type aliases and recursive modules

type aliases and recursive modules

[Josh Berdine]

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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


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

[Philippe Wang]

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

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

[Josh Berdine]

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Hi,

 

Josh Berdine wrote:

> 

> Hi,

> 

> There's something I'm not understanding about recursive modules. 

> Consider the following distilled example:

> 

 

      [snip buggy simplification]

 

> 

> 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

 

 

Thanks Philippe and Christopher, yes, mea culpa, I forgot, e.g., a call to
choose.  But my confusion remains:

 

 

# 

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 (C.choose x))

end

;;

                                      Characters 350-370:

    let get_its_elements x = C.elements (A.get (C.choose x))

                                        ^^^^^^^^^^^^^^^^^^^^

This expression has type ASet.t but is here used with type

  C.t = Set.Make(A).t

#

 

 

Am I still doing something stupid?

 

Cheers,  Josh


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

[Philippe Wang]

Josh Berdine wrote:
>
> Thanks Philippe and Christopher, yes, mea culpa, I forgot, e.g., a 
> call to choose.  But my confusion remains:
>
>  
>
> #
>
> 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 (C.choose x))
>
> end
>
> ;;
>
>                                       Characters 350-370:
>
>     let get_its_elements x = C.elements (A.get (C.choose x))
>
>                                         ^^^^^^^^^^^^^^^^^^^^
>
> This expression has type ASet.t but is here used with type
>
>   C.t = Set.Make(A).t
>
> #
>
>  
>
>  
>
> Am I still doing something stupid?
>
>  
>
> Cheers,  Josh
>
Hi,

Your A.get function takes something of the form "It of something", which 
is of type A.t = It of ASet.t.
There is no such a constructor (It) in,
let get_its_elements x = C.elements (A.get (C.choose x))
So it can't go right when you call A.get...
;-)

Cheers,
--
  Philippe Wang
  mail@philippewang.info

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

[Andreas Rossberg]

"Josh Berdine" <berdine@dcs.qmul.ac.uk>:
> #
> 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 (C.choose x))
> end
>
>                                      Characters 350-370:
>    let get_its_elements x = C.elements (A.get (C.choose x))
>                                        ^^^^^^^^^^^^^^^^^^^^
> This expression has type ASet.t but is here used with type
>  C.t = Set.Make(A).t

You are suffering from the "double vision" problem. This arises when you 
have a recursive module with an abstract type and attempt to cross the 
abstraction boundary recursively. The type system does not make the abstract 
type recursively transparent. Here is a simpler example:

# module rec A : sig type t val f : t -> t end =
  struct type t = int let f (x : t) = A.f x end;;
This expression has type t = int but is here used with type A.t

You cannot do that in current Ocaml. The only way to avoid this problem is 
by making the type t transparent in the ascribed signature.

This problem actually is not straightforward to address on the 
type-theoretic level, and one of the reasons why recursive modules are 
pretty much an open research topic.

Hope this helps,
- Andreas