Cse321, Programming Languages and Compilers 1 6/12/2015 Lecture #10, Feb. 14, 2007 Modified sets of item construction Rules for building LR parse tables The Action rules The GOTO rules Conflicts and ambiguity Shift-reduce and reduce-reduce conflicts Parser generators and ambiguity Ambiguous expression grammar Ambiguous if-then-else grammar Ml-yacc

Cse321, Programming Languages and Compilers 2 6/12/2015 Assignments Homework –Assignment 7 will be accepted til the end of the week »good review for exam! –Assignment 8 (paper and pencil) is posted & due Mon. Feb 19 »good review for exam! –Assignment 9 (programming) is posted & due Wed. Feb 21 »in case your interested Project 1 is Due today. – me the code –Name files with your last name as discussed in the Project 1 description. Midterm Exam will be Monday, Feb 19, 2007 –Exam will be closed book –Exam will take 60 minutes –We will have a short lecture after the exam. Project 2 will be assigned next Monday Feb 19, 2006

Cse321, Programming Languages and Compilers 3 6/12/2015 To facilitate Table building To facilitate Table building we modify the sets of items construction slightly Each item now has three components. –A production –A location for the dot –A terminal symbol that indicates a valid terminal that could follow the production. This is similar to, but not quite like the Non- terminals that are in the Follow set. Examples: [ Start →. Exp, EOF] [ F → T. * F, +] Start → E E → E + T | T T → T * F | F F → ( E ) | id

Cse321, Programming Languages and Compilers 4 6/12/2015 Modified Closure Let I be a set of it modified items Then Closure(I) = For each i  I, where i = [A → . B β, x] For each p  Productions where p = B →  For each t  Terminals where t  First(βx) Add [B →. , t] to Closure(I) if its not already there

Cse321, Programming Languages and Compilers 5 6/12/2015 Modified GOTO GOTO(I,X) = For each item in I of the form [A → . X β, a] »i.e. the dot comes just before the X Let J be the set of items [A →  X. β, a] »i.e. move the dot after the X Return the Closure(J)

Cse321, Programming Languages and Compilers 6 6/12/2015 Modified Sets of items Construction Start with a grammar with a Start symbol with only 1 production. Start → E –If the grammar isn’t of that form create a new grammar that is of that form with a new start symbol that accepts the same set of strings. C := Closure( { [ Start →. E, EOF] }) For each set of items I  C For Each X  NonTerminal union Terminal Compute new := GOTO(I,X) –If new is not empty, and new is not already in C, add it to C Until no new sets of items can be added to C

Cse321, Programming Languages and Compilers 7 6/12/2015 Building Tables for LR parsers Once the sets of items have been constructed, then the tables can be constructed by using –The set of items –The GOTO construction –The grammar Each set of items corresponds to a state. States and Terminals index the ACTION table States and Non-Terminals index the GOTO table

Cse321, Programming Languages and Compilers 8 6/12/2015 Construction of ACTION table Let C be the sets of items constructed for a grammar. There is one state “i” for each set c i  C 1.If [A → . a β, b]  c i, and GOTO(c i, a) = c j Then set ACTION[ i, a ] to shift j note “a” is a terminal symbol 2.If [A → ., a]  c i Then set ACTION[ i, a ] to reduce( A →  ) 3.If [Start → S., EOF ]  c i Then set ACTION[ i, EOF ] to accept Any conflict in these rules means the grammar is ambiguous.

Cse321, Programming Languages and Compilers 9 6/12/2015 Construction of GOTO table If GOTO(c i,A) = c j Then set GOTO(I,A) to j Note that “A” is a Non-Terminal symbol All other entries are error entries The Start state of the parser is the state derived from Closure( { [ Start →. E, EOF] })

Cse321, Programming Languages and Compilers 10 6/12/2015 Parser generators Programs that analyze grammars to produce efficient winning strategies ml-yacc uses a LALR(1) table-driven parser –Look-Ahead 1 symbol –Left to right processing of input –Right-most derivation ml-yacc reads a grammar, produces a table ml-yacc attaches semantic actions to reduce moves

Cse321, Programming Languages and Compilers 11 6/12/2015 ml-yacc makes a virtue out of Ambiguity Why not: expr -> expr + expr | expr * expr | Number Ambiguity ! ! ! expr Number + 17 Number 3 expr * 2

Cse321, Programming Languages and Compilers 12 6/12/2015 Factoring - A hard solution Fix the grammar E : E + T | T ; T : T * F | F ; F : ( E ) | Number ; Problems –Grammar is harder to understand –Grammar is bigger E E +T T T*F F Number

