Re: [Caml-list] Default value in module

[Martin Jambon <mjambon@gmail.com>]

On 09/12/2017 02:49 PM, Paul A. Steckler wrote:
> On Tue, Sep 12, 2017 at 5:40 PM, Alain Frisch <alain.frisch@lexifi.com> wrote:
>> What would be allowed as a default value is not completely clear (you don't
>> want to carry runtime values with module types).
> 
> Yes, propagating the default value sounds like the main implementation obstacle.

A possible solution inspired from atdgen is to support only expressions 
that come with no uncertain dependency and no "computation", i.e. 
literal constants of core types. Core types here would include unit, 
bool, int, float, string, and list/option of a core type, and a few more 
(int64 etc.) and would exclude mutable values, records, variants other 
than list/option, and objects.

Then we could write, using atd's conventions for `~` and `?`:

module type A = sig
   val ~x : int = 42 (* just a literal,
                        whose type can be inferred syntactically *)
end

module A1 : A = struct
   let x = 17
end

module A2 : A = struct
   (* implicit `let x = 42` *)
end

Additionally, we could also have canonical defaults:

bool: false
int: 0
float: 0.
string: ""
option: None
list: []

These being common and intuitive defaults, it makes sense to support 
them. So we could write:

module type A = sig
   val ~x : int
end

module A1 : A = struct
   let x = 17
end

module A3 : A = struct
   (* implicit `let x = 0` *)
end

I think this feature would be most useful if it was supported on record 
types.

A record definition equivalent to the module signature above would be:

type a = {
   ~x: int;         (* default value is 0 *)
   ~y: int = 42;    (* default value is 42 *)
   ?z: int option;  (* default value is None *)
}

let a1 : a = {}
   (* equals `{ x = 0; y = 42; z = None }` *)

Martin