# shift-reduce parsing

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1 Parsing #2

2 Bottom-up Parsing Rightmost derivations; use of rules from right to left Uses a stack to push symbols the concatenation of the stack symbols with the rest of the input forms a valid bottom-up derivation E - num x-2*y\$ stack input Derivation: E-num*id\$ Two operations: reduction: if a postfix of the stack matches the RHS of a rule (called a handle), replace the handle with the LHS nonterminal of the rule eg, reduce the stack x * E + E by the rule E ::= E + E new stack: x * E shifting: if no handle is found, push the current input token on the stack and read the next symbol Also known as shift-reduce parsing

3 0) S :: = E \$ 1) E ::= E + T 2) E ::= E - T 3) E ::= T 4) T ::= T * F 5) T ::= T / F 6) T ::= F 7) F ::= num 8) F ::= id Example Input: x-2*y\$ Stack rest-of-the-input Action 1) id - num * id \$ shift 2) id - num * id \$ reduce by rule 8 3) F - num * id \$ reduce by rule 6 4) T - num * id \$ reduce by rule 3 5) E - num * id \$ shift 6) E - num * id \$ shift 7) E - num * id \$ reduce by rule 7 8) E - F * id \$ reduce by rule 6 9) E - T * id \$ shift 10) E - T * id \$ shift 11) E - T * id \$ reduce by rule 8 12) E - T * F \$ reduce by rule 4 13) E - T \$ reduce by rule 2 14) E \$ shift 15) S accept (reduce by 0)

4 Machinery Need to decide when to shift or reduce, and if reduce, by which rule use a DFA to recognize handles Example 0) S ::= R \$ 1) R ::= R b 2) R ::= a state 2: accept (reduce by 0) state 3: reduce by 2 state 4: reduce by 1 The DFA is represented by an ACTION and a GOTO table The stack contains state numbers now ACTION GOTO a b \$ S R 0 s3 1 1 s4 s2 2 a a a 3 r2 r2 r2 4 r1 r1 r1

5 The Shift-Reduce Parser push(0); read_next_token(); for(;;) { s = top(); /* current state is taken from top of stack */ if (ACTION[s,current_token] == 'si') /* shift and go to state i */ { push(i); read_next_token(); } else if (ACTION[s,current_token] == 'ri') /* reduce by rule i: X ::= A1...An */ { perform pop() n times; s = top(); /* restore state before reduction from top of stack */ push(goto[s,x]); /* state after reduction */ } else if (ACTION[s,current_token] == 'a') success!! else error();

6 Example Example: parsing abb\$ ACTION GOTO a b \$ S R 0 s3 1 1 s4 s2 2 a a a 3 r2 r2 r2 4 r1 r1 r1 Stack rest-of-input Action 0 abb\$ s3 0 3 bb\$ r2 (pop, push GOTO[0,R] since R ::= a) 0 1 bb\$ s b\$ r1 (pop twice, push GOTO[0,R] since R ::= R b) 0 1 b\$ s \$ r1 (pop twice, push GOTO[0,R] since R ::= R b) 0 1 \$ s accept

7 Table Construction Problem: given a CFG, construct the finite automaton (DFA) that recognizes handles The DFA states are itemsets (sets of items) An item is a rule with a dot at the RHS eg, possible items for the rule E ::= E + E: E ::=. E + E E ::= E. + E E ::= E +. E E ::= E + E. The dot indicates how far we have progressed using this rule to parse the input eg, the item E ::= E +. E indicates that 1)we are using the rule E ::= E + E 2)we have parsed E, we have seen the token +, and we are ready to parse another E

8 Table Construction (cont.) The items in a itemset indicate different possibilities that will have to be resolved later by reading more input tokens eg, the itemset: T ::= ( E. ) E ::= E. + T coresponds to a DFA state where we don't know whether we are looking at an ( E ) handle or an E + T handle it will be ( E ) if the next token is ) it will be E + T if the next token is + When the dot is at the end of an item, we have found a handle eg, T ::= ( E ). it corresponds to a reduction by T ::= ( E ) reduce/reduce conflict: an itemset should never have more than one item with a dot at the end otherwise we can't choose a handle

