lecture5-2x2

Make the nonterminal functions return semantic values

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Unformatted text preview: #5 3 Last modified: Mon Sep 17 17:06:09 2012 CS164: Lecture #5 4 Expression Recognizer with Actions • Can make the nonterminal functions return semantic values. • Assume lexer somehow supplies semantic values for tokens, if elist : ǫ | sexp elist { $$ = emptyList; } { $$ = cons($1, $2); } def elist (): if next() in [SYM, NUM, STRING, ’(’, ’\’’]: v1 = sexp(); v2 = elist(); return cons(v1,v2) else: return emptyList Last modified: Mon Sep 17 17:06:09 2012 CS164: L Grammar Problems I Grammar Problems II In a recursive-descent parser, what goes wrong here? p : e ’⊣’ e:t | e ’/’ t | e ’*’ t Well then: What goes wrong here? p : e ’⊣’ e:t | t ’/’ e | t ’*’ e { $$ = $1; } { $$ = makeTree(DIV, $1, $3); } { $$ = makeTree(MULT, $1, $3); } { $$ = $1; } { $$ = makeTree(DIV, $1, $3); } { $$ = makeTree(MULT, $1, $3); } If we choose the second of third alternative for e, we’ll get an infinite recursion. If we choose the first, we’ll miss ’/’ and ’*’ cases. No infinite recursion, but we still don’t know which right-hand side to choose for e. Last modified: Mon Sep 17 17:06:09 2012 Last modified: Mon Sep 17 17:06:09 2012 CS164: Lecture #5 5 FIRST and FOLLOW string. For example: : : : : 6 Using FIRST and FOLLOW • If α is any string of terminals and nonterminals (like the right side of a production) then FIRST(α) is the set of terminal symbols that start some string that α produces, plus ǫ if α can produce the empty p e s t CS164: Lecture #5 • In a r...
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