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Unformatted text preview: CS 421 Lecture 7: Grammars and parsing s Announcements s MP2 review s Lecture outline s Contextfree grammars s Topdown, a.k.a. recursive descent, parsing 6/14/2009 1 Announcements s TA office hours s I2CS: Tue, Thu 45pm CDT s Oncampus: Wed 45pm CDT s MP2 solutions posted 6/14/2009 2 MP2 review s Problem 7 flatten : ‘a list list > ‘a list flatten [[1;2;3]; [4;5]; [8;2;3;4]];; let rec flatten lst = match lst with … 6/14/2009 3 MP2 review s Problem 7 flatten : ‘a list list > ‘a list flatten [[1;2;3]; [4;5]; [8;2;3;4]];; let rec flatten lst = match lst with >  ::xs > flatten xs  (x::xs)::ys > x::(flatten (xs::ys));; 6/14/2009 4 Review: compiler frontend 6/14/2009 5 Tokens Source Lexer AST Parser Intro to grammars and languages s Grammar s Finite set of t e r m i n a l s s Finite set of n o n  t e r m i n a l s s Finite set of p r o d u c t i o n r u l e s s Start symbol s Language s Set of strings recognized by a grammar 6/14/2009 6 Grammars: Chomsky hierarchy s Unrestricted s Recursivelyenumerable languages s Recognized by a Turing machine s Contextsensitive s Contextsensitive languages s Recognized by a linear bounded automaton (LBA) s Contextfree s Contextfree languages s Recognized by a pushdown automaton (PDA) s Regular s Regular languages s Recognized by a finite state automaton (FSA) 6/14/2009 7 Contextfree grammar s Given: s Set of terminals (tokens) T s Set of nonterminals (variables) V s A cfg G is a set of p r o d u c t i o n s of the form s A → X 1 … X n ( n ≥ 0) where s A ∈ V , X 1 … X n ∈ G = V ∪ T s One symbol designated as “start symbol” 6/14/2009 8 Notation s A → X 1 … X n s Also written A ::= X 1 … X n s When n = 0, write A → ε s Instead of A → s When there is more than one production from A , say s A → X 1 … X n and A → Y 1 … Y n s Instead write: A → X 1 … X n  Y 1 … Y n 6/14/2009 9 Example s Expressions s Exp → intlit  variable  Exp + Exp  Exp * Exp s Sentences include s 3 s x s 3+x s 3+x*y s Tree representation 6/14/2009 10 Example s Method definition: MethodDef → Type ident ‘(’ Args ‘)’ ‘{‘ Stmtlist ‘}’ Args → ε  NonEmptyArgs NonEmptyArgs → Type ident  Type ident ‘,’ NonEmptyArgs Stmtlist → ε  Stmt Stmtlist Type → ident  int  boolean s Sentence: int fun(boolean b) { } s Tree representation s ??...
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 Summer '09
 KirillMechitov
 Formal grammar, Somé, Contextfree grammar, match parseC tls

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