[Caml-list] 'b is unbound

[Caml-list] 'b is unbound

[briand]

I'm getting the following error:

      method op :
        (float, 'a, 'b) Bigarray.Array2.t ->
        float array -> float array -> unit
    end
The method op has type
  (float, 'a, 'b) Bigarray.Array2.t -> float array -> float array -> unit
where 'b is unbound

Is 'b really unbound, or is the inferencer simply unable to figure out
what the type should be ?

I tried explicitly setting the type for the first argument, but could
not create a type which would not give me some sort of error, e.g.

method op (j: (float, Bigarray.float64, Bigarray.c_layout) Bigarray.Array2.t) ...

gives me an error :

Unbound type constructor Bigarray.float64

Any help appreciated...




Brian

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Re: [Caml-list] 'b is unbound

[Damien]

On Tue, 11 May 2004 23:31:48 -0700 briand@aracnet.com wrote:
 
> I'm getting the following error:
> 
>       method op :
>         (float, 'a, 'b) Bigarray.Array2.t ->
>         float array -> float array -> unit
>     end
> The method op has type
>   (float, 'a, 'b) Bigarray.Array2.t -> float array -> float array ->
>   unit
> where 'b is unbound
> 
> Is 'b really unbound, or is the inferencer simply unable to figure out
> what the type should be ?

polymorphic methods require to be explicitly typed
(this kind of type inference should lead to the one for system F)

# class o = object method id x = x end;;
Some type variables are unbound in this type:
  class o : object method id : 'a -> 'a end
The method id has type 'a -> 'a where 'a is unbound

# class o = object method id: 'a.'a -> 'a = fun x -> x end;;
class o : object method id : 'a -> 'a end
# let o = new o in o#id 5, o#id "toto";;
- : int * string = (5, "toto")


or maybe you want the whole class to be parametrized wrt 'a :

# class ['a] o = object method id (x:'a) = x end;;
class ['a] o : object method id : 'a -> 'a end
# let o = new o in o#id 5, o#id "toto";;
This expression has type string but is here used with type int

hope this helps,
damien

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Re: [Caml-list] 'b is unbound

[briand]

>>>>> "Damien" == Damien  <Damien.Pous@ens-lyon.fr> writes:

  Damien> On Tue, 11 May 2004 23:31:48 -0700 briand@aracnet.com wrote:
  >> I'm getting the following error:
  >> 
  >> method op :
  >> (float, 'a, 'b) Bigarray.Array2.t ->
  >> float array -> float array -> unit
  >> end
  >> The method op has type
  >> (float, 'a, 'b) Bigarray.Array2.t -> float array -> float array ->
  >> unit
  >> where 'b is unbound
  >> 
  >> Is 'b really unbound, or is the inferencer simply unable to figure out
  >> what the type should be ?

  Damien> polymorphic methods require to be explicitly typed
  Damien> (this kind of type inference should lead to the one for system F)

I'm not intending the method to be polymorphic.
op is simply a method which takes a Bigarray.Array2 type as an argument.

So it would seem that declaring the argument explicitly should solve
the problem.


  Damien> # class o = object method id: 'a.'a -> 'a = fun x -> x end;;
  Damien> class o : object method id : 'a -> 'a end
  Damien> # let o = new o in o#id 5, o#id "toto";;
  Damien> - : int * string = (5, "toto")

Here you are making the argument and return types consistent.
I believe that my problem is different.

It doesn't help that I don't understand what the problem is...


Brian

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Re: [Caml-list] 'b is unbound

[briand]

>>>>> "Damien" == Damien  <Damien.Pous@ens-lyon.fr> writes:

  Damien> polymorphic methods require to be explicitly typed (this
  Damien> kind of type inference should lead to the one for system F)

After little more investigation I found that the following will work :

    method op (j:(float, Bigarray.float64_elt, Bigarray.c_layout) Bigarray.Array2.t) f x =

Seems very straightforward.  However there is one confusing aspect :

    method op (j:(float, Bigarray.float64, Bigarray.c_layout) Bigarray.Array2.t) f x =

   error -> Unbound type constructor Bigarray.float64

? Aha ! float64 in bigarray.mli is not a type but a val.

So now my method is no longer polymorphic, right ?


So everything makes sense until I go back to my bigarray test file :

   let a = Bigarray.Array2.create Bigarray.float64 Bigarray.c_layout n n;;

is OK.

   let a = Bigarray.Array2.create Bigarray.float64_elt Bigarray.c_layout n n;;

   error -> Unbound value Bigarray.float64_elt

Then I looked at the definition for create and it finally makes sense.

However I would argue that it is also _very_ confusing.

Why doesn't create simply take the type, i.e. ..._elt instead of 
('a, 'b) kind ??


Brian

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