9 Closure of an Itemset The closure of an item X ::= a. t b is the singleton set that contains the item X ::= a. t b only X ::= a. Y b is the set consisting of the item itself, plus all rules for Y with the dot at the beginning of the RHS, plus the closures of these items eg, the closure of the item E ::= E +. T is the set: E ::= E +. T T ::=. T * F T ::=. T / F T ::=. F F ::=. num F ::=. id The closure of an itemset is the union of closures of all items in the itemset

10 Constructing the DFA The initial state of the DFA (state 0) is the closure of the item S ::=. a, where S ::= a is the first rule of the grammar For each itemset, if there is an item X ::= a. s b in an itemset, where s is a symbol, we have a transition labeled by s to an itemset that contains X ::= a s. b But 1)if we have more than one item with a dot before the same symbol s, say X ::= a. s b and Y ::= c. s d, then the new itemset contains both X ::= a s. b and Y ::= c s. d 2)we need to get the closure of the new itemset 3)we need to check if this itemset has appeared before so that we don't create it again

11 Table construction for LR(0)

12 Table construction for LR(0) X For each edge I J where X is a terminal, we put the action shift J at position (I, X) of the table; if X is a nonterminal, we put goto J at position (I, X). For each state I containing an item S S.\$ we put an accept action at (I, \$). Finally, for a state containing an item A γ. (production n with the dot at the end), we put a reduce n action at (I, Y) for every token Y.

13 Example #1 0) S ::= R \$ 1) R ::= R b 2) R ::= a ACTION GOTO a b \$ S R 0 s3 1 1 s4 s2 2 a a a 3 r2 r2 r2 4 r1 r1 r1

14 Example #2 0) S' ::= S \$ 1) S ::= B B 2) B ::= a B 3) B ::= c

15 Example #3 S ::= E \$ E ::= ( L ) E ::= ( ) E ::= id L ::= L, E L ::= E

16 Example #4

17 LR(0) If an itemset has more than one reduction (an item with the dot at the end), it is a reduce/reduce conflict If an itemset has at least one shifting (an outgoing transition to another state) and at least one reduction, it is a shift/reduce conflict A grammar is LR(0) if it doesn't have any reduce/reduce or shift/reduce conflict 1) S ::= E \$ 2) E ::= E + T 3) T 4) T ::= T * F 5) F 6) F ::= id 7) ( E ) S ::=. E \$ T E ::= T. E ::=. E + T T ::= T. * F E ::=. T T ::=. T * F T ::=. F F ::=. id F ::=. ( E )

18 Another example of Shift/Reduce conflict

19 SLR(1) There is an easy fix for some of the shift/reduce or reduce/reduce errors requires to look one token ahead (called the lookahead token) Steps to resolve the conflict of an itemset: 1) for each shift item Y ::= b. c you find FIRST(c) 2) for each reduction item X ::= a. you find FOLLOW(X) 3) if each FOLLOW(X) do not overlap with any of the other sets, you have resolved the conflict! eg, for the itemset with E ::= T. and T ::= T. * F FOLLOW(E) = { \$, +, ) } FIRST(* F) = { * } no overlapping! This is a SLR(1) grammar, which is more powerful than LR(0)

20 SLR parsing table for the previous example

21 LR(1) The SLR(1) trick doesn't always work S ::= E \$ E ::= L = R R L ::= * R id S ::=. E \$ E ::=. L = R L E ::= L. = R E ::=. R R ::= L. L ::=. * R L ::=. id R ::=. L R ::= L For a reduction item X ::= a. we need a smaller (more precise) set of lookahead tokens to tell us when to reduce called expected lookahead tokens must be a subset of or equal to FOLLOW(X) (hopefully subset) they are context-sensitive => finer control

22 LR(1) Items Now each item is associated with expected lookahead tokens eg, L ::= *. R =\$ the tokens = and \$ are the expected lookahead tokens the are only useful in a reduction item: L ::= * R. =\$ it indicates that we reduce when the lookahead token from input is = or \$ LR(1) grammar: at each shift/reduce reduce/reduce conflict, the expected lookahead tokens of the reductions must not overlap with the first tokens after dot (ie, the FIRST(c) in Y ::= b. c) Rules for constructing the LR(1) DFA: for a transition from A ::= a. s b by a symbol s, propagate the expected lookaheads when you add the item B ::=. c to form the closure of A ::= a. B b with expected lookahead tokens t, s,..., the expected lookahead tokens of B ::=. c are FIRST(bt) U FIRST(bs) U...

