Chapter 8: Add variable declarations

src/com/craftinginterpreters/lox/AstPrinter.java

@@ -3,7 +3,7 @@ package com.craftinginterpreters.lox;
 /**
  * AstPrinter
  */
-class AstPrinter implements Expr.Visitor<String> {
+class AstPrinter implements Expr.Visitor<String>, Stmt.Visitor<String> {
     String print(Expr expr) {
         return expr.accept(this);
     }
@@ -30,6 +30,26 @@ class AstPrinter implements Expr.Visitor<String> {
         return parenthesize(expr.operator.lexeme, expr.right);
     }
 
+    @Override
+    public String visitVariableExpr(Expr.Variable expr) {
+        return expr.name.lexeme;
+    }
+
+    @Override
+    public String visitVarStmt(Stmt.Var statement) {
+        return parenthesize("define " + statement.name.lexeme + " = ", statement.initializer);
+    }
+
+    @Override
+    public String visitPrintStmt(Stmt.Print statement) {
+        return parenthesize("print", statement.expression);
+    }
+
+    @Override
+    public String visitExpressionStmt(Stmt.Expression expression) {
+        return expression.expression.accept(this);
+    }
+
     private String parenthesize(String name, Expr... exprs) {
         StringBuilder builder = new StringBuilder();
 

src/com/craftinginterpreters/lox/CMakeLists.txt

@@ -7,7 +7,7 @@ set(GENERATED_JAVA_FILENAMES
 
 add_custom_command(OUTPUT ${GENERATED_JAVA_FILENAMES}
   COMMAND java -jar ${GENERATE_AST_JAR} ${CMAKE_CURRENT_BINARY_DIR}
-  DEPENDS GenerateAst)
+  DEPENDS GenerateAst ${GENERATE_AST_JAR})
 
 add_jar(Lox
   Lox.java
@@ -19,4 +19,5 @@ add_jar(Lox
   Parser.java
   Interpreter.java
   RuntimeError.java
+  Environment.java
   ENTRY_POINT com/craftinginterpreters/lox/Lox)

src/com/craftinginterpreters/lox/Environment.java

@@ -0,0 +1,20 @@
+package com.craftinginterpreters.lox;
+
+import java.util.HashMap;
+import java.util.Map;
+
+class Environment {
+    private final Map<String, Object> values = new HashMap<>();
+
+    Object get(Token name) {
+        if (values.containsKey(name.lexeme)) {
+            return values.get(name.lexeme);
+        }
+
+        throw new RuntimeError(name, "Undefined variable '" + name.lexeme + "'.");
+    }
+
+    void define(String name, Object value) {
+        values.put(name, value);
+    }
+}

src/com/craftinginterpreters/lox/Interpreter.java

@@ -3,6 +3,8 @@ package com.craftinginterpreters.lox;
 import java.util.List;
 
 class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
+    private Environment environment = new Environment();
+
     @Override
     public Object visitLiteralExpr(Expr.Literal expr) {
         return expr.value;
@@ -24,6 +26,11 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Object visitVariableExpr(Expr.Variable expr) {
+        return environment.get(expr.name);
+    }
+
     private void checkNumberOperand(Token operator, Object operand) {
         if (operand instanceof Double)
             return;
@@ -95,6 +102,17 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Void visitVarStmt(Stmt.Var stmt) {
+        Object value = null;
+        if (stmt.initializer != null) {
+            value = evaluate(stmt.initializer);
+        }
+
+        environment.define(stmt.name.lexeme, value);
+        return null;
+    }
+
     @Override
     public Object visitBinaryExpr(Expr.Binary expr) {
         Object left = evaluate(expr.left);

src/com/craftinginterpreters/lox/Parser.java

@@ -20,7 +20,7 @@ class Parser {
         List<Stmt> statements = new ArrayList<>();
 
         while (!isAtEnd()) {
-            statements.add(statement());
+            statements.add(declaration());
         }
 
         return statements;
@@ -30,6 +30,18 @@ class Parser {
         return equality();
     }
 
+    private Stmt declaration() {
+        try {
+            if (match(VAR))
+                return varDeclaration();
+
+            return statement();
+        } catch (ParseError error) {
+            synchronize();
+            return null;
+        }
+    }
+
     private Stmt statement() {
         if (match(PRINT))
             return printStatement();
@@ -43,6 +55,18 @@ class Parser {
         return new Stmt.Print(value);
     }
 
+    private Stmt varDeclaration() {
+        Token name = consume(IDENTIFIER, "Expect variable name.");
+
+        Expr initializer = null;
+        if (match(EQUAL)) {
+            initializer = expression();
+        }
+
+        consume(SEMICOLON, "Expect ';' after variable declaration.");
+        return new Stmt.Var(name, initializer);
+    }
+
     private Stmt expressionStatement() {
         Expr expr = expression();
         consume(SEMICOLON, "Expect ';' after expression.");
@@ -119,6 +143,10 @@ class Parser {
             return new Expr.Literal(previous().literal);
         }
 
+        if (match(IDENTIFIER)) {
+            return new Expr.Variable(previous());
+        }
+
         if (match(LEFT_PAREN)) {
             Expr expr = expression();
             consume(RIGHT_PAREN, "Expect ')' after expression.");

src/com/craftinginterpreters/tool/GenerateAst.java

@@ -14,9 +14,11 @@ public class GenerateAst {
 
         String outputDir = args[0];
 
-        defineAst(outputDir, "Expr", Arrays.asList("Binary   : Expr left, Token operator, Expr right",
-                "Grouping : Expr expression", "Literal  : Object value", "Unary    : Token operator, Expr right"));
-        defineAst(outputDir, "Stmt", Arrays.asList("Expression : Expr expression", "Print : Expr expression"));
+        defineAst(outputDir, "Expr",
+                Arrays.asList("Binary : Expr left, Token operator, Expr right", "Grouping : Expr expression",
+                        "Literal : Object value", "Unary : Token operator, Expr right", "Variable : Token name"));
+        defineAst(outputDir, "Stmt", Arrays.asList("Expression : Expr expression", "Print : Expr expression",
+                "Var : Token name, Expr initializer"));
     }
 
     private static void defineAst(String outputDir, String baseName, List<String> types) throws IOException {