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Parsing #1

A parser recognizes sequences of tokens according to some grammar and generates Abstract Syntax Trees (ASTs). It uses context-free grammar and performs top-down parsing.

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Parsing #1

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  1. Leonidas Fegaras Parsing #1

  2. Parser • A parser recognizes sequences of tokens according to some grammar and generates Abstract Syntax Trees (ASTs)‏ • A context-free grammar (CFG) has • a finite set of terminals (tokens)‏ • a finite set of nonterminals from which one is the start symbol • and a finite set of productions of the form: A ::= X1 X2 ... Xn where A is a nonterminal and each Xi is either a terminal or nonterminal symbol get token get next character AST scanner parser source file token

  3. Example • Expressions: E ::= E + T | E - T | T T ::= T * F | T / F | F F ::= num | id • Nonterminals: E T F • Start symbol: E • Terminals: + - * / id num • Example: x+2*y • ... or equivalently: E ::= E + T E ::= E - T E ::= T T ::= T * F T ::= T / F T ::= F F ::= num F ::= id

  4. Derivations • Notation: • terminals: t, s, ... • nonterminals: A, B, ... • symbol (terminal or nonterminal): X, Y, ... • sequence of symbols: a, b, ... • Given a production: A ::= X1 X2 ... Xn the form aAb => aX1 X2 ... Xnb is called a derivation • eg, using the production T ::= T * F we get T / F + 1 - x => T * F / F + 1 - x • Leftmost derivation: when you always expand the leftmost nonterminal in the sequence • Rightmost derivation: ... rightmost nonterminal

  5. Top-down Parsing • It starts from the start symbol of the grammar and applies derivations until the entire input string is derived • Example that matches the input sequence id(x) + num(2) * id(y)‏ E => E + T use E ::= E + T => E + T * F use T ::= T * F => T + T * F use E ::= T => T + F * F use T ::= F => T + num * F use F ::= num => F + num * F use T ::= F => id + num * F use F ::= id => id + num * id use F ::= id • You may have more than one choice at each derivation step: • my have multiple nonterminals in each sequence • for each nonterminal in the sequence, may have many rules to choose from • Wrong predictions will cause backtracking • need predictive parsing that never backtracks

  6. Bottom-up Parsing • It starts from the input string and uses derivations in the opposite directions (from right to left) until you derive the start symbol • Previous example: id(x) + num(2) * id(y)‏ <= id(x) + num(2) * F use F ::= id <= id(x) + F * F use F ::= num <= id(x) + T * F use T ::= F <= id(x) + T use T ::= T * F <= F + T use F ::= id <= T + T use T ::= F <= E + T use E ::= T <= E use E ::= E + T • At each derivation step, need to recognize a handle (the sequence of symbols that matches the right-hand-side of a production)‏

  7. Parse Tree • Given the derivations used in the top-down/bottom-up parsing of an input sequence, a parse tree has • the start symbol as the root • the terminals of the input sequence as leafs • for each production A ::= X1 X2 ... Xn used in a derivation, a node A with children X1 X2 ... Xn E E T T T F F F id(x) + num(2) * id(y)‏ • E => E + T • => E + T * F • => T + T * F => T + F * F => T + num * F => F + num * F => id + num * F => id + num * id

  8. Playing with Associativity • What about this grammar? E ::= T + E | T - E | T T ::= F * T | F / T | F F ::= num | id • Right associative • Now x+y+z is equivalent to x+(y+z)‏ E T E F T E F T F id(x) + id(y) + id(z)‏

  9. Ambiguous Grammars • What about this grammar? E ::= E + E | E - E | E * E | E / E | num | id • Operators + - * / have the same precedence! • It is ambiguous: has more than one parse tree for the same input sequence (depending which derivations are applied each time)‏ E E E E E id(x) * id(y) + id(z)‏ E E E E E id(x) * id(y) + id(z)‏

  10. Predictive Parsing • The goal is to construct a top-down parser that never backtracks • Always leftmost derivations • left recursion is bad! • We must transform a grammar in two ways: • eliminate left recursion • perform left factoring • These rules eliminate most common causes for backtracking although they do not guarantee a completely backtrack-free parsing

  11. Left Recursion Elimination • For example, the grammar A ::= A a | b recognizes the regular expression ba*. • But a top-down parser may have hard time to decide which rule to use • Need to get rid of left recursion: A ::= b A' A' ::= a A' | ie, A' parses the RE a*. • The second rule is recursive, but not left recursive

  12. Left Recursion Elimination (cont.)‏ • For each nonterminal X, we partition the productions for X into two groups: • one that contains the left recursive productions • the other with the rest That is: X ::= X a1 ... X ::= X an where a and b are symbol sequences. • Then we eliminate the left recursion by rewriting these rules into: X ::= b1 X' ... X ::= bm X' X ::= b1 ... X ::= bm X' ::= a1 X' ... X' ::= an X' X' ::=

  13. Example E ::= T E' E' ::= + T E' | - T E' | T ::= F T' T' ::= * F T' | / F T' | F ::= num | id E ::= E + T | E - T | T T ::= T * F | T / F | F F ::= num | id

  14. Example • A grammar that recognizes regular expressions: R ::= R R | R bar R | R * | ( R )‏ | char • After left recursion elimination: R ::= ( R ) R' | char R' R' ::= R R' | bar R R' | * R' |

