The future of Haskell discussion

Yoann Padioleau
17 Sep 2001 18:12:42 +0200

Alastair David Reid <> writes:

> In the case of the Draw monad (which is identical to the
> IO monad except that it carries a "device context" around as an
> implicit parameter), the different feel comes from aggressive
> use of continuations (actually, they're not quite continuations
> but I don't have a better word for them).  For example, you might
> normally write code like this:
>   do
>     old_color <- selectColor blue
>     old_font  <- selectFont  helvetica
>     write_text "Hello World"
>     selectFont  old_font
>     selectColor old_color
> (Reselecting the old color and font at the end is recommended Win32
> programming style.)
> In the HGL, you instead write:
>   setPicture window (withColor blue (withFont helvetica (text "Hello World")))

you can achieve the same in many langage such as c++. 
I dont really see what is haskell specific in your code.

pseudo code (i dont remember exactly c++ :) ):

class Draw {
 Color color;
 Font  font;
 Widget wid;
 Draw(Widget w) { wid = w }
 draw(window) { Color old; Font old;  
	        oldc = setColor color; oldf = setFont font; 
	        setColor oldc; setFont oldf;
 withColor (Color c) {color = c}
 withFont  (Font f)  {font = f}

new Draw(new textWidget("Hello World"))->withColor(blue)->withFont("helvetica")->draw(window)
there are plenty of way to achieve what you do.

> or, equivalently but allegedly easier to read,
>   setPicture window    $
>     withColor blue     $
>     withFont helvetica $
>     text "Hello World"
> where withColor and withFont are defined like this:
>   withColor :: Color -> Draw a -> Draw a
>   withColor c m = do{ old <- selectColor c; a <- m; selectColor old; return a }
>   withFont  :: Font  -> Draw a -> Draw a
>   withFont f m = do{ old <- selectFont f; a <- m; selectFont old; return a }
> and setPicture exploits the fact that an object of type "Draw a" is a
> first class object which can be stored in the window state and
> executed when appropriate (e.g., when the window is uniconified).
> What I'm saying is that Haskell's standard abstraction facilities mean
> that even in the IO monad your programming experience can
> significantly better than that of a C programmer.  (Of course your
> experience can also be worse if you don't or can't bring Haskell's
> strengths to bear on your problem.)

Yoann  Padioleau,  INSA de Rennes, France,
Opinions expressed here are only mine. Je n'écris qu'à titre personnel.
**____   Get Free. Be Smart.  Simply use Linux and Free Software.   ____**