MiniJava Abstract Syntax

CS 5470
Compiler Techniques and Principles
January 29, 2010 — LECTURE 8
Parsing MiniJava
Abstract syntax for MiniJava

MiniJava Grammar (1)
ClassDecl class id { VarDecl* MethodDecl* }
ClassDecl class id extends id { VarDecl* MethodDecl* }
●
Is this fragment of the grammar LL(1)?
●
LL(2)?
CS 5470—Lecture 8 2

MiniJava Grammar (2)
MethodDecl public ... { VarDecl* Statement* ... }
VarDecl Type id ;
Type int []
Type boolean
Type int
Type id
Statement { Statement* }
Statement if ( Exp ) Statement else Statement
Statement while ( Exp ) Statement
Statement System.out.println ( Exp ) ;
Statement id = Exp ;
Statement id [ Exp ] = Exp ;
Is this fragment of the grammar LL(1)? LL(2)?
CS 5470—Lecture 8 3

MiniJava Grammar (3)
Exp Exp op Exp
Exp Exp [ Exp ]
Exp Exp . length
Exp Exp . id ( ExpList )
Exp INTEGER_LITERAL
Exp true
Exp false
Exp id
Exp this
Exp new int [ Exp ]
Exp new id ( )
Exp ! Exp
Exp ( Exp )
●
Is this fragment of the
grammar LL(1)?
●
LL(2)?
CS 5470—Lecture 8 4

MiniJava Abstract Syntax
Abstract classes for some nonterminals:
ClassDecl, Type, Statement, Exp
Subclasses for each of their production rules:
such as ClassDeclSimple, IntegerType, If, Not
Classes for nonterminals with only one production rule:
Program, MainClass, VarDecl, MethodDecl, Formal
List classes:
ClassDeclList, ExpList, FormalList,
MethodDeclList, StatementList, VarDeclList
CS 5470—Lecture 8 5

An Abstract Syntax Class
Object field variables representing the nonterminals, identifiers,
and integer literals in right side of the production rule.
Constructor with arguments corresponding to the field variables.
Several accept() methods, used in the parser, checker, and IRtree
generator.
** For a production rule whose right side has only terminals that
are not identifiers or integer literals, its syntax class has no field
variables.
CS 5470—Lecture 8 6

List Classes
All list classes are treated similarly, using the Vector object.
public class StatementList {
private Vector list;
public StatementList() {list = new Vector();}
public void addElement(Statement n)
{list.addElement(n);}
public Statement elementAt(int i)
{return (Statement)list.elementAt(i);}
public int size() {return list.size();}
}
** Notice the lack of an accept() method.
CS 5470—Lecture 8 7

Example: MiniJava Statement
if (x < 0) {
y = my_obj.my_method(1);
z = new my_class();
}
else
x = 0;
What Java code will build the AST for this statement?
CS 5470—Lecture 8 8

if (x < 0) {
else
y = my_obj.my_method(1);
z = new my_class(); }
x = 0;
ExpList el = new ExpList();
el.addElement(new IntegerLiteral(1));
StatementList sl = new StatementList();
sl.addElement(new Assign(new Identifier(“y”),
new Call(new IdentifierExp(“my_obj”),
new Identifier(“my_method”), el)));
sl.addElement(new Assign(new Identifier(“z”),
new NewObject(new Identifier(“my_class”))));
Statement s = new If(new LessThan(
new IdentifierExp(“x”),
new IntegerLiteral(0)), new Block(sl),
new Assign(new Identifier(“x”),
new IntegerLiteral(0)));

Provided Visitors
The interface Visitor, containing a visit() method for
each syntax class that returns void.
PrettyPrintVisitor implements Visitor
●
each visit() method for a syntax class calls the
accept() method for each of its object field
variables
●
list classes have no accept() methods, uses
elementAt() method inside a for loop
●
prints formatted version of code for each syntax class
CS 5470—Lecture 8 10

PrettyPrintVisitor
public void visit(ArrayAssign n) {
// call accept for Identifier i
n.i.accept(this);
System.out.print("[");
// call accept for Exp e1
n.e1.accept(this);
System.out.print("] = ");
// call accept for Exp e2
n.e2.accept(this);
System.out.print(";");
}
CS 5470—Lecture 8 11

PrettyPrintVisitor
public void visit(Block n) {
System.out.println("{ ");
// call accept for each Statement in list
for ( int i = 0; i < n.sl.size(); i++ ) {
System.out.print(" ");
n.sl.elementAt(i).accept(this);
System.out.println();
}
System.out.print(" } ");
}
CS 5470—Lecture 8 12