LR(1) grammars The Chinese University of Hong Kong Fall 2011 CSCI 3130: Formal languages and automata theory LR(1) grammars Andrej Bogdanov http://www.cse.cuhk.edu.hk/~andrejb/csc3130

LR(0) parsing review A  a•Ab A  a•b A  •aAb A  •ab A  aA•b A  aAb• A  ab• A b a A •ab 1 2 3 5 4 A  aAb A  ab parser generator CFG G “PDA” for parsing G error if G is not LR(0) Motivation: Fast parsing for programming languages

Parsing computer programs if (n == 0) { return x; } else { return x + 1; } Statement Block ... else Statement if ParExpression Statement ( Expression ) Block ... ... CFGs of programming languages are not LR(0)

LR(0) parsing review A  aAb | ab A a b A a A b A  a•Ab A  a•b 2 3 4 A b A  a•Ab A  a•b A  •aAb A  •ab A  aA•b A  aAb• 1 A  •aAb A •ab a 5 A  ab• b stack state action  1 S A  aAb | ab 1 2 S A • • 12 2 S a b A 122 5 R • • • • 12 3 S • • • 123 4 R

Meaning of LR(0) items PDA transitions: A  aX•b X  •g A  a•Xb A undiscovered part move past subtree rooted at X a • X b focus X  •g A  a•Xb shift focus to subtree rooted at X

Outline of LR(0) parsing algorithm LR(0) parser has two kinds of actions: What if: no complete item is valid shift (S) there is one valid item, and it is complete reduce (R) some valid items complete, some not S / R conflict more than one valid complete item R / R conflict

Hierarchy of context-free grammars LR(1) grammars allow some conflicts conflicts can be resolved by lookahead LR(0) grammars

A CFG that is not LR(0) input: valid LR(0) items: S  A | Bc A  aA | a B  a | ab input: a valid LR(0) items: update S  •A , S  •Bc, A  •aA, A  •a, B  •a, B  •ab

A CFG that is not LR(0) input: valid LR(0) items: S/R, R/R conflicts! S  A | Bc A  aA | a B  a | ab input: a peek inside! valid LR(0) items: A  a•A, A  a• B  a•, B  a•b, A  •aA, A  •a B S c a • b … A S/R, R/R conflicts! possible parse trees

Lookahead input: valid LR(0) items: action: shift S  A | Bc A  aA | a B  a | ab input: a peek inside! a B S c a • b … A valid LR(0) items: A  a•A, A  a• B  a•, B  a•b, A  •aA, A  •a action: shift possible parse trees

Lookahead input: valid LR(0) items: S/R conflict action: shift S  A | Bc A  aA | a B  a | ab input: a a peek inside! a … A a S • valid LR(0) items: A  a•A, A  a• A  •aA, A  •a S/R conflict possible parse trees action: shift

Lookahead input: valid LR(0) items: action: reduce S  A | Bc A  aA | a B  a | ab input: a a a e … A a S • valid LR(0) items: A  a•A, A  a• A  •aA, A  •a action: reduce possible parse trees

LR(0) items vs. LR(1) items A a b • LR(1) A a b • A  a•Ab [A  a•Ab, b] A  aAb | ab

LR(1) items A A a b x • a • b [A  a•b, x] [A  a•b, e]

Generating an LR(1) parser S  A | Bc A  aA | a B  a | ab NFA states are LR(1) items DFA with stack may have S/R, R/R conflicts In an LR(1) CFG conflicts can always be resolved with one symbol lookahead

NFA for LR(0) parsing e q0 S  •a For every LR(0) item S  •a X a, b: terminals A, B, C: variables a, b, d: mixed strings X: terminal or variable notation e q0 S  •a For every LR(0) item S  •a X A  •X A  X• For every LR(0) item A  •X e A  •C C  •d For every pair of LR(0) items A  •C, C  •d

NFA for LR(1) parsing e q0 [S  •a, e] For every item S  •a X a, b: terminals A, B, C: variables a, b, d: mixed strings X: terminal or variable notation q0 e [S  •a, e] For every item S  •a X [A  X•, x] [A  •X, x] For every LR(1) item [A  •X, x] e [C  •d, y] [A  •C, x] For every LR(1) item [A  a•Cb, x] and production C  d and every y in FIRST(bx)

Explaining the transitions • X b x a X • b x X [A  •X, x] [A  X•, x] C b A y a • C b x • d e [A  •C, x] [C  •d, y] y ∈ FIRST(bx)

FIRST sets FIRST(g) are all leftmost terminals in derivations g ⇒ ... [C  •d, y] [A  •C, x] S  A | cB A  aA | a B  a | ab For every y in FIRST(bx) a A S cA BA e g FIRST(g) A {a} {a} a • C b x {a, c} {c} {a} FIRST(g) are all leftmost terminals in derivations g ⇒ ... ∅

Example: Construct the NFA [S  A•, e] A S  A(1) | Bc(2) A  aA(3) | a(4) B  a(5) | ab(6) [A  •aA, e] e [A  •a, e] [S  •A, e] e . . . q0 [S  B•c, e] B [S  •Bc, e] e [B  •a, c] [B  •ab, c] e

Example: Construct the NFA S  A(1) | Bc(2) A  aA(3) | a(4) B  a(5) | ab(6) [S  A•, e] A a e A [S  •A, e] [A  •aA, e] [A  a•A, e] [A  aA•, e] e e a e [A  •a, e] [A  a•, e] q0 e c [S  B•c, e] [S  Bc•, e] B e a [S  •Bc, e] [B  •a, c] [B  a•, c] e a b [B  •ab, c] [B  a•b, c] [B  ab•, c]

Example: Convert NFA to DFA LEGEND S  A | Bc A  aA | a B  a | ab shift variable shift terminal reduce A 2 1 [A  a•A, e] [S  •A, e] 3 [S  •Bc, e] [A  •aA, e] [A  a•A, e] 4 [A  •a, e] [A  •aA, e] A [A  •aA, e] a a [A  aA•, e] [B  a•b, c] [A  •a, e] [A  •a, e] [B  •a, c] [A  a•, e] [A  a•, e] [B  a•, c] [B  •ab, c] a b A B 5 6 7 8 c [S  A•, e] [S  B•c, e] [S  Bc•, e] [B  ab•, c]

Example: Resolve conflicts by lookahead LEGEND S  A(1) | Bc(2) A  aA(3) | a(4) B  a(5) | ab(6) shift variable shift terminal reduce 2 next action 3 next action [A  a•A, e] [A  a•A, e] a shift a shift [A  •aA, e] [A  •aA, e] [A  •a, e] b shift [A  •a, e] b error [B  a•b, c] c reduce B [A  a•, e] c error [A  a•, e] e reduce A e reduce A [B  a•, c]

Example: Reconstruct the parse tree 1 2 stack state action [S  •A, e] [A  a•A, e] [S  •Bc, e]  1 S [A  •aA, e] [A  •aA, e] [A  •a, e] 1 2 S a A [A  •a, e] [B  a•b, c] [B  •a, c] 12 8 R [A  a•, e] [B  •ab, c] [B  a•, c] 1 6 S A 5 16 7 R a B [S  A•, e] 3  [A  a•A, e] 6 [S  B•c, e] b [A  •aA, e] A [A  •a, e] S c 7 [A  a•, e] B [S  Bc•, e] a A • a b c • 8 4 • • [A  aA•, e] [B  ab•, c]