Re: [Haskell-cafe] elementary Maybe Monad problem .. sigh

1 May 2008

      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