diff --git a/src/twopass.rs b/src/twopass.rs
index cb146be..9975875 100644
--- a/src/twopass.rs
+++ b/src/twopass.rs
@@ -1,6 +1,6 @@
 use std::cell::RefCell;
 
-use crate::ast;
+use crate::{ast, types};
 
 #[derive(Debug, Clone)]
 pub struct OpenFunction(Function);
@@ -240,3 +240,21 @@ fn build_function(parent: &BuildContext<'_>, code: &ast::Function) -> Function {
 pub fn build(code: &ast::Function) -> OpenFunction {
 	OpenFunction(build_function(&BuildContext::new_toplevel(), &code))
 }
+
+impl Function {
+	fn call(&self, upvalues: &[types::Value], args: &[types::Value]) -> types::Value {
+		assert_eq!(upvalues.len(), self.upvalues.len());
+		assert_eq!(args.len(), self.args.len());
+		todo!()
+	}
+}
+
+impl OpenFunction {
+	pub fn call(&self, env: types::Value, args: &[types::Value]) -> types::Value {
+		match self.0.upvalues.as_slice() {
+			[] => self.0.call(&[], args),
+			[up] if up.0 == "_ENV" => self.0.call(&[env], args),
+			_ => unreachable!(),
+		}
+	}
+}