closures

callable.go

@@ -1,36 +1,44 @@
 package main
 
-type Callable struct {
-	arity int
-	call  func(*Interpreter, ...any) any
+type Callable interface {
+	arity() int
+	call(i *Interpreter, args ...any) (ret any)
 }
 
-func newCallable(f *FunStmt) *Callable {
-	return &Callable{
-		arity: len(f.args),
-		call: func(i *Interpreter, args ...any) (ret any) {
+type Function struct {
+	definition *FunStmt
+	closure    *Environment
+}
 
-			defer func() {
-				if err := recover(); err != nil {
-					re, ok := err.(Return)
+func (f *Function) call(i *Interpreter, args ...any) (ret any) {
 
-					if !ok {
-						panic(err)
-					}
+	defer func() {
+		if err := recover(); err != nil {
+			re, ok := err.(Return)
 
-					ret = re.val
-				}
-			}()
-
-			env := newEnvironment(i.globals)
-
-			for idx, arg := range f.args {
-				env.define(arg.lexeme, args[idx])
+			if !ok {
+				panic(err)
 			}
 
-			i.executeBlock(f.body, env)
+			ret = re.val
+		}
+	}()
 
-			return nil
-		},
+	env := newEnvironment(f.closure)
+
+	for idx, arg := range f.definition.args {
+		env.define(arg.lexeme, args[idx])
 	}
+
+	i.executeBlock(f.definition.body, env)
+
+	return nil
+}
+
+func (f *Function) arity() int {
+	return len(f.definition.args)
+}
+
+func newFunction(fun *FunStmt, env *Environment) Callable {
+	return &Function{fun, env}
 }

globals.go

@@ -2,12 +2,16 @@ package main
 
 import "time"
 
-func defineGlobals(env *Environment) {
+type ClockFun struct{}
 
-	env.define("clock", &Callable{
-		arity: 0,
-		call: func(i *Interpreter, arg ...any) any {
-			return time.Now().Unix()
-		},
-	})
+func (cf *ClockFun) call(i *Interpreter, args ...any) any {
+	return time.Now().Unix()
+}
+
+func (cf *ClockFun) arity() int {
+	return 0
+}
+
+func defineGlobals(env *Environment) {
+	env.define("clock", &ClockFun{})
 }

interpreter.go

@@ -171,18 +171,18 @@ func (i *Interpreter) visitCall(c *Call) any {
 		args = append(args, i.evaluate(arg))
 	}
 
-	callable, ok := callee.(*Callable)
+	callable, ok := callee.(Callable)
 
 	if !ok {
 		i.panic(&RuntimeError{c.paren, "Can only call function and classes."})
 	}
 
-	if callable.arity != len(args) {
+	if callable.arity() != len(args) {
 		i.panic(&RuntimeError{
 			c.paren,
 			fmt.Sprintf(
 				"Expected %d arguments  but got %d",
-				callable.arity,
+				callable.arity(),
 				len(args),
 			),
 		})
@@ -192,7 +192,7 @@ func (i *Interpreter) visitCall(c *Call) any {
 }
 
 func (i *Interpreter) visitFunStmt(f *FunStmt) {
-	i.env.define(f.name.lexeme, newCallable(f))
+	i.env.define(f.name.lexeme, newFunction(f, i.env))
 }
 
 func (i *Interpreter) visitReturnStmt(r *ReturnStmt) {

tests/functions.lox

@@ -42,3 +42,20 @@ fun fib(n) {
 for (var i = 1; i <= 10; i = i + 1) {
     print fib(i);
 }
+
+
+fun makeCounter() {
+    var i = 0;
+
+    fun incr() {
+        i = i + 1;
+        print i;
+    }
+
+    return incr;
+}
+
+var counter = makeCounter();
+
+counter();
+counter();