23 Example S ::= E \$ E ::= L = R R L ::= * R id R ::= L S ::=. E \$? E ::=. L = R \$ L E ::= L. = R \$ E ::=. R \$ R ::= L. \$ L ::=. * R =\$ L ::=. id =\$ R ::=. L \$

24 LR(1) parsing table construction The algorithm for constructing an LR(1) parsing table is similar to that for LR(0), but the notion of an item is more sophisticated. An LR(1) item consists of a grammar production, a right-hand-side position (represented by the dot), and a lookahead symbol The idea is that an item (A α.β, x) indicates that the sequence α is on top of the stack, and at the head of the input is a string derivable from βx.

25 LR(1) parsing table construction (cont.) An LR(1) state is a set of LR(1) items, and there are Closure and Goto operations for LR(1) that incorporate the lookahead: The start state is the closure of the item (S.S \$,?), where the lookahead symbol? will not matter, because the end-of-file marker will never be shifted. Reduce actions:

26 Example The following grammar is not SLR but LR(1): S S \$ S V = E S E E V V x V * E

27 LALR(1) If the lookaheads s1 and s2 are different, then the items A ::= a s1 and A ::= a s2 are different this results to a large number of states since the combinations of expected lookahead symbols can be very large. We can combine the two states into one by creating an item A := a s3 where s3 is the union of s1 and s2 LALR(1) is weaker than LR(1) but more powerful than SLR(1) LALR(1) and LR(0) have the same number of states Easy construction of LALR(1) itemsets: start with LR(0) items and propagate lookaheads as in LR(1) don't create a new itemset if the LR(0) items are the same just union together the lookaheads of the corresponding items you may have to propagate lookaheads by looping through the same itemsets until you cannot add more Most parser generators are LALR(1)

28 Example The following grammar is not SLR but LR(1): S S \$ S V = E S E E V V x V * E LR(1) Table LALR(1) Table

29 Hierarchy of Grammar Classes Any reasonable programming language has a LALR(1) grammar, and there are many parser-generator tools available for LALR(1) grammars. For this reason, LALR(1) has become a standard for programming languages and for automatic parser generators.

30 Using parser generators JavaCC is an LL(K) parse generator Productions are of the form: void Assignment() : {} { Identifier() "=" Expression() ";" } As shown the grammar should be written to a specific format so that we can use parser generator JavaCC reports like left-recursion in the grammar SableCC is an LALR(1) parser generator. Productions are of the form: assignment = identifier assign expression semicolon ; SableCC reports errors like shift-reduce or reduce-reduce conflicts Read more in the book and documentations for the tools

31 Error Recovery All the empty entries in the ACTION and GOTO tables correspond to syntax errors We can either report it and stop the parsing continue parsing finding more errors (error recovery) Recovery using the error symbol: exp ID exp exp + ext exp ( exps ) exps exp exps exps ; exp we can specify that if a syntax error is encountered in the middle of an expression, the parser should skip to the next semicolon or right parenthesis (these are called synchronizing tokens) and resume parsing. error

32 Error Recovery (cont.) Global Error repair: Global error repair finds the smallest set of insertions and deletions that would turn the source string into a syntactically correct string, even if the insertions and deletions are not at a point where an LL or LR parser would first report an error. Burke-Fisher error repair: a limited but useful form of global error repair, which tries every possible single-token insertion, deletion, or replacement at every point that occurs no earlier than K tokens before the point where the parser reported the error.

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