Lec8 - CMPSC 461 Programming Language Concepts Lecture 8...

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CMPSC 461: Programming Language Concepts Lecture 8 Instructor: Prof. Swarat Chaudhuri
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A context-free grammar A context-free grammar for {a m b n : m > n} S X Y X a X | є Y a Y b | є
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A grammar for C-- Terminals: IF, WHILE, NUM, ID, REL Nonterminals: Comm Rules: AtomicStmt ::= IfStmt | WhileStmt | Assgn MS ::= AtomicStmt | MS AtomicStmt Assgn ::= ID ‘=‘ Exp ‘;’ Exp ::= NUM | ID | ‘(‘ Exp REL Exp ‘)’ IfStmt ::= IF ‘( Exp ‘) ‘{‘ MS ‘} WhileStmt ::= WHILE ‘( Exp ‘) ‘{‘ MS ‘}’
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But what about NUM, STRING, etc? We could of course define them using grammars. But we usually don’t as they are not abstract. That is, their encodings can vary. We assume that there is a lower-level specification for strings, numbers, etc. We will come back to this point of abstraction shortly.
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The top-down view When does an element belong to a set? When does a word belong to a formal language? When is a C command really a C command? Algorithmic question.
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