COP 5021 Lecture -*- Outline -*-

* Abstract Syntax Trees (ASTs) in JastAdd

** importance of ASTs
------------------------------------------
          IMPORTANCE OF ASTS

ASTs are:
   the key interface
 between parser
 and rest of compiler (or static analysis)
------------------------------------------

** overview in JastAdd
------------------------------------------
     ABSTRACT SYNTAX TREES IN JASTADD

ASTs are described in .ast files
   (e.g., WHILE.ast)

JastAdd tool converts to java classes
   (e.g., AST/IfS.java, AST/BExpr.java)

At runtime, each AST is a Java object.
   (constructed by the parser)

------------------------------------------

** WHILE.ast file
------------------------------------------
        // WHILE.ast file

Program ::= S;

abstract Op ::= <Contents>;
Op_b : Op;
Op_r : Op;
Op_a : Op;

abstract Expr;

VarRefExpr: Expr ::= <Contents>;
NumLitExpr: Expr ::= <Contents>;
ABinaryExpr: Expr ::= Left:Expr Op:Op_a Right:Expr;

BoolLitExpr:Expr ::= <Contents>;
NotExpr: Expr ::= Expr;
LogicExpr: Expr ::= Left:Expr Op:Op_b Right:Expr;
RelExpr: Expr ::= Left:Expr Op:Op_r Right:Expr;
LabeledExpr: Expr ::= LabelAST:Label Expr; 

// statements
abstract S; 
CompoundS: S ::= SList:S* ;
AssignS:S ::= LabelAST:Label <Var> Expr;
SkipS:S ::= LabelAST:Label;
IfS:S ::= LabeledExpr S1:S S2:S ;
WhileS:S ::= LabeledExpr S;
AssertS:S ::= LabeledExpr;

abstract Label;
NumLabel:Label ::= <Num>;

------------------------------------------

        Q: What's the syntax?
        Q: What does "abstract" mean?

        might want to look at some of the generated output .java files

*** picture of runtime AST
------------------------------------------
          PARSED AST EXAMPLE

Parsing the program

    if x < 5 then y := 2 else skip

gives the AST:

  [Program | S: * ]
                |
                v
     [IfS | LabeledExpr: * | S1: * |   S2: *]
                         /        |          \
                        v         v           v
      [LabeledExpr  |        [AssignS |     [SkipS |
                 LabelAST: *    LabelAST: *    LabelAST: * ]
                 Expr:  * ]|     Var: *                    \
                       /   |     Expr:  * ]                 \
                           v                                 v
      [NumLabel | LabelAST: *]   [NumLabel |  [NumLabel | Num: * ]
                           /       Num: * ]                    |
                          v             |                      v
               [String | "1"]           v               [String | "3"]
                                   [String | "2"]

      [RelExpr | Left: *-----\        
               | Op_r: *-----|----->[Op_r | Contents: * ]
               | Right: * ]  |                        |
                            /                         v
                           v                    [String | "<" ]
 [VarRefExpr | Contents * ]   
                        |
                        v
                  [String | "x" ]


 [NumLitExpr | Contents: * ]
                         |
                         v
                   [String | "5"]
------------------------------------------