The Easiest Way to Build a Type Checker

I recently made a toy type checker for Python and had a lot of fun.

https://austinhenley.com/blog/babytypechecker.html

I grabbed the code from the article and annotated it with the different cases from the famous picture*

  switch (expr.kind) {
    case "number"/"string"/"var":
      ... [Var]
    case "call":
      ... [App]
    case "function":
      throw new Error("...[Abs]")
    case "let":
      ... [Let]

* https://wikimedia.org/api/rest_v1/media/math/render/svg/10d6...

I thought the implementation here was how hindley Milner worked? I guess not?