CODEWITHSUNDEEP
ocaml
Kostya
Kostya

Reputation: 1102

Include submodule of a functor application

I have this functor that has a submodule:

module type PT = sig
    type t       
    val to_string: t -> string
end

module A(P:PT) = struct
    module SubA = struct
        type t = T of P.t | F of float
    end
end

And I want to extend it using an include. This even works: 

module Aplus(P:PT) = struct
    include A(P)
    let print x = print_string (P.to_string x)
end

But for some reason it doesn't work for submodules.

module Aplus(P:PT) = struct
    include A(P)
    let print x = print_string (P.to_string x)
    module SubAplus = struct
        include A(P).SubA (* Syntax error here *)
    end
end

It fails with a Syntax error on the submodule reference after a functor application. That is quite strange, since it looks like the language grammar allows that. Is there any specific reason for disallowing that?

Upvotes: 2

Views: 135

Answers (2)

Drup
Drup

Reputation: 3739

Easy enough:

module M = F(A)
include M.SubModule

There are various possible reason, but one simple is: if the module resulting of the functor application F(A) is not named, you will not be able to refer to it. If the function introduces type equalities in the context, it will lead to issues (and in error messages in particular ...)

Note that it's possible to do it with types, module types and class types. F(M).M.t is a perfectly valid type expression.

To finish, in your case, you don't need to reapply the functor, you already did earlier, so include SubA will be enough.

Upvotes: 2

ivg
ivg

Reputation: 35210

OCaml syntax in a module path, allows you only to reference to module types, type constructors and types:

For referring to type constructors, module types, or class types, the prefix can also contain simple functor applications (as in the syntactic class extended-module-path above) in case the defining module is the result of a functor application.

In other words to access to real values, you need to instantiate your module, and then you can reference any values. See Drup's answer for a complete example.

Upvotes: 2

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