type checking and error handling for builtins improved.

object/function.go

@@ -12,9 +12,14 @@ type Function struct {
 	Env        *Environment
 }
 
+func (f *Function) Bool() bool {
+	return false
+}
+
 func (f *Function) Type() ObjectType {
 	return FUNCTION_OBJ
 }
+
 func (f *Function) Inspect() string {
 	var out bytes.Buffer
 
@@ -32,6 +37,7 @@ func (f *Function) Inspect() string {
 
 	return out.String()
 }
+
 func (f *Function) String() string {
 	return f.Inspect()
 }
@@ -40,12 +46,18 @@ type ReturnValue struct {
 	Value Object
 }
 
+func (rv *ReturnValue) Bool() bool {
+	return true
+}
+
 func (rv *ReturnValue) Type() ObjectType {
 	return RETURN_VALUE_OBJ
 }
+
 func (rv *ReturnValue) Inspect() string {
 	return rv.Value.Inspect()
 }
+
 func (rv *ReturnValue) String() string {
 	return rv.Inspect()
 }