[Haskell] A riddle...

[Andreas Abel]

Today a student came to me with a piece of code that worked it executed 
by itself, but produced different result in the context of his larger 
problem.  We cut down the example to the following:

> import Graphics.UI.Gtk
>
> -- should produce [(26471,0,65535),...
> rgbliste =
>  (map (\ i ->
>   let rb = 5 * (mod (mod 181935629 (4534+i)) 100)-250+128 in
>   let gb = 5 * (mod (mod 128872693 (5148+i)) 100)-250+128 in
>   let bb = 5 * (mod (mod 140302469 (7578+i)) 100)-250+128 in
>   let r = min 255 $ max 0 rb in
>   let g = min 255 $ max 0 gb in
>   let b = min 255 $ max 0 bb in
>   (r*257,g*257,b*257)) [0..])
>
> --farbliste = map (\ (r,g,b) -> Color r g b) rgbliste
>
> main :: IO ()
> main = do
>   print $ head rgbliste

If you run it, it prints (26471,0,65535).
If you uncomment the def. of farbliste, it prints (44461,65535,65535).

I was surprised.  What is going on?

Cheers,
Andreas

-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY

andreas.abel at ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/