Re: why do i have to repeat so much here?

Hi Martin,

A frequently used trick is to define your module type in an ml file to
avoid one repetition:

environment_sig.ml:

module type ENV = sig
  type dict

  type env = {
    dict : dict;
    op : Types.uop
  }
end

environment.mli:

module Make (Dict : sig type t end) : Environment_sig.ENV with type dict =
Dict.t


environment.ml:

module Make (Dict : sig type t end) = struct
  type dict = Dict.t

  type env = {
    dict : dict;
    op : Types.uop
  }
end

Cheers,
  Philippe




2014-08-19 8:22 GMT+02:00 Martin DeMello [email protected]
[ocaml_beginners] <[email protected]>:

>
>
> interesting thread. i usually don't mind repeating myself in the .mli file
> - i think what is annoying me the most here is that i don't really have an
> "implementation"; all i want to do is define a single record type with a
> type parameter. i know i could delete the .mli file and have everything
> work, but that feels wrong when theoretically i want to delete the .ml file
> instead.
>
> thanks for the pointer to ppx_import - at some point i'll probably get
> annoyed enough to use it :) though as you advise, i'll maintain the current
> code for a while first, and see how painful it is to add fields to the
> record in three places every time.
>
> martin
>
>
> On Mon, Aug 18, 2014 at 10:54 PM, Gabriel Scherer
> [email protected] [ocaml_beginners] <
> [email protected]> wrote:
>
>>
>>
>> You may be interested in this long caml-list thread about precisely this
>> question:
>>   https://sympa.inria.fr/sympa/arc/caml-list/2012-10/msg00205.html
>> The short answer is "there is no well-understood way to do better without
>> design hacks".
>>
>> Some people have designed meta-programming tools to macro-away some of
>> the boilerplate (eg. recently https://github.com/whitequark/ppx_import
>> ). I would recommend first trying to do it the usual way, to get a feeling
>> of how sufferable it actually is.
>>
>>
>> On Tue, Aug 19, 2014 at 7:50 AM, Martin DeMello [email protected]
>> [ocaml_beginners] <[email protected]> wrote:
>>
>>>
>>>
>>> I'm learning my way around the module system, and trying to write a
>>> small Environment module that creates a record based on a given type. My
>>> code works, but it looks like i had to essentially write out the record
>>> definition thrice. Is there a better way to do this?
>>>
>>> Here's the definition of the module
>>>
>>> /--------------------------------
>>> $ cat environment.mli
>>>
>>> module type ENV = sig
>>>   type dict
>>>
>>>   type env = {
>>>     dict : dict;
>>>     op : Types.uop
>>>   }
>>> end
>>>
>>> module Make (Dict : sig type t end) : ENV with type dict = Dict.t
>>>
>>> $ cat environment.ml
>>>
>>> module type ENV = sig
>>>   type dict
>>>
>>>   type env = {
>>>     dict : dict;
>>>     op : Types.uop
>>>   }
>>> end
>>>
>>> module Make (Dict : sig type t end) = struct
>>>   type dict = Dict.t
>>>
>>>   type env = {
>>>     dict : dict;
>>>     op : Types.uop
>>>   }
>>> end
>>>
>>> \--------------------------------
>>>
>>> and I'm using it in the following code:
>>>
>>> /--------------------------------
>>> module Evaluator =
>>>   functor (Env : ENV) ->
>>>   functor (E : ENGINE with type dict = Env.dict) ->
>>>   struct ... end
>>>
>>> module Env = Environment.Make (Trie)
>>>
>>> module Eval = Evaluator.Evaluator (Env) (TrieEngine)
>>> \--------------------------------
>>>
>>> martin
>>>
>>>
>>>
>>>
>>
>
>
> 
>