CODEWITHSUNDEEP
ocaml
jp.rider63
jp.rider63

Reputation: 584

Can I constrain a module's type to be a tuple?

I'm trying to accomplish the following:

module type S = sig
       type 'a t
end

module A = struct
       type 'a t  = 'a list
end

module B = struct
       type ('a,'b) t  = ('a * 'b) list
end

module Make (P : S) = struct
       type 'a t = 'a P.t
end

module Single = struct
       include Make (A)
end

module Tuple = struct
       include Make (B)
end

Basically, I want to reuse the make functor, except for in Tuple, force the type to be a tuple. Is this possible?

I think the module type S is messing it up, which is giving me the error: They have different arities. Maybe it is possible to get this working using first class modules?

Thanks!

Upvotes: 0

Views: 128

Answers (1)

rgrinberg
rgrinberg

Reputation: 9878

What are you trying to accomplish? If you forget about functors for a second you will see that your module B cannot satisfy the signature S. For example:

module B1 = (B : S)

Will give you the same error. The problem here is that your type t in your singature S only accepts 1 type variable. You cannot apply this signature to a module with type t with 2 type variables.

Upvotes: 1

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