Re: [Caml-list] type aliases and recursive modules

[Philippe Wang <lists@philippewang.info>]

Josh Berdine wrote:
>
> Hi,
>
> There’s something I’m not understanding about recursive modules. 
> Consider the following distilled example:
>
> #
>
> module rec A : sig
>
> type t = It of ASet.t
>
> val compare : t -> t -> int
>
> val get : t -> ASet.t
>
> end = struct
>
> type t = It of ASet.t
>
> let compare = compare
>
> let get = function It(x) -> x
>
> end
>
> and ASet : sig
>
> type t
>
> val get_its_elements : t -> A.t list
>
> end = struct
>
> module C = Set.Make(A)
>
> type t = C.t
>
> let get_its_elements x = C.elements (A.get x)
>
> end
>
> ;;
>
> Characters 350-359:
>
> let get_its_elements x = C.elements (A.get x)
>
> ^^^^^^^^^
>
> This expression has type ASet.t but is here used with type
>
> C.t = Set.Make(A).t
>
> #
>
> Why doesn’t the typechecker know that ASet.t and C.t are the same 
> type? Anyone know a workaround?
>
> This is with the 3.10.0 beta version, but seems to be the same back to 
> 3.07.
>
> Cheers, Josh
>

Hi,

There is an incoherency in your definition of ASet.get_its_elements.

In the signature of ASet, get_its_elements take an argument of type 
ASet.t, but then in its definition, it takes an argument of type A.t (as 
A.get takes an argument of type A.t, and you give ASet.get_its_elements' 
argument to A.get)

--
Philippe Wang
mail@philippewang.info