# List with GADTs
**URL:** https://discuss.ocaml.org/t/list-with-gadts/10104
**Category:** Learning
**Created:** [July 1, 2022, 6:11am UTC](https://discuss.ocaml.org/t/list-with-gadts/10104 "2022-07-01T06:11:01Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@Jazz](https://discuss.ocaml.org/u/Jazz)
#### Post date: [July 1, 2022, 6:11am UTC](https://discuss.ocaml.org/t/list-with-gadts/10104/1 "2022-07-01T06:11:01Z")
Hi,
I am learning GADTs and found the below snippet at [ocamlgadt](https://sites.google.com/site/ocamlgadt/). I find that using GADTs, OCaml can check if a list passed to a function is empty or not at compile time!
I have an issue with the snippet though - the list is heterogeneous.
How could i make it homogeneous without breaking `hd` function?
Thanks.
```auto
module GList = struct
type zero
and _ t =
| [] : zero t
| ( :: ) : 'a * 'b t -> ('a * 'b) t
let hd : type a. (a * _) t -> a = function (h :: _) -> h ;;
end ;;
GList.hd (["hello"; 1.1; 1; 'a']: (_ * _) GList.t) ;;
GList.hd ([] : (_ * _) GList.t) ;;
```
---
### Author:  [@silene](https://discuss.ocaml.org/u/silene)
#### Post date: [July 1, 2022, 8:18am UTC](https://discuss.ocaml.org/t/list-with-gadts/10104/2 "2022-07-01T08:18:18Z")
There are lots of different ways to do it. Here is a possibility. It changes the type family `t` so that its first argument is the type of the list elements and the second one is a type-level boolean that indicates whether the list has elements:
```ocaml
type (_, _) t =
| [] : ('a, float) t
| ( :: ) : 'a * ('a, 'b) t -> ('a, unit) t
let hd : type a. (a, unit) t -> a = function (h :: _) -> h
```