[Caml-list] Typing problem with polymorphic variants

[Caml-list] Typing problem with polymorphic variants

[Alessandro Baretta]

This just baffles me.

 > Values do not match:
 >   val feature :
 >     [> `Clear | `Length of int | `Pitch of int
 >               | `Width of 'a] -> string
 > is not included in
 >   val feature : [> `Clear] -> string

Why are "match-anything" functions of the kind of

let f = function
   | `Foo -> "foo"
   | `Bar -> "bar"
   | _ -> "?"

incompatible with signatures like

val f : [> `Foo ] -> string
?


There clearly is no case when a value of type [> `Foo] will 
not be acceptable for f. So why does the type checker reject 
this?

Alex

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Re: [Caml-list] Typing problem with polymorphic variants

[Alessandro Baretta]

Ville-Pertti Keinonen wrote:
> 
> On Tuesday, Apr 1, 2003, at 20:34 Europe/Helsinki, Alessandro Baretta 
> wrote:
> 
>> This just baffles me.
>>
>> > Values do not match:
>> >   val feature :
>> >     [> `Clear | `Length of int | `Pitch of int
>> >               | `Width of 'a] -> string
>> > is not included in
>> >   val feature : [> `Clear] -> string
> 
> 
> Consider what would happen if you tried to pass a `Length of float, 
> which is acceptable according to the latter signature.
> 
> 

Thank you very much.

This is actually a meaningful comment. I wonder if there is 
any sensible way to do what I'm trying to do.

I'll think it over.

Alex

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Re: [Caml-list] Typing problem with polymorphic variants

[Ville-Pertti Keinonen]

On Tuesday, Apr 1, 2003, at 20:34 Europe/Helsinki, Alessandro Baretta 
wrote:

> This just baffles me.
>
> > Values do not match:
> >   val feature :
> >     [> `Clear | `Length of int | `Pitch of int
> >               | `Width of 'a] -> string
> > is not included in
> >   val feature : [> `Clear] -> string

Consider what would happen if you tried to pass a `Length of float, 
which is acceptable according to the latter signature.

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Re: [Caml-list] Typing problem with polymorphic variants

[David Brown]

On Tue, Apr 01, 2003 at 07:34:24PM +0200, Alessandro Baretta wrote:

> Why are "match-anything" functions of the kind of
> 
> let f = function
>   | `Foo -> "foo"
>   | `Bar -> "bar"
>   | _ -> "?"
> 
> incompatible with signatures like
> 
> val f : [> `Foo ] -> string
> ?
> 
> There clearly is no case when a value of type [> `Foo] will 
> not be acceptable for f. So why does the type checker reject 
> this?

I would apply the same question to the object system: why can a
signature not hide methods of a class.

Does making the signature more restrictive somehow hurt the type system?

e.g.:

  class foo : object method baz : int end =
    object
      method baz = 5
      method bar = 2
    end

gives:

  The class type object method bar : int method baz : int end
  is not matched by the class type object method baz : int end
  The public method bar cannot be hidden

It seems to me this is the same kind of thing where struct's can contain
definitions that are not in the signature, and that can be used to hide
them.

Is there some underlying reason that I'm missing?  Perhaps assumptions
the type system makes about the signature being complete?

Dave Brown

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Re: [Caml-list] Typing problem with polymorphic variants

[Jacques Garrigue]

From: David Brown <caml-list@davidb.org>

> I would apply the same question to the object system: why can a
> signature not hide methods of a class.
> 
> Does making the signature more restrictive somehow hurt the type system?
> 
> e.g.:
> 
>   class foo : object method baz : int end =
>     object
>       method baz = 5
>       method bar = 2
>     end
> 
> gives:
> 
>   The class type object method bar : int method baz : int end
>   is not matched by the class type object method baz : int end
>   The public method bar cannot be hidden


Just consider now a variant on your example:

  class foo : object method baz : int end =
    object (self)
      method baz = self#bar + 3
      method bar = 2
    end

  class bar = object
    inherit foo
    method bar = "hello"
  end

If you can forget the method bar, the class bar becomes typable.
However (new bar)#baz lead to computing ["hello" + 3] which is
unsound.

You can try to change the problem in many ways, but the conclusion is
that there is no way to allow the hiding of a public method without a
notion of generative types for objects or views. There is a paper by
Jerome Vouillon on the subject
  http://www.pps.jussieu.fr/~vouillon/publi.html
  Combining subsumption and binary methods: An object calculus with views.

Jacques Garrigue

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