Re: [Caml-list] Polymorphic graph widget problem

[Michal Moskal <malekith@pld-linux.org>]

On Fri, Aug 29, 2003 at 12:54:46PM +0100, Richard Jones wrote:
> 
> 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
> 
> 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.
> 
> What I need is some way to isolate the parts which break the
> polymorphism.  How do I do that?

Put this 'a array inside an object with string_of_int and (+) functions.
Then parametrize chart with 'a, where 'a :> that object. Finally provide
functions to embed int array (or float array) in this kind of object.
You can also try to wait for generics to fix this problem.

-- 
: Michal Moskal :: http://www.kernel.pl/~malekith : GCS {C,UL}++++$ a? !tv
: When in doubt, use brute force. -- Ken Thompson : {E-,w}-- {b++,e}>+++ h

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