Namespacing Variants in ML

I'm also a fan of namespacing variants, and I used it extensively on an OCaml representation of the SpiderMonkey AST. (Coincidentally, up until writing our own parser we were using pfff's JavaScript parser! Go pfff!). Anyway, here are some other useful things I found in OCaml along the way:

Without namespacing, you might be using mutually recursive type definitions like

  type statement = ExpressionStatement of expression
  and expression = FunctionExpression of statement list

to do this with modules you need to use recursive modules, like

  module rec Statement : sig
    type t = Expression of Expression.t
  end = struct
    type t = Expression of Expression.t
  end

  and Expression : sig
    type t = Function of Statement.t list
  end = struct
    type t = Function of Statement.t list
  end

One of the sucky things about recursive modules is that you cannot omit the signatures, however if your modules only include types then there is a shortcut that looks like

  module rec Statement : sig
    type t = Expression of Expression.t
  end = Statement

  and Expression : sig
    type t = Function of Statement.t list
  end = Expression

Links: Big real world example: https://github.com/facebook/flow/blob/3a5b4040b7d2a648f97a06...

Recursive module docs: http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.htm...

Or you can use polymorphic variants in OCaml:

   type color = `Red | `Green | `Blue | `Black | `Brown
   type hair_color = `Black | `Brown | `Blond

One thing to consider though is the grepability of a codebase. By relying more on nested modules, module aliases, opens, local opens, then many constructors have more than one way to be written and it makes it harder for grep to find what you look for. Of course people should not use grep and instead rely on IDE or tools that understand deeply the programming language being edited ... which makes the problem I mentioned go away. But still it's nice to have the possibility to use simple tools like grep and that they would simply work.