From: Olivier Andrieu <andrieu@ijm.jussieu.fr>
From: Olivier Andrieu <andrieu@ijm.jussieu.fr>
To: Richard Jones <rich@annexia.org>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Polymorphic graph widget problem
Date: Fri, 29 Aug 2003 15:40:20 +0200 [thread overview]
Message-ID: <16207.22468.534018.193998@karryall.dnsalias.org> (raw)
In-Reply-To: <20030829115446.GA879@redhat.com>
Hi, Richard
Richard Jones [Friday 29 August 2003] :
> As part of a project I'm doing at the moment, I've written a Gtk graph
> widget using lablgtk. The graph currently plots ints, so the type
> looks something like this:
>
> class chart :
> ?width:int ->
> ?height:int ->
> ?packing:(GObj.widget -> unit) ->
> ?show:bool ->
> int array -> (* the data being plotted *)
> object
> method repaint : Gdk.Rectangle.t option -> unit
> end
>
> All fine, but now I'd like to generalise this so it can plot floating
> point values as well as int, thus:
>
> class ['a] chart :
> ?width:int ->
> ?height:int ->
> ?packing:(GObj.widget -> unit) ->
> ?show:bool ->
> 'a array -> (* the data being plotted *)
> object
> method repaint : Gdk.Rectangle.t option -> unit
> end
first, you do not need to parametrize your `chart' class if the type
variable doesn't appear in a method signature. I.e. you can do :
class chart : 'a array -> object method repaint : ... end
or even :
class chart : 'a array ->
object val data : 'a array method repaint : ... end
> The problem with this is that at various places in the implementation
> we need to break the polymorphism. eg. To plot Y labels we call
> 'string_of_int', and to work out the height of the Y axis we do some
> sums on the values using the (+) operator.
Here's one solution :
class virtual chart ?width ?height ?packing ?show (data : 'a array) =
object (self)
method private virtual sum : 'a array -> 'a
method private virtual to_string : 'a -> string
method repaint =
(* code using self#to_string and self#sum *)
end
class type chart_t = object method repaint : Gdk.Rectangle.t option -> unit end
class int_chart ?w ?h ?p ?s data : chart_t =
object
inherit chart ?w ?h ?p ?s data
method private to_string = string_of_int
method private sum a = Array.fold_left (+) 0 a
end
class float_chart ?w ?h ?p ?s data : chart_t =
object
inherit chart ?w ?h ?p ?s data
method private to_string = string_of_float
method private sum a = Array.fold_left (+.) 0. a
end
--
Olivier
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next prev parent reply other threads:[~2003-08-29 14:33 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-08-29 11:54 Richard Jones
2003-08-29 13:24 ` Remi Vanicat
2003-08-29 13:27 ` Michal Moskal
2003-08-29 13:40 ` Olivier Andrieu, Olivier Andrieu [this message]
2003-08-29 17:01 ` Issac Trotts
2003-08-29 18:30 ` Matt Gushee
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