Local type definition

Learning
1

Hello everyone,

As far as I know, there is no way to locally define a type, is there a reason for that? I know you can always create polymorphic variants on the fly, but I feel it might be nice to be able to do that.

Example of application:

type t = | A1 | A2 | A3 | B1 | B2 | B3

let f (x:t): string * int =
  let type radix = | A | B in
  let radix = function
  | A1 | A2 | A3 -> A | _ -> B
  in
  begin match radix with
  | A -> do_something ()
  | B -> do_something_else ()
  end ;
  do_yet_something_else () ;
  begin match radix with
  | A -> return_something
  | B -> return_something_else

Obviously, one can find a lot of alternative ways to solve this. My question is “Why would we not allow that” rather than “How else can we do that”.

Any attempt at an answer appreciated :slight_smile:

3

Oh yes of course, local modules, I don’t know how I didn’t think about it…

Thank you!

4

Directly doing let type ... is indeed not possible at the moment, but might be in the future as there is an open PR about this (on which I probably need to spend time to give it more chances of being merged)

5

Note that such way to define a new local type involves an allocations which can be hard to track (and probably uneeded for the purpose).

6

No it does not. Perhaps you are confusing this construct with the one used to extend an already existing extensible type (e.g., exceptions)?

7

That’s indeed true if you don’t define & use functions into this module.

8

Almost three years alter, note that this will become possible in 5.5! Along with many other let xxx ... in constructs.