Re: [Caml-list] Default value in module

[Martin Jambon <mjambon@gmail.com>]

On 09/12/2017 02:29 PM, Paul A. Steckler wrote:
> On Tue, Sep 12, 2017 at 5:21 PM, Yotam Barnoy <yotambarnoy@gmail.com> wrote:
>> The easiest thing to do is to create a prototype module with the value
>> you want, and to include that module (include Foo) in any module you
>> want to have that basic structure.
> 
> Yes, but that means I have to write "include Foo" to get the default
> value. My goal is to avoid writing anything extra when I'm using the
> default value.

I personally find this solution quite reasonable. You'd write this:

module type A = sig
   val x : int
end

module A_defaults = struct
   let x = 42
end

module A1 : A = struct
   include A_defaults
   let x = 17
end

module A2 : A = struct
   include A_defaults
end

which is really nicely written as:

module A2 : A = A_defaults

Now if you have extra module items to add or to override, the cost of 
inserting the line `include A_defaults` becomes quickly negligible, as 
in the `B1` definition below:

module type B = sig
   val x : int
   val y : int
   val z : int
end

module B_defaults = struct
   let x = 42
   let y = 17
end

module B1 : B = struct
   include B_defaults
   let y = 21
   let z = -1
end

Martin