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

[Edward Z. Yang]

Excerpts from sean's message of Fri Jan 21 04:09:23 -0500 2011:
> 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?

A more direct answer is that you need to use a guard in this case:

case x of
  x | x == nullPtr -> ...

Or maybe in this case, using a regular conditional would be better.

You can pattern match against constructors (things that are capitalized);
variable names (lower-cased) will just assign new variables.

Cheers,
Edward