[Haskell-cafe] [Haskell] A riddle...
Lyndon Maydwell
maydwell at gmail.com
Mon Jul 16 17:59:17 CEST 2012
I'd guess that type-inference is allowing rgbliste to use Integers
when farbliste is commented out, but restricting it to 16 Bit types
when it is uncommented. I don't have the GTK module installed on this
machine, but try adding some type annotations to see if this is the
case.
The maxBound of Word16 appears to confirm my suspicion:
Prelude GHC.Word> maxBound :: Word16
65535
On Mon, Jul 16, 2012 at 11:36 PM, Vo Minh Thu <noteed at gmail.com> wrote:
> Oh the mailing list was haskell@ instead of café. Sorry. (Now cc'ing café.)
>
> Andreas, please post this kind of mail to the haskell-cafe instead of
> the haskell mailing list.
>
> 2012/7/16 Vo Minh Thu <noteed at gmail.com>:
>> It seems like the infered type (and thus bounds) is different when you
>> force the result to be a Color or not. Just give explicit type
>> signatures and conversion functions.
>>
>> Cheers,
>> Thu
>>
>> 2012/7/16 Andreas Abel <andreas.abel at ifi.lmu.de>:
>>> 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/
>>>
>>>
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>
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