[Haskell-beginners] Improve my FFI code please...

[Henk-Jan van Tuyl]

On Fri, 21 Jan 2011 10:09:23 +0100, <sean at objitsu.com> wrote:

> Edward, Daniel, thanks.
>
>> On Friday 21 January 2011 00:35:02, Sean Charles wrote:
>> wiiOpen :: String -> IO (Maybe (Ptr Word8))
>> wiiOpen wid = do
>> ? ?handle <- withCString wid c_wiimote_open
>> ? ?case handle of
>> ? ? ?nullPtr -> return Nothing
>> ? ? ?handle -> return (Just handle)
>> Unless nullPtr is a macro that gets replaced with a literal by the
>> preprocessor, that won't do what you want. "nullPtr" is a generic name  
>> and
>> matches everything, so you'll always take the first branch (with -Wall,  
>> ghc
>> should warn about overlapping patterns in that case).
>
> Daniel, you have confused me. In my RWH book there is a test from the  
> PCRE example of nullPtr == x, so I just did my code using a case  
> instead, I am sure it's correct!?!?!?   If I run my program with the  
> Wiimote off, it prints FAIL and if I press the red sync. button it then  
> prints out OK and the memory address of the allocated structure so it  
> surely must be workng as I expected?
>
> Anybody?
>

The Haskell 2010 Language Report[0] says that case expressions work with  
patterns; the nullPtr is regarded as a pattern that matches anything.

It can be demonstrated by the following:

Program Case.lhs:

> c = 1

> f =  case 0 of
>     c -> print 0
>     1 -> print 1

GHCi session:

Prelude> :load "Case.lhs"
[1 of 1] Compiling Main             ( Case.lhs, interpreted )


Case.lhs:8:4:
     Warning: Pattern match(es) are overlapped
              In a case alternative: 1 -> ...
Ok, modules loaded: Main.
*Main> f
0

If the c in the case expression was a constant, f would have printed 0

Regards,
Henk-Jan van Tuyl


[0]  
http://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-460003.13


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