  15. Left Factoring • Factors out common prefixes: X ::= a b1 ... X ::= a bn • becomes: X ::= a X' X' ::= b1 ... X' ::= bn • Example: E ::= T + E | T - E | T E ::= T E' E' ::= + E | - E |

  16. Recursive Descent Parsing static void E () { T(); Eprime(); } static void Eprime () { if (current_token == PLUS)‏ { read_next_token(); T(); Eprime(); } else if (current_token == MINUS)‏ { read_next_token(); T(); Eprime(); }; } static void T () { F(); Tprime(); } static void Tprime() { if (current_token == TIMES)‏ { read_next_token(); F(); Tprime(); } else if (current_token == DIV)‏ { read_next_token(); F(); Tprime(); }; } static void F () { if (current_token == NUM || current_token == ID)‏ read_next_token(); else error(); } E ::= T E' E' ::= + T E' | - T E' | T ::= F T' T' ::= * F T' | / F T' | F ::= num | id

  17. Predictive Parsing Using a Table • The symbol sequence from a derivation is stored in a stack (first symbol on top)‏ • if the top of the stack is a terminal, it should match the current token from the input • if the top of the stack is a nonterminal X and the current input token is t, we get a rule for the parse table: M[X,t] • the rule is used as a derivation to replace X in the stack with the right-hand symbols push(S); read_next_token(); repeat X = pop(); if (X is a terminal or '$')‏ if (X == current_token)‏ read_next_token(); else error(); else if (M[X,current_token] == "X ::= Y1 Y2 ... Yk")‏ { push(Yk); ... push(Y1); } else error(); until X == '$';

  18. Parsing Table Example 1) E ::= T E' $ 2) E' ::= + T E' 3) | - T E' 4) | 5) T ::= F T' 6) T' ::= * F T' 7) | / F T' 8) | 9) F ::= num 10) | id num id + - * / $ E 1 1 E' 2 3 4 T 5 5 T' 8 8 6 7 8 F 9 10

  19. Example: Parsing x-2*y$ top Stack current_token Rule E x M[E,id] = 1 (using E ::= T E' $)‏ $ E' T x M[T,id] = 5 (using T ::= F T')‏ $ E' T' F x M[F,id] = 10 (using F ::= id)‏ $ E' T' id x read_next_token $ E' T' - M[T',-] = 8 (using T' ::= )‏ $ E' - M[E',-] = 3 (using E' ::= - T E')‏ $ E' T - - read_next_token $ E' T 2 M[T,num] = 5 (using T ::= F T')‏ $ E' T' F 2 M[F,num] = 9 (using F ::= num)‏ $ E' T' num 2 read_next_token $ E' T' * M[T',*] = 6 (using T' ::= * F T')‏ $ E' T' F * * read_next_token $ E' T' F y M[F,id] = 10 (using F ::= id)‏ $ E' T' id y read_next_token $ E' T' $ M[T',$] = 8 (using T' ::= )‏ $ E' $ M[E',$] = 4 (using E' ::= )‏ $ $ stop (accept)‏

  20. Constructing the Parsing Table • FIRST[a] is the set of terminals t that result after a number of derivations on the symbol sequence a • ie, a => ... => tb for some symbol sequence b • FIRST[ta]={t} eg, FIRST[3+E]={3} • FIRST[X]=FIRST[a1]  …  FIRST[an] for each production X ::= ai • FIRST[Xa]=FIRST[X] but if X has an empty derivation then FIRST[Xa]=FIRST[X]  FIRST[a] • FOLLOW[X] is the set of all terminals that follow X in any legal derivation • find all productions Z ::= a X b in which X appears at the RHS; then • FIRST[b] must be included in FOLLOW[X] • if b has an empty derivation, FOLLOW[Z] must be included in FOLLOW[X]

  21. Example 1) E ::= T E' $ 2) E' ::= + T E' 3) | - T E' 4) | 5) T ::= F T' 6) T' ::= * F T' 7) | / F T' 8) | 9) F ::= num 10) | id FIRST FOLLOW E {num,id} {} E' {+,-} {$} T {num,id} {+,-,$} T' {*,/} {+,-,$} F {num,id} {+,-,*,/,$}

  22. Constructing the Parsing Table (cont.)‏ • For each rule X ::= a do: • for each t in FIRST[a], add X ::= a to M[X,t] • if a can be reduced to the empty sequence, then for each t in FOLLOW[X], add X ::= a to M[X,t] FIRST FOLLOW E {num,id} {} E' {+,-} {$} T {num,id} {+,-,$} T' {*,/} {+,-,$} F {num,id} {+,-,*,/,$} 1) E ::= T E' $ 2) E' ::= + T E' 3) | - T E' 4) | 5) T ::= F T' 6) T' ::= * F T' 7) | / F T' 8) | 9) F ::= num 10) | id num id + - * / $ E 1 1 E' 2 3 4 T 5 5 T' 8 8 6 7 8 F 9 10

  23. Another Example 0) G := S $ 1) S ::= ( L )‏ 2) S ::= a 3) L ::= S L' 4) L' ::= , S L' 5) L' ::= G ::= S $ S ::= ( L )‏ | a L ::= L , S | S ( ) a , $ G 0 0 S 1 2 L 3 3 L' 5 4

  24. LL(1)‏ • A grammar is called LL(1) if each element of the parsing table of the grammar has at most one production element • the first L in LL(1) means that we read the input from left to right • the second L means that it uses left-most derivations only • the number 1 means that we need to look one token ahead from the input

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