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class fields and members can be hidden?
f#

Hello.

Field definitions and member definitions can be hidden.
This behavior, combined with inheritance, confuses me.
Could someone explain to me?
Or is this documented somewhere?

(* This does not work *)
> type c1 = class
val x:int
new (init) = { x = init }
member self.x = 0
end

(* But this works *)
> type c2 = class
val x:int
new (init) = { x = init }
end

> type c3 = class
inherit c2
new (init) = { inherit c2 (init) }
member self.x = 0
end

> let x2 = new c2 3;;
> let x3 = new c3 3;;

> x2.x;;
val it : int = 3

(* the field x inherited from c2 seems to be hidden *)
> x3.x;;
val it : int = 0

(*
A similar behavior is observed with members; this looks more complex...
*)

type c4 = class
new () = {}
member self.f x = x + 1
member self.g (x:int) = self.f x
end;;

type c5 = class
inherit c4
new () = {}
member self.f x = x + 2
member self.g (x:int) = self.f x
end

let x4 = new c4 ();;
let x5 = new c5 ();;

x4.f 0;;
val it : int = 1

x5.f 0;;
val it : int = 2

x4.g 0;;
val it : int = 1

(* internally, i.e., through self, c5 refers to c4's f ? *)
x5.g 0;;
val it : int = 1

Best regards,
keiko

.

Hello Keiko.

As far as I can get, the problem seems to be that in the definition of c3:

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type c3 = class
    inherit c2
    new (init) = { inherit c2 (init) }
    member self.x = 1
end;;

The line "member self.x = 1" does not redefine the field x in class c2, it defines a new member x for class c3 and this member "hides" the field x in class c2.

If you try to evaluate

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let x3 = new c3 4;;
// val x3: c3

x3.x;;
//val it : int = 1
// but...

(x3 :> c2).x;;
// val it : int = 4

So the value of field x is there as it should.

Hope it helps.

By on 11/27/2007 1:13 AM ()

Hello.

Thank you for replies.
I have been thinking for a while, but I still do not understand.

Continuing my previous post:

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type c6 = class
   inherit c5
   new () = {}
   member self.f x = x + 3
   member self.g (x:int) = self.f x
end;;

type c7 = class
  inherit c5
  new () = {}
  member self.f x = x + 3
  member self.g (x:int) = (self :> c7).f x
end;;

let x6 = new c6 ();;
let x7 = new c7 ();;

x6.g 0;;
//val it : int = 2
x7.g 0;;
//val it : int = 2

As seen even I coerce self into c7, c7'self still looks for a method f within c5.
Is there a way to make it search within c7, for instance?
Could you please explain to me?

With best regards,
keiko

By on 11/29/2007 2:13 AM ()

Hi keiko,

I reworked your definitions for c5, c6 and c7. I think it works as you expect now.

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type c5 = class
    inherit c4
    new () = {}
    abstract f: int -> int default self.f x = x + 2
    abstract g: int -> int default self.g (x:int) = self.f x
end;;

type c6 = class
    inherit c5
    new () = {}
    override self.f x = x + 3
    override self.g (x:int) = self.f x
end;;

type c7 = class
    inherit c5
    new () = {}
    override self.f x = x + 3
    override self.g (x:int) = (self :> c7).f x
end;;

let x6 = new c6 ();;

let x7 = new c7 ();;

x6.g 0;;
//val it : int = 3

x7.g 0;;
//val it : int = 3

Also, the class c7 gives a warning "FS0066: This upcast is unnecessary - the types are identical".

Hope it helps.

By on 11/29/2007 3:20 AM ()

Thank you for such a prompt reaction.

Yes, I understand that everything go as expected
if I use this overriding mechanism.
Yet I also want to understand the hiding behavior I have been observing
to know how I can exploit this behavior in my programming practice.

Best regards,
Keiko

By on 11/29/2007 4:15 AM ()

BTW,

I thought that the line "member self.x = 1" would raise an error, because "self" is not automatically defined. The fact that there as no error really surprised me.

Could somebody explain that?

Thanks.

By on 11/27/2007 1:17 AM ()

You don't need to the same reference name for all methods, nor do you need declaring what reference name(s) you will use at the beginning of the class (as in OCaml where you specify it at the "top" of the class, but never reference it afterwards). The following works :

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type foo() =
  member x.doit() = print_string "subliminal message"
  member y.run() = ()
  member z.run_again() = ()

If you want to use references to inherited class, you simply have to give an alias for that class, as in :

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type bar()=
  inherit foo () as foo
  member x.doit() = foo.doit() ; print_string " : use F# !!!"
By on 11/27/2007 6:18 AM ()

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