’ | ’-’ ::= ’*’ | ’/’• Are the parse trees for expressions with two or more adjacent occurrences of the same precedencecorrect?• Example3+4+5 = (3+4)+5 (left associative)3^4^5 = 3^(4^5) (right associative)• New grammar7. Dangling else ::= ::= ::= ::= ::= ’(’ ’)’ ::= num | id ::= ’+’ | ’-’ ::= ’*’ | ’/’• The following grammar is ambiguous ::= | whatever ::= if then else ::= if then • The unambiguous grammar. key observation: between a then and its matching else, there cannot be an if without an else.4
::= | ::= if then else | whatever ::= if then | if then else • In practice, the dangling-else ambiguity is intentionally left unresolved in the language’s grammar;explicitly fixing the grammar (as above) makes the grammar less readable. It is assumed thatthe programmer and language implementor is aware of the ambiguity and knows how resolvethe problem. A good example is the grammar listed in the appendix the famous Kernighan andRitchie book The C Programming Language.• Ada’s solution to the dangling-else problemSince it’s hard to construct an unambiguous grammar for if-then-else statements in C or Pascal,does this hint at a bad language design? Ada’s solution to the “dangling else” problem (seep. 122 in text) is to syntactically differentiate between “nested if statements” and “cascaded ifstatements” (note use of elsif).if Score < 50 then -- cascaded if-statement in Adagrade := ’F’;elsif Score < 60 thengrade := ’C’;elsif Score < 70 thengrade := ’B’;elsegrade := ’A’;endif;• Perl forces you to use block statements (i.e., curly braces) for if statements and uses an elsifkeyword for cascaded if statements:if ($score < 50) {$grade = ’F’;} elsif ($score < 60) {$grade = ’C’;...}8. Static semantics• Many of the rules about what makes a legal program can not be fully specified via BNF or otherCFG representations, e.g.:– In Java or C++, all variables must be declared before use.– The target of a goto statement in C must be matched with the label of the same name.• Many other rules that could be specified using BNF would make the grammar to large andcumbersome, e.g:– Typing rules in Cstruct {int i; float f} vara, varb;struct {int i; int j} varc;vara = varb; /* legal struct copy */vara = varc; /* illegal struct copy */• These types of rules that are/can not be specified by the grammar, but can be checked at compiletime (i.e. statically) are called static semantics.5
Page 1 and 2: Language Syntax and SemanticsCS 355
Page 3: • Note that rules can be “recur
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[PDF] Syntax and Semantics

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