[Haskell-cafe] Getting the x out

[Mattias Bengtsson]

On Tue, 2009-04-21 at 17:49 -0700, michael rice wