Parsing III Bottom-up ParsingParsing Techniques Top-down parsers (LL(1), recursive descent) • Start at the root of the parse tree and grow toward leaves • Pick a production & try to match the input • Bad “pick” ⇒ may need to backtrack • Some grammars are backtrack-free (predictive parsing) Bottom-up parsers (LR(1), operator precedence) • Start at the leaves and grow toward root • As input is consumed, encode possibilities in an internal state • Start in a state valid for legal first tokens • Bottom-up parsers handle a large class of grammarsBottom-up Parsing (definitions) The point of parsing is to construct a derivation A derivation consists of a series of rewrite steps S ⇒ γ0 ⇒ γ1 ⇒ γ2 ⇒ … ⇒ γn–1 ⇒ γn ⇒ sentence • Each γi is a sentential form → If γ contains only terminal symbols, γ is a sentence in L(G) → If γ contains ≥ 1 non-terminals, γ is a sentential form • To get γi from γi–1, expand some NT A ∈ γi–1 by using A →β&→ Replace the occurrence of A ∈ γi–1 with β to get γi → In a leftmost derivation, it would be the first NT A ∈ γi–1 A left-sentential form occurs in a leftmost derivation. A right-sentential form occurs in a rightmost derivation.Bottom-up Parsing A bottom-up parser builds a derivation by working from the input sentence back toward the start symbol S S ⇒ γ0 ⇒ γ1 ⇒ γ2 ⇒ … ⇒ γn–1 ⇒ γn ⇒ sentence To reduce γi to γi–1 (assuming the production A→β) match some rhs β against γi then replace β with its corresponding lhs, A. In terms of the parse tree, this is working from leaves to root • Nodes with no parent in a partial tree form its frontier • Since each replacement of β with A shrinks the current frontier, we call it a reduction. bottom-upFinding Reductions Consider the simple grammar And the input string abbcde The trick is scanning the input and finding the next reduction The mechanism for doing this should be efficientFinding Reductions (Handles) The parser must find a substring β of the tree’s frontier that matches some production A → β that occurs as one step in the rightmost derivation We call this substring β a handle Formally, A handle is a pair <A→β,k> where A→β ∈ P and k is position in tree’s current frontier of β’s rightmost (last) symbol. Replacing β at k with A in the bottom-up parse represents the next step in the reverse rightmost derivation.Finding Reductions (Handles) Critical Insight If G is unambiguous, then every right-sentential form has a unique handle. If we can find those handles, we can build a derivation ! Sketch of Proof: 1 G is unambiguous ⇒ rightmost derivation is unique 2 ⇒ a unique production A → β applied to derive γi from γi–1 3 ⇒ a unique position k at which A→β is applied 4 ⇒ a unique handle <A→β,k> This all follows from the definitionsHandle-pruning, Bottom-up Parsers The process of discovering a handle & reducing it to the appropriate left-hand side is called handle pruning Handle pruning forms the basis for a bottom-up parsing method To construct a rightmost derivation S ⇒ γ0 ⇒ γ1 ⇒ γ2 ⇒ … ⇒ γn–1 ⇒ γn ⇒ sentence Apply the following simple algorithm for i ← n to 1 by –1 Find the handle <Ai →βi , ki > in γi Replace βi with Ai to generate γi–1Handle-pruning, Bottom-up Parsers One implementation technique is the shift-reduce parser push INVALID word ← NextWord( ) repeat until (top of stack = Goal and word = EOF) if a handle for A→β on top of the stack then // reduce β to A pop |β| symbols off the stack push A onto the stack else if (word ≠ EOF) then // shift push word word ← NextWord( ) else // either no handle or no input report an error Figure 3.11 in EAC How do errors show up? • failure to find a handle • hitting EOF & needing to shift (final else clause) Either generates an errorExample The expression grammar <id,x> - <num,2> * <id,y>Example <id,x> - <num,2> * <id,y> Goal <id,x> Term Fact. Expr – Expr <id,y> <num,2> Fact. Fact. Term Term *Shift-reduce Parsing Shift reduce parsers are easily built and easily understood A shift-reduce parser has just four actions • Shift — next word is shifted onto the stack • Reduce — right end of handle is at top of stack Locate left end of handle within the stack Pop handle off stack & push appropriate lhs • Accept — stop parsing & report success • Error — call an error reporting/recovery routine Accept & Error are simple Shift is just a push and a call to the scanner Reduce takes |rhs| pops & 1 push Handle finding is key • handle is on stack • finite set of handles ⇒ use a DFA !An Important Lesson about Handles To be a handle, a substring of a sentential form γ must have two properties: → It must match the right hand side β of some rule A → β → There must be some rightmost derivation from the goal symbol that produces the sentential form γ with A → β as the last production applied • Simply looking for right hand sides that match strings is not good enough • Critical Question: How can we know when we have found a handle without generating lots of different derivations? → Answer: we use look ahead in the grammar along with tables produced as the result of analyzing the grammar. → LR(1) parsers build a DFA that runs over the stack & finds
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