Chapter 7: Evaluating Expressions

jlox/src/main/java/com/craftinginterpreters/lox/Interpreter.java

@@ -0,0 +1,145 @@
+package com.craftinginterpreters.lox;
+
+import com.craftinginterpreters.lox.Expr.Binary;
+import com.craftinginterpreters.lox.Expr.Grouping;
+import com.craftinginterpreters.lox.Expr.Literal;
+import com.craftinginterpreters.lox.Expr.Unary;
+
+class Interpreter implements Expr.Visitor<Object> {
+
+  void interpret(Expr expression) {
+    try {
+      Object value = evaluate(expression);
+      System.out.println(stringify(value));
+    } catch (RuntimeError error) {
+      Lox.runtimeError(error);
+    }
+  }
+
+  @Override
+  public Object visitBinaryExpr(Binary expr) {
+    Object left = evaluate(expr.left);
+    Object right = evaluate(expr.right);
+
+    switch (expr.operator.type) {
+      case GREATER:
+        checkNumberOperands(expr.operator, left, right);
+        return (double) left > (double) right;
+      case GREATER_EQUAL:
+        checkNumberOperands(expr.operator, left, right);
+        return (double) left >= (double) right;
+      case LESS:
+        checkNumberOperands(expr.operator, left, right);
+        return (double) left < (double) right;
+      case LESS_EQUAL:
+        checkNumberOperands(expr.operator, left, right);
+        return (double) left <= (double) right;
+      case BANG_EQUAL:
+        return !isEqual(left, right);
+      case EQUAL_EQUAL:
+        return isEqual(left, right);
+      case MINUS:
+        checkNumberOperands(expr.operator, left, right);
+        return (double) left - (double) right;
+      case PLUS:
+        if (left instanceof Double && right instanceof Double) {
+          return (double) left + (double) right;
+        }
+
+        if (left instanceof String && right instanceof String) {
+          return (String) left + (String) right;
+        }
+
+        throw new RuntimeError(expr.operator, "Operands must be two numbers or two strings.");
+      case SLASH:
+        checkNumberOperands(expr.operator, left, right);
+        return (double) left / (double) right;
+      case STAR:
+        checkNumberOperands(expr.operator, left, right);
+        return (double) left * (double) right;
+    }
+
+    // Uncreachable
+    return null;
+  }
+
+  @Override
+  public Object visitGroupingExpr(Grouping expr) {
+    return evaluate(expr.expression);
+  }
+
+  @Override
+  public Object visitLiteralExpr(Literal expr) {
+    return expr.value;
+  }
+
+  @Override
+  public Object visitUnaryExpr(Unary expr) {
+    Object right = evaluate(expr.right);
+
+    switch (expr.operator.type) {
+      case BANG:
+        return !isTruthy(right);
+      case MINUS:
+        checkNumberOperand(expr.operator, right);
+        return -(double) right;
+    }
+
+    return null;
+  }
+
+  private void checkNumberOperand(Token operator, Object operand) {
+    if (operand instanceof Double) {
+      return;
+    }
+    throw new RuntimeError(operator, "Operand must be a number.");
+  }
+
+  private void checkNumberOperands(Token operator, Object left, Object right) {
+    if (left instanceof Double && right instanceof Double) {
+      return;
+    }
+
+    throw new RuntimeError(operator, "Operands must be numbers.");
+  }
+
+  private boolean isTruthy(Object object) {
+    if (object == null) {
+      return false;
+    }
+    if (object instanceof Boolean) {
+      return (boolean) object;
+    }
+    return true;
+  }
+
+  private boolean isEqual(Object a, Object b) {
+    if (a == null && b == null)
+      return true;
+    if (a == null)
+      return false;
+
+    return a.equals(false);
+  }
+
+  private String stringify(Object object) {
+    if (object == null) {
+      return "nil";
+    }
+
+    if (object instanceof Double) {
+      String text = object.toString();
+      if (text.endsWith(".0")) {
+        text = text.substring(0, text.length() - 2);
+      }
+      return text;
+    }
+
+    return object.toString();
+  }
+
+  private Object evaluate(Expr expr) {
+    return expr.accept(this);
+  }
+
+}

jlox/src/main/java/com/craftinginterpreters/lox/Lox.java

@@ -9,7 +9,9 @@ import java.nio.file.Paths;
 import java.util.List;
 
 public class Lox {
+    private static final Interpreter interpreter = new Interpreter();
     static boolean hadError = false;
+    static boolean hadRuntimeError = false;
 
     public static void main(String[] args) throws IOException {
         if (args.length > 1) {
@@ -29,6 +31,10 @@ public class Lox {
         if (hadError) {
             System.exit(65);
         }
+
+        if (hadRuntimeError) {
+            System.exit(70);
+        }
     }
 
     private static void runPrompt() throws IOException {
@@ -53,13 +59,18 @@ public class Lox {
 
         if (hadError) return;
 
-        System.out.println(new AstPrinter().print(expression));
+        interpreter.interpret(expression);
     }
 
     static void error(int line, String message) {
         report(line, "", message);
     }
 
+    static void runtimeError(RuntimeError error) {
+        System.err.println(error.getMessage() + "\n[line " + error.token.line + "]");
+        hadRuntimeError = true;
+    }
+
     private static void report(int line, String where, String message) {
         System.err.println("[line " + line + "] Error" + where + ": " + message);
         hadError = true;

jlox/src/main/java/com/craftinginterpreters/lox/RuntimeError.java

@@ -0,0 +1,10 @@
+package com.craftinginterpreters.lox;
+
+class RuntimeError extends RuntimeException{
+  final Token token;
+
+  RuntimeError(Token token, String message) {
+    super(message);
+    this.token = token;
+  }
+}