Chapter 8: Add assignment

src/com/craftinginterpreters/lox/AstPrinter.java

@@ -35,6 +35,11 @@ class AstPrinter implements Expr.Visitor<String>, Stmt.Visitor<String> {
         return expr.name.lexeme;
     }
 
+    @Override
+    public String visitAssignExpr(Expr.Assign expression) {
+        return parenthesize(expression.name.lexeme + " = ", expression.value);
+    }
+
     @Override
     public String visitVarStmt(Stmt.Var statement) {
         return parenthesize("define " + statement.name.lexeme + " = ", statement.initializer);

src/com/craftinginterpreters/lox/Environment.java

@@ -14,6 +14,15 @@ class Environment {
         throw new RuntimeError(name, "Undefined variable '" + name.lexeme + "'.");
     }
 
+    void assign(Token name, Object value) {
+        if (values.containsKey(name.lexeme)) {
+            values.put(name.lexeme, value);
+            return;
+        }
+
+        throw new RuntimeError(name, "Undefined variable '" + name.lexeme + "'.");
+    }
+
     void define(String name, Object value) {
         values.put(name, value);
     }

src/com/craftinginterpreters/lox/Interpreter.java

@@ -113,6 +113,13 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Object visitAssignExpr(Expr.Assign expr) {
+        Object value = evaluate(expr.value);
+        environment.assign(expr.name, value);
+        return value;
+    }
+
     @Override
     public Object visitBinaryExpr(Expr.Binary expr) {
         Object left = evaluate(expr.left);

src/com/craftinginterpreters/lox/Parser.java

@@ -27,7 +27,7 @@ class Parser {
     }
 
     private Expr expression() {
-        return equality();
+        return assignment();
     }
 
     private Stmt declaration() {
@@ -73,6 +73,24 @@ class Parser {
         return new Stmt.Expression(expr);
     }
 
+    private Expr assignment() {
+        Expr expr = equality();
+
+        if (match(EQUAL)) {
+            Token equals = previous();
+            Expr value = assignment();
+
+            if (expr instanceof Expr.Variable) {
+                Token name = ((Expr.Variable) expr).name;
+                return new Expr.Assign(name, value);
+            }
+
+            error(equals, "Invalid assignment target.");
+        }
+
+        return expr;
+    }
+
     private Expr equality() {
         Expr expr = comparison();
 

src/com/craftinginterpreters/tool/GenerateAst.java

@@ -15,7 +15,7 @@ public class GenerateAst {
         String outputDir = args[0];
 
         defineAst(outputDir, "Expr",
-                Arrays.asList("Binary : Expr left, Token operator, Expr right", "Grouping : Expr expression",
+                  Arrays.asList("Assign : Token name, Expr value", "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"));