Re: [Caml-list] small typing problem with modules ...

[Andreas Rossberg <rossberg@mpi-sws.org>]

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On May 26, 2009, at 14.37 h, Julien Signoles wrote:
>
>> My problem here is that I'd like to have a module type T for the  
>> functor
>> Make in b.ml and to avoid the signature duplication in b.mli ...  
>> ideally
>> I'd like to write in b.mli something like :
>> module type T = sig ???? end
>> module Make : functor (Extra : A.Extra) -> functor (A : A.T with  
>> type extra  = Extra.t) -> T with ...
>> include T with ...
>> It's clear that I cannot write a module type as follows because the  
>> the "open A" will consider the default type of A and not it's
>> parametrized version ...
>> module type T = sig
>>  open A
>>  val f : extra -> unit
>> end
>> how can I parametrize this signature ? I can write something like :
>> module type T = functor (A : A.T) ->   sig
>>    open A
>>    val f : extra -> unit end ;;
>> but then I'm a bit lost putting all this together ... I know I'm  
>> close ... a small hint ?
>
> Hum, module types cannot be parameterized.

Actually, they can. You just have to put them into a functor. For  
example:

# module TF (A : sig type t end) =
   struct
     module type T = sig open A val x : t end
   end;;
module TF :
   functor (A : sig type t end) -> sig module type T = sig val x : A.t  
end end
# module I = struct type t = int end;;
module I : sig type t = int end
# module M : TF(I).T = struct let x = 9 end;;
module M : TF(I).T

- Andreas


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