Cse321, Programming Languages and Compilers 13 6/12/2015 Using Ambiguity Ambiguity means the parser can’t decide between –Shifting a terminal, or reducing a handle to a Non-Terminal –Reducing a handle to one or more Non-terminals »T → rhs and S → rhs are both in the grammar and rhs is the handle. This choice means we can’t construct a unique parse tree for any string. But what if we could direct the parser to always prefer one choice over the other. Then –The parse tree would always be unique –The grammar might even be smaller

Cse321, Programming Languages and Compilers 14 6/12/2015 Ambiguous Expression Grammar Contrast the two grammars. Convince yourself they both accept the same set of strings. Which one is ambiguous? Which one is simpler? Which one is smaller? Start → E E → E + E | E * E | ( E ) | id Start → E E → E + T | T T → T * F | F F → ( E ) | id

Cse321, Programming Languages and Compilers 15 6/12/2015 An LR parser for the ambiguous EXP grammar The sets of item construction has 11 states It has 4 shift-reduce ambiguities Start → E E → E + E | E * E | ( E ) | id

Cse321, Programming Languages and Compilers 16 6/12/ shift reduce ambiguities State 8 { [E → E. + E, _ ], [E → E. * E, _ ], [E → E * E., _ ] } Action(8,+) shift 5 Action(8,+) reduce by 2 State 8 { [E → E. + E, _ ], [E → E. * E, _ ], [E → E * E., _ ] } Action(8,*) shift 4 Action(8,*) reduce by 2 State 9 { [E → E. PLUS E, _ ], [E → E PLUS E., _ ], [E → E. TIMES E, _ ] } Action(9,*) shift 4 Action(9,*) reduce by 1 State 9 { [E → E. PLUS E, _ ], [E → E PLUS E., _ ], [E → E. TIMES E, _ ] } Action(9,+) shift 5 Action(9,+) reduce by 1

Cse321, Programming Languages and Compilers 17 6/12/2015 State of the stack 1 Stack input... Exp * Exp + 3 EOF Choices Action(8,+) shift 5 Action(8,+) reduce by 2 Reducing by 2 means (E * E) has higher precedence than (E + E) Generally this is what we want. { [E → E. + E, _ ], [E → E. * E, _ ], [E → E * E., _ ] }

Cse321, Programming Languages and Compilers 18 6/12/2015 State of the stack 2 Stack input... Exp * Exp * 3 EOF Choices Action(8,*) shift 4 Action(8,*) reduce by 2 Reducing by 2 means that (E * E) is left associative Shifting * means (E * E) is right associative The other 2 shift reduce errors are similar but talk about the precedence and associativity of (E + E) { [E → E. + E, _ ], [E → E. * E, _ ], [E → E * E., _ ] }

Cse321, Programming Languages and Compilers 19 6/12/2015 ml-yacc, a Better Solution ml-yacc allows ambiguous grammars to be disambiguated via declarations of precedence and associativity For example: –%left ‘+’ –%left ‘*’ Declares that * has higher precedence than + and that both are left associative If ambiguity remains the following rules are used –always shift on a shift/reduce conflict –do the first reduction listed in the grammar on a reduce/reduce conflict

Cse321, Programming Languages and Compilers 20 6/12/2015 Partial Ml-yacc file %left TIMES %left PLUS % Start: E EOF ( E ) E : E PLUS E ( Add(E1,E2) ) | E TIMES E ( Mult(E1,E2) ) | LP E RP ( E ) | id ( Id id ) Much more about ml-yacc next time!

Cse321, Programming Languages and Compilers 21 6/12/2015 If-then-else example Goal → Stmt Stmt → IF Exp THEN Stmt | IF Exp THEN Stmt ELSE Stmt | ID := Exp State 9 {[Stmt → IF EXP THEN Stmt., _],[Stmt → IF EXP THEN Stmt. ELSE Stmt, _ ] } Action(9,ELSE) = Shift 10 Action(9,ELSE) = Reduce by 1

Cse321, Programming Languages and Compilers 22 6/12/2015 State of machine Stack input... IF Exp THEN Stmt ELSE x := 3 … EOF State 9 {[Stmt → IF EXP THEN Stmt., _],[Stmt → IF EXP THEN Stmt. ELSE Stmt, _ ] } Action(9,ELSE) = Shift 10 Else associated with closest IF on stack Action(9,ELSE) = Reduce by 1 Else associated with further away IF

Cse321, Programming Languages and Compilers 23 6/12/2015 Ml-yacc file %nonassoc THEN %nonassoc ELSE % Goal: Stmt EOF ( Stmt ) Stmt: IF EXP THEN Stmt ( IfThen(E,Stmt) ) | IF EXP THEN Stmt ELSE Stmt ( IfThenElse(E,Stmt1,Stmt2) ) | ID ASSIGNOP EXP ( Assign(ID,E) )

Cse321, Programming Languages and Compilers 24 6/12/2015 Some sample ambiguous grammars These examples can be found in the directory