[Caml-list] signature mismatch

[Caml-list] signature mismatch

[Pietro Abate]

Hi all,
I get this error trying to compile the file below:

File "pp.ml", line 28, characters 22-24:
Signature mismatch:
Modules do not match:
  sig type t = BB.t val toast : t -> t -> bool end
is not included in
  sig type t = AA.t val toast : t -> t -> bool end
Type declarations do not match:
  type t = BB.t
is not included in
  type t = AA.t

how can I force BB.t to be the same as AA.t ?
why it's not enforced by the functor declaration (with type...) ?

tnx,
p

file :

-----------------
module type A1 = sig
    type t 
    val test : t -> t -> bool
end

module type A2 = sig
    type t 
    val toast : t -> t -> bool
end

module Make (A : A1 ) (B : A2 with type t = A.t) = struct
    let fifi a b c =
        if c > 1 then A.test a b 
        else B.toast b a
end

module AA : A1 = struct 
    type t = int
    let test f1 f2 = true
end

module BB : A2 = struct
    type t = int
    let toast f1 f2 = false
end

module C = Make (AA) (BB)

---------------------

-- 
Civilization advances by extending the number
of important operations which we can perform 
without thinking. (Alfred North Whitehead)

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Re: [Caml-list] signature mismatch

[Damien Pous]

AA.t and BB.t are two distinct abstract types, so they can't be unified,

you should export the type definitions in AA and BB : 

module AA : A1 with type t = int = struct 
    type t = int
    let test f1 f2 = true
end

module BB : A2 with type t = int = struct
    type t = int
    let toast f1 f2 = false
end

module C = Make (AA) (BB)


damien


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Re: [Caml-list] signature mismatch

[Jean-Christophe Filliatre]

Pietro Abate writes:
 > Hi all,
 > I get this error trying to compile the file below:
 > 
 > File "pp.ml", line 28, characters 22-24:
 > Signature mismatch:
 > Modules do not match:
 >   sig type t = BB.t val toast : t -> t -> bool end
 > is not included in
 >   sig type t = AA.t val toast : t -> t -> bool end
 > Type declarations do not match:
 >   type t = BB.t
 > is not included in
 >   type t = AA.t
 > 
 > how can I force BB.t to be the same as AA.t ?
 > why it's not enforced by the functor declaration (with type...) ?

Types AA.t and BB.t are abstract, due to the declarations "module AA :
A1" and "module BB : A2" and  to the fact that "t" is abstract in both
interfaces A1 and A2. You should write instead

	module AA : A1 with type t = int = ...
	module BB : A2 with type t = int = ...
	module C = Make (AA) (BB)

(Another possibility is  to declare "type t =  int" in both signatures
A1 and A2 but it is probably not what you want to do.)

-- 
Jean-Christophe Filliâtre (http://www.lri.fr/~filliatr)


 > -----------------
 > module type A1 = sig
 >     type t 
 >     val test : t -> t -> bool
 > end
 > 
 > module type A2 = sig
 >     type t 
 >     val toast : t -> t -> bool
 > end
 > 
 > module Make (A : A1 ) (B : A2 with type t = A.t) = struct
 >     let fifi a b c =
 >         if c > 1 then A.test a b 
 >         else B.toast b a
 > end
 > 
 > module AA : A1 = struct 
 >     type t = int
 >     let test f1 f2 = true
 > end
 > 
 > module BB : A2 = struct
 >     type t = int
 >     let toast f1 f2 = false
 > end
 > 
 > module C = Make (AA) (BB)
 > 
 > ---------------------
 > 
 > -- 
 > Civilization advances by extending the number
 > of important operations which we can perform 
 > without thinking. (Alfred North Whitehead)
 > 
 > -------------------
 > To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
 > Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
 > Beginner's list: http://groups.yahoo.com/group/ocaml_beginners

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Re: [Caml-list] signature mismatch

[Andreas Rossberg]

Pietro Abate wrote:
> 
> how can I force BB.t to be the same as AA.t ?

module BB : A2 with type t = A1.t = struct
     type t = A1.t
     let toast f1 f2 = false
end

 > why it's not enforced by the functor declaration (with type...) ?

Well, it is, but probably in a sense different from what you think. 
That's why the compiler complains, because the constraint is not 
satisfied for the arguments you pass to the functor.

When you write

> module AA : A1 = ...

You declare module AA and introduce an abstract type t. That means that 
AA.t is a fresh type, distinct from any other type in your program. 
Likewise with

> module BB : A2 = ...

In particular, both types are distinct from each other (and int!). 
Consequently,

> module C = Make (AA) (BB)

is not well typed, because the constraint you state on Make's arguments 
requires AA.t and BB.t to be equivalent, which they aren't.

The solution, as sketched above, is to avoid type abstraction for at 
least one of the modules.

Hope this helps,

| Andreas

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

"Computer games don't affect kids; I mean if Pac Man affected us
  as kids, we would all be running around in darkened rooms, munching
  magic pills, and listening to repetitive electronic music."
  - Kristian Wilson, Nintendo Inc.

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Re: [Caml-list] signature mismatch

[Julien Signoles]

On Tue, 17 Jun 2003, Andreas Rossberg wrote:

> Pietro Abate wrote:
> >
> > how can I force BB.t to be the same as AA.t ?
>
> module BB : A2 with type t = A1.t = struct
>      type t = A1.t
>      let toast f1 f2 = false
> end

It does not work because A1 is used as a structure identifier.
It's ok if you replace A1 by AA.

Julien
-- 
mailto : Julien.Signoles@lri.fr ; http : www.lri.fr/~signoles
"In theory, practice and theory are the same,
but in practice they are different" (Larry McVoy)

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Re: [Caml-list] signature mismatch

[Andreas Rossberg]

Julien Signoles wrote:
> On Tue, 17 Jun 2003, Andreas Rossberg wrote:
> 
>>Pietro Abate wrote:
>>
>>>how can I force BB.t to be the same as AA.t ?
>>
>>module BB : A2 with type t = A1.t = struct
>>     type t = A1.t
>>     let toast f1 f2 = false
>>end
> 
> It does not work because A1 is used as a structure identifier.
> It's ok if you replace A1 by AA.

Of course, you are completely right. Should have reread my reply before 
posting...

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

"Computer games don't affect kids; I mean if Pac Man affected us
  as kids, we would all be running around in darkened rooms, munching
  magic pills, and listening to repetitive electronic music."
  - Kristian Wilson, Nintendo Inc.

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