SML-SYNTAX-LANGUAGE INTERPRETER IN JAVA. Jiahao Yuan Supervisor: Dr. Vijay Gehlot

Transcription

1 SML-SYNTAX-LANGUAGE INTERPRETER IN JAVA Jiahao Yuan Supervisor: Dr. Vijay Gehlot

2 Target Design SML-Like-Syntax Build Parser in ANTLR Abstract Syntax Tree Representation ANLTER Integration In Interpreter In Java

3 Step One: Syntax

4 Step Two: Build Parser In ANTLR Parsers can be automatically generated from a CFG description (Model Driven Development MDD) ANTLR is one such tool that generates a (recursive descent) parser (in Java by default) Other tools: Yacc, Bison, SableCC, JavaCC, MLYacc, etc. Grammar file ANTLR Tool Java Code

5 ANTLR Demo

6 ANLTR Demo

7 Project Interpreter Details Key idea: Syntactic constructs can be represented as an AST ASTs can be implemented in Java as a recursive type Interpreter is nothing but an function that takes an AST and an environment and traverses the tree and evaluates each (sub) construct as defined by the underlying semantics of the construct. n Eval: Expr x Env Value Under OO view, the eval function will be part of the object representing an AST. Modify the parser so as to return an AST upon successful parse of the input expression.

8 Step Three: AST Representation Can be visualized (and coded) in SML fairly easily: datatype BinOp = PLUS MINUS TIMES DIV EQ LT AND OR datatype Expr = IntConst of int BoolConst of bool VarExp of BinExp of BinOp Expr Expr NotExp of Expr AssnExp of string Expr IfExp of Expr Expr Expr WhileExp of LetValExp of string Expr Expr LetFunExp of string string Expr Expr AppExp of string Expr SeqExp of Expr Expr

9 Step Three: AST Representation datatype Expr = IntConst of int BoolConst of bool VarExp of string BinExp of BinOp * Expr * Expr NotExp of Expr AssnExp of string * Expr IfExp of Expr * Expr * Expr WhileExp of Expr * Expr LetValExp of string * Expr * Expr LetFunExp of string * string * Expr * Expr AppExp of string * Expr SeqExp of Expr * Expr Can be translated in Java (or any other language) easily.

10 Step Four: ANLTER Integration In Interpreter In Java o Define the following subclasses of Expr: n IntConst, BoolConst, VarExp, BinExp,NotExp, AssnExp, IfExp, WhileExp, LetValExp, LetFunExp, AppExp, SeqExp. o There is a enum type (class) BinOp for binary operations.

11 Step Four: ANLTER Integration In Interpreter In Java public abstract class Value { abstract public String tostring(); Define the following subclasses of Value: IntVal, BoolVal, FunVal.

12 Step Four: ANLTER Integration In Interpreter In Java public interface Env { public Value lookup(string id) throws UnboundVar; public Env addbinding(string id, Value v); public Env updatebinding(string id, Value v) throws UnboundVar; public String tostring();

13 Examples: IntVal.java public class IntVal extends Value { public final int value; public IntVal(int _value) { value = _value; public String tostring() { return String.valueOf(value);

14 Examples: BinOp.java public enum BinOp { PLUS, MINUS, TIMES, DIV, EQ, LT, AND, OR

15 Examples: IntConst.java public class IntConst extends Expr { public final int value; public IntConst(int _value) { value = _value; public Value eval(env e) throws EvalError { return new IntVal(value); public String tostring() { return String.valueOf(value);

16 Examples: NotExp.java public class NotExp extends Expr { public final Expr arg; public NotExp(Expr _arg) { arg = _arg; public Value eval(env e) throws EvalError { Value v = arg.eval(e); if (v instanceof BoolVal) return new BoolVal(!((BoolVal)v).value); else throw new EvalError("Evaluation error: Incompatible arg types"); public String tostring() { return "!" + arg.tostring();

17 Product:InterpretMain.java public class InterpretMain { public static void main(string[] args) throws Exception { System.setErr(System.out); ANTLRInputStream input = new ANTLRInputStream(System.in); ProjLangLexer lexer = new ProjLangLexer(input); CommonTokenStream tokens = new CommonTokenStream(lexer); ProjLangParser parser = new ProjLangParser(tokens); try { Expr e = parser.input(); Env emptyenv = new EnvImp(); System.out.println(e.eval(emptyEnv)); catch(exception e) { System.out.println(e.getMessage());

18 Demo SML Interpreted In Java: IfExp

19 Demo SML Interpreted In Java: LetValExp

20 Demo SML Interpreted In Java: LetFunExp

21 Demo SML Interpreted In Java: VarExp