第6天 通过解释器执行程序

解释器从抽象语法树的根节点开始遍历该树直至叶节点，并计算各节点的内容

6.1 eval方法与环境对象

eval方法：eval是evaluate（求值）的缩写。eval方法将计算与该节点为根的子树对应的语句、表达式及子表达式，并返回执行结果。
eval方法递归调用子节点的eval方法
不同类型的节点的类，对eval方法有着不同的定义

eval方法的简化版本

public Object eval(Environment env){
    Object left = left().eval(env);
    Object right = right().eval(env);
    return (Integer)letf + (Integer)right;
}

public Object eval(Environment env){ return value(); }  // value()将返回该对象表示的整型字面量

类似深度优先树节点搜索算法

代码清单 6.1 环境对象的接口Environment.java

package chap6;

public interface Environment {
    void put(String name, Object value);
    Object get(String name);
}

代码清单 6.2 环境对象的类BasicEnv.java

package chap6;
import java.util.HashMap;

public class BasicEnv implements Environment {
    protected HashMap<String,Object> values;
    public BasicEnv() { values = new HashMap<String,Object>(); }
    public void put(String name, Object value) { values.put(name, value); }
    public Object get(String name) { return values.get(name); }
}

6.2 各种类型的eval方法

代码清单 6.3 新增的eval方法 （BasicEvaluator.java）

package chap6;

import stone.StoneException;
import stone.Token;
import stone.ast.*;
import javassist.gluonj.*;

import java.util.List;

@Reviser public class BasicEvaluator {
    public static final int TRUE = 1;
    public static final int FALSE = 0;
    @Reviser public static abstract class ASTreeEx extends ASTree {
        public abstract Object eval(Environment env);
    }
    @Reviser public static class ASTListEx extends ASTList {
        public ASTListEx(List<ASTree> c) { super(c); }
        public Object eval(Environment env) {
            throw new StoneException("cannot eval: " + toString(), this);
        }
    }
    @Reviser public static class ASTLeafEx extends ASTLeaf {
        public ASTLeafEx(Token t) { super(t); }
        public Object eval(Environment env) {
            throw new StoneException("cannot eval: " + toString(), this);
        }
    }
    @Reviser public static class NumberEx extends NumberLiteral {
        public NumberEx(Token t) { super(t); }
        public Object eval(Environment e) { return value(); }
    }
    @Reviser public static class StringEx extends StringLiteral {
        public StringEx(Token t) { super(t); }
        public Object eval(Environment e) { return value(); }
    }
    @Reviser public static class NameEx extends Name {
        public NameEx(Token t) { super(t); }
        public Object eval(Environment env) {
            Object value = env.get(name());
            if (value == null)
                throw new StoneException("undefined name: " + name(), this);
            else
                return value;
        }
    }
    @Reviser public static class NegativeEx extends NegativeExpr {
        public NegativeEx(List<ASTree> c) { super(c); }
        public Object eval(Environment env) {
            Object v = ((ASTreeEx)operand()).eval(env);
            if (v instanceof Integer)
                return new Integer(-((Integer)v).intValue());
            else
                throw new StoneException("bad type for -", this);
        }
    }
    @Reviser public static class BinaryEx extends BinaryExpr {
        public BinaryEx(List<ASTree> c) { super(c); }
        public Object eval(Environment env) {
            String op = operator();
            if ("=".equals(op)) {
                Object right = ((ASTreeEx)right()).eval(env);
                return computeAssign(env, right);
            }
            else {
                Object left = ((ASTreeEx)left()).eval(env);
                Object right = ((ASTreeEx)right()).eval(env);
                return computeOp(left, op, right);
            }
        }
        protected Object computeAssign(Environment env, Object rvalue) {
            ASTree l = left();
            if (l instanceof Name) {
                env.put(((Name)l).name(), rvalue);
                return rvalue;
            }
            else
                throw new StoneException("bad assignment", this);
        }
        protected Object computeOp(Object left, String op, Object right) {
            if (left instanceof Integer && right instanceof Integer) {
                return computeNumber((Integer)left, op, (Integer)right);
            }
            else
                if (op.equals("+"))
                    return String.valueOf(left) + String.valueOf(right);
                else if (op.equals("==")) {
                    if (left == null)
                        return right == null ? TRUE : FALSE;
                    else
                        return left.equals(right) ? TRUE : FALSE;
                }
                else
                    throw new StoneException("bad type", this);
        }
        protected Object computeNumber(Integer left, String op, Integer right) {
            int a = left.intValue(); 
            int b = right.intValue();
            if (op.equals("+"))
                return a + b;
            else if (op.equals("-"))
                return a - b;
            else if (op.equals("*"))
                return a * b;
            else if (op.equals("/"))
                return a / b;
            else if (op.equals("%"))
                return a % b;
            else if (op.equals("=="))
                return a == b ? TRUE : FALSE;
            else if (op.equals(">"))
                return a > b ? TRUE : FALSE;
            else if (op.equals("<"))
                return a < b ? TRUE : FALSE;
            else
                throw new StoneException("bad operator", this);
        }
    }
    @Reviser public static class BlockEx extends BlockStmnt {
        public BlockEx(List<ASTree> c) { super(c); }
        public Object eval(Environment env) {
            Object result = 0;
            for (ASTree t: this) {
                if (!(t instanceof NullStmnt))
                    result = ((ASTreeEx)t).eval(env);
            }
            return result;
        }
    }
    @Reviser public static class IfEx extends IfStmnt {
        public IfEx(List<ASTree> c) { super(c); }
        public Object eval(Environment env) {
            Object c = ((ASTreeEx)condition()).eval(env);
            if (c instanceof Integer && ((Integer)c).intValue() != FALSE)
                return ((ASTreeEx)thenBlock()).eval(env);
            else {
                ASTree b = elseBlock();
                if (b == null)
                    return 0;
                else
                    return ((ASTreeEx)b).eval(env);
            }
        }
    }
    @Reviser public static class WhileEx extends WhileStmnt {
        public WhileEx(List<ASTree> c) { super(c); }
        public Object eval(Environment env) {
            Object result = 0;
            for (;;) {
                Object c = ((ASTreeEx)condition()).eval(env);
                if (c instanceof Integer && ((Integer)c).intValue() == FALSE)
                    return result;
                else
                    result = ((ASTreeEx)body()).eval(env);
            }
        }
    }
}

6.3 关于GluonJ

在GluonJ中，标有@Reviser的类称为修改器（reviser）。修改器看起来和子类很相似，实则不然，它将直接修改（reviser）所继承的类的定义。

6.4 执行程序

eval方法是Stone语言解释器的核心完成了eval方法的实现之后解释器只要读取程序并调用eval方法，就能执行Stone语言程序。

代码清单 6.4 Stone语言的解释器 BasicInterpreter.java

package chap6;
import stone.*;
import stone.ast.ASTree;
import stone.ast.NullStmnt;

public class BasicInterpreter {
    public static void main(String[] args) throws ParseException {
        run(new BasicParser(), new BasicEnv());
    }
    public static void run(BasicParser bp, Environment env)
        throws ParseException
    {
        Lexer lexer = new Lexer(new CodeDialog());
        while (lexer.peek(0) != Token.EOF) {
            ASTree t = bp.parse(lexer);
            if (!(t instanceof NullStmnt)) {
                Object r = ((BasicEvaluator.ASTreeEx)t).eval(env);
                System.out.println("=> " + r);
            }
        }
    }
}