[Caml-list] Polymorphic graph widget problem

[Caml-list] Polymorphic graph widget problem

[Richard Jones]

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?

Rich.

-- 
Richard Jones. http://www.annexia.org/ http://freshmeat.net/users/rwmj
Merjis Ltd. http://www.merjis.com/ - all your business data are belong to you.
 All new technology is irrelevant until it is taken up by the public.

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Re: [Caml-list] Polymorphic graph widget problem

[Remi Vanicat]

Richard Jones <rich@annexia.org> writes:

> 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?

Well, as defined, your class should work not only on int and real but
also on string, Array of file descriptor, or whatever. It seem hard. 

In fact, you need to give to your class not only the data, but also
the function for manipulating those data :

class ['a] chart :
   ?width:int ->
   ?height:int ->
   ?packing:(GObj.widget -> unit) ->
   ?show:bool ->
   'a array ->  (* the data being plotted *)
   ('a -> string) -> (* a function to have the string representation
                        of the data *)
   ('a -> int) -> (* a function to have an aproximation of the data *)
   object
     method repaint : Gdk.Rectangle.t option -> unit
   end


(well, you might want to choose some other type but you have the idea)

-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat

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Re: [Caml-list] Polymorphic graph widget problem

[Michal Moskal]

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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Re: [Caml-list] Polymorphic graph widget problem

[Olivier Andrieu, Olivier Andrieu]

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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Re: [Caml-list] Polymorphic graph widget problem

[Issac Trotts]

If you'll only be graphing ints and floats, why not just use floats and 
cast to them?  To support both, you could try this:


type number = Int of int | Float of float

class chart :
...
  number array ->  (* the data being plotted *)
  object
    method repaint : Gdk.Rectangle.t option -> unit
  end



It might also help to think of the array as a map from ints to floats.  
Then you could write this instead:



class chart :
...
  (int * int * (int -> float)) ->  (* x0, x1, the data being plotted *)
  object
    method repaint : Gdk.Rectangle.t option -> unit
  end



- ijt


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?
>
>Rich.
>
>  
>


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Re: [Caml-list] Polymorphic graph widget problem

[Matt Gushee]

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:

Hi, Richard--

I am working on a vaguely similar application. I say vaguely, because
the problem domain is very similar, but the practical constraints are
very different: my application also creates charts based on numeric (and
textual) data, but the input will usually be represented as strings (SQL
results and maybe XML data sets) and creates bitmap graphics for the
web. So my approach is necessarily quite different from yours, and I
don't know if any of my techniques will be useful to you. But just in
case it will help, let me briefly describe what I am doing.

 * The two main active components in the system are the module types
   CHART_TYPE and RENDERER, defined as follows:

   module type CHART_TYPE =
     sig
       type t
       val create : string list list -> t
       val drawing : t -> cp_drawing
       val raw_drawing : t -> cp_drawing   (* W/O labels, axes, etc. -
                                              useful for composite
                                              charts *)
     end

   module type RENDERER =
     sig
       type t
       val create : output_spec -> style -> t
       val reset : t -> output_spec -> t
       val render : cp_drawing -> unit
     end

 * All modules conforming to CHART_TYPE will produce an instance of
   the 'cp_drawing' type, which is then passed to a RENDERER to
   produce an actual image. The 'cp_drawing' type represents an 
   abstract drawing--by which I mean that it includes shapes such as
   Rectangle, Ellipse, and so on, but all dimensions are expressed in
   an abstract way, and graphical properties such as line width, fill,
   etc. are unspecified (see example below).

 * All numbers are handled internally as floats. Whether they are 
   *displayed* as floats or integers will be based on a style specified
   by the user, who presumably knows which is most appropriate to the
   data being charted.


EXAMPLE (this is an actual toplevel session, edited for clarity)

    (* Create a chart, then a drawing based on it. *)
    let data = [["A";"121.8"];["B";"84.3"];["C";"98"];["D";"108.5"]];;
    let ch = SimpleColumnChart.create data;;
    let dr = SimpleColumnChart.drawing ch;;
    val dr : Chartpak.cp_drawing =
      {drawing_id = "simple_column_chart"; components = <abstr>}

    (* Here are the components--first the four columns *)
    {id = "rect-0"; tags = []; label = SingleLabel "A";
      figure = Rectangle (Slot 1, Start, AutoSizeCentered, Length 121.8)}
    {id = "rect-1"; tags = []; label = SingleLabel "B";
      figure = Rectangle (Slot 2, Start, AutoSizeCentered, Length 84.3)}
    {id = "rect-2"; tags = []; label = SingleLabel "C";
      figure = Rectangle (Slot 3, Start, AutoSizeCentered, Length 98.)}
    {id = "rect-3"; tags = []; label = SingleLabel "D";
      figure = Rectangle (Slot 4, Start, AutoSizeCentered, Length 108.5)}

    (* And here are the vertical and horizontal axes. Note that the 
       length of the vertical axis is wrong -- it should be at least as 
       long as the longest column. Thanks for helping me discover this 
       bug! *)
    {id = "line-0"; tags = []; label = FloatSeries (0., 100., 10.);
      figure = Line (Start, Start, AutoSizeCentered, Length 100.)}
    {id = "line-1"; tags = []; label = NoLabel;
      figure = Line (Start, Start, SpanTo (Slot 4), AutoSizeCentered)}
                       
                       
-- 
Matt Gushee                 When a nation follows the Way,
Englewood, Colorado, USA    Horses bear manure through
mgushee@havenrock.com           its fields;
http://www.havenrock.com/   When a nation ignores the Way,
                            Horses bear soldiers through
                                its streets.
                                
                            --Lao Tzu (Peter Merel, trans.)

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