ast.go: presetting of system variables before starting evaluation process

ast.go

@@ -79,9 +79,44 @@ func (self *ast) insert(tree, node *term) (root *term, err error) {
 
 func (self *ast) eval(ctx exprContext) (result any, err error) {
 	if self.root != nil {
+		initDefaultVars(ctx)
 		result, err = self.root.compute(ctx)
 	} else {
 		err = errors.New("empty expression")
 	}
 	return
 }
+
+// Preset variables
+const (
+	preset_bool_shortcut = "_bool_shortcut"
+)
+
+func initDefaultVars(ctx exprContext) {
+	ctx.SetValue(preset_bool_shortcut, true)
+}
+
+func enable(ctx exprContext, name string) {
+	if strings.HasPrefix(name, "_") {
+		ctx.SetValue(name, true)
+	} else {
+		ctx.SetValue("_"+name, true)
+	}
+}
+
+func disable(ctx exprContext, name string) {
+	if strings.HasPrefix(name, "_") {
+		ctx.SetValue(name, false)
+	} else {
+		ctx.SetValue("_"+name, false)
+	}
+}
+
+func isEnabled(ctx exprContext, name string) (status bool) {
+	if v, exists := ctx.GetValue(name); exists {
+		if b, ok := v.(bool); ok {
+			status = b
+		}
+	}
+	return
+}