Here is a simpler case of what I want to do .. 1) To function1 pass in (Maybe Int). 2) If "Nothing" then pass nullPtr to C function. 3) If "Just 1", then pass a pointer to a "1" to teh same C function. Thanks, Vasili On Thu, May 1, 2008 at 8:18 PM, Galchin, Vasili wrote:

Sorry .. my example was bad. I want to use "x" .. in then branch where it occur ...

e.g. bonzo :: Maybe Bozo -> IO () bonzo maybe_bozo = do case maybe_bozo of Just (Bozo x) -> x ........ _ -> .........

??

Thanks, V.

On Thu, May 1, 2008 at 7:50 PM, Luke Palmer wrote:

...

2008/5/2 Galchin, Vasili :

...

data Bozo = Bozo { id :: Int }

bonzo :: Maybe Bozo -> IO () bonzo maybe_bozo = do if maybe_bozo == (Just (Bozo x)) then return () else return ()

bonzo maybe_bozo = case maybe_bozo of Just (Bozo x) -> return () _ -> return ()

Or equivalently:

bonzo (Just (Bozo x)) = return () bonzo _ = return ()

You should watch out for your use of id as a field name, since id is a builtin function and you will get ambiguity errors.

Luke

2008/5/1 Galchin, Vasili :

Sorry .. my example was bad. I want to use "x" .. in then branch where it occur ...

e.g. bonzo :: Maybe Bozo -> IO () bonzo maybe_bozo = do case maybe_bozo of Just (Bozo x) -> x ........ _ -> .........

??

Sure, after pattern-matching on the x (using a case, or a top-level pattern match), you are free to use x in the resulting branch. For example: bonzo (Just (Bozo x)) = print (show x) >> return x -- or whatever bonzo Nothing = return () -Brent