Learning OCaml: Module Aliases

OCaml is famous for allow you to do a lot of things like modules. Like really a lot! Advanced features like functors, aside, it’s really common to either alias module names to something shorter or localize open Module_name to a smaller scope:

(* module alias *)
module P = Printf
P.printf "Hello, %s!" "world"

(* open module for subsequent scope *)
let open Printf in
let portfolio = List.map parse_line portfolio_lines in
List.iter (fun (ticker, shares, price) ->
  printf "%s: %d shares at $%.2f\n" ticker shares price
) portfolio;
let total = total_value portfolio in
printf "Total portfolio value: $%.2f\n" total

(* open module for an expression *)
List.([1; 2; 3; 4; 5] |> map (fun x -> x * 2) |> fold_left (+) 0);;

All of them have their uses, but I’d like to also mention one less known approach - namely a scoped module alias:

let module P = Printf in
let portfolio = List.map parse_line portfolio_lines in
List.iter (fun (ticker, shares, price) ->
  P.printf "%s: %d shares at $%.2f\n" ticker shares price
) portfolio;
let total = total_value portfolio in
P.printf "Total portfolio value: $%.2f\n" total

I think in some way that’s the best of both worlds as it makes it obvious that certain functions are coming from a module, and you’re still not doing that much extra typing. Finding the right balance between conciseness, readability and maintainability is never easy, though.

Note: Interestingly OCaml’s younger sibling F# chose not to implement scoped module opens at all. I’m guessing this happened due to maintainability concerns.

What are your thoughts on the subject? In which situations would you prefer let open Module in over a local module alias and vice versa?

That’s all I have for you today. Keep hacking!