java interpreter

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Interpreter.java

@@ -0,0 +1,136 @@
+package ch.vanwa.lox_interpreter;
+
+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 visitLiteralExpr(Expr.Literal expr) {
+    return expr.value;
+  }
+
+  @Override
+  public Object visitGroupingExpr(Expr.Grouping expr) {
+    return evaluate(expr.expression);
+  }
+
+  private Object evaluate(Expr expr) {
+    return expr.accept(this);
+  }
+
+  @Override
+  public Object visitBinaryExpr(Expr.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;
+    }
+
+    // Unreachable.
+    return null;
+  }
+
+  @Override
+  public Object visitUnaryExpr(Expr.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;
+    }
+
+    // Unreachable.
+    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(b);
+  }
+
+  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();
+  }
+}

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Lox.java

@@ -12,11 +12,18 @@ import java.util.List;
  * A Lox interpreter.
  */
 public class Lox {
+  private static final Interpreter interpreter = new Interpreter();
+
   /**
    * True if a run produced an error, false otherwise.
    */
   static boolean hadError = false;
 
+  /**
+   * True if a run produced a runtime error, false otherwise.
+   */
+  static boolean hadRuntimeError = false;
+
   /**
    * Run a file with Lox source code.
    *
@@ -28,6 +35,9 @@ public class Lox {
     if (hadError) {
       System.exit(65);
     }
+    if (hadRuntimeError) {
+      System.exit(70);
+    }
   }
 
   /**
@@ -60,10 +70,11 @@ public class Lox {
     List<Token> tokens = scanner.scanTokens();
     Parser parser = new Parser(tokens);
     Expr expression = parser.parse();
-    if (hadError)
+    if (hadError) {
       return;
+    }
 
-    System.out.println(new AstPrinter().print(expression));
+    interpreter.interpret(expression);
   }
 
   /**
@@ -90,6 +101,16 @@ public class Lox {
     }
   }
 
+  /**
+   * Print a runtime error.
+   *
+   * @param error Runtime error to print.
+   */
+  static void runtimeError(RuntimeError error) {
+    System.err.println(String.format("%s\n[line %d]", error.getMessage(), error.token.line()));
+    hadRuntimeError = true;
+  }
+
   /**
    * Print a message to stderr.
    *

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/RuntimeError.java

@@ -0,0 +1,13 @@
+package ch.vanwa.lox_interpreter;
+
+/**
+ * Represent an error at runtime (evaluating).
+ */
+class RuntimeError extends RuntimeException {
+  final Token token;
+
+  RuntimeError(Token token, String message) {
+    super(message);
+    this.token = token;
+  }
+}