finish off chapter 5

src/lox/ASTPrinter.java

@@ -0,0 +1,42 @@
+package lox;
+
+class ASTPrinter implements Expr.Visitor<String> {
+  String print(Expr expr) {
+    return expr.accept(this);
+  }
+
+  @Override
+  public String visitBinaryExpr(Expr.Binary expr) {
+    return parenthesize(expr.operator.lexeme, expr.left, expr.right);
+  }
+
+  @Override
+  public String visitGroupingExpr(Expr.Grouping expr) {
+    return parenthesize("group", expr.expression);
+  }
+
+  @Override
+  public String visitLiteralExpr(Expr.Literal expr) {
+    if (expr.value == null) {
+      return "nil";
+    }
+    return expr.value.toString();
+  }
+
+  @Override
+  public String visitUnaryExpr(Expr.Unary expr) {
+    return parenthesize(expr.operator.lexeme, expr.right);
+  }
+
+  private String parenthesize(String name, Expr... exprs) {
+    StringBuilder builder = new StringBuilder();
+
+    builder.append("(").append(name);
+    for (Expr expr : exprs) {
+      builder.append(" ").append(expr.accept(this));
+    }
+    builder.append(")");
+
+    return builder.toString();
+  }
+}

src/lox/Expr.java

@@ -1,6 +1,15 @@
 package lox;
 
+import java.util.List;
+
 abstract class Expr {
+  interface Visitor<R> {
+    R visitBinaryExpr(Binary expr);
+    R visitGroupingExpr(Grouping expr);
+    R visitLiteralExpr(Literal expr);
+    R visitUnaryExpr(Unary expr);
+  }
+
   static class Binary extends Expr {
     Binary(Expr left, Token operator, Expr right) {
       this.left = left;
@@ -8,6 +17,11 @@ abstract class Expr {
       this.right = right;
     }
 
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitBinaryExpr(this);
+    }
+
     final Expr left;
     final Token operator;
     final Expr right;
@@ -18,6 +32,11 @@ abstract class Expr {
       this.expression = expression;
     }
 
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitGroupingExpr(this);
+    }
+
     final Expr expression;
   }
 
@@ -26,6 +45,11 @@ abstract class Expr {
       this.value = value;
     }
 
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitLiteralExpr(this);
+    }
+
     final Object value;
   }
 
@@ -35,7 +59,14 @@ abstract class Expr {
       this.right = right;
     }
 
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitUnaryExpr(this);
+    }
+
     final Token operator;
     final Expr right;
   }
+
+  abstract <R> R accept(Visitor<R> visitor);
 }

src/tool/GenerateAST.java

@@ -34,16 +34,40 @@ public class GenerateAST {
     writer.println();
     writer.println("abstract class " + baseName + " {");
 
+    defineVisitor(writer, baseName, types);
+
     for (String type : types) {
       String className = type.split(":")[0].trim();
       String fields = type.split(":")[1].trim();
       defineType(writer, baseName, className, fields);
     }
 
+    writer.println("  abstract <R> R accept(Visitor<R> visitor);");
+
     writer.println("}");
     writer.close();
   }
 
+  private static void defineVisitor(PrintWriter writer, String baseName, List<String> types) {
+    writer.println("  interface Visitor<R> {");
+
+    for (String type : types) {
+      String typeName = type.split(":")[0].trim();
+      writer.println(
+          "    R visit"
+              + typeName
+              + baseName
+              + "("
+              + typeName
+              + " "
+              + baseName.toLowerCase()
+              + ");");
+    }
+
+    writer.println("  }");
+    writer.println();
+  }
+
   private static void defineType(
       PrintWriter writer, String baseName, String className, String fieldList) {
     writer.println("  static class " + className + " extends " + baseName + " {");
@@ -59,6 +83,12 @@ public class GenerateAST {
     writer.println("    }");
     writer.println();
 
+    writer.println("    @Override");
+    writer.println("    <R> R accept(Visitor<R> visitor) {");
+    writer.println("      return visitor.visit" + className + baseName + "(this);");
+    writer.println("    }");
+    writer.println();
+
     for (String field : fields) {
       writer.println("    final " + field + ";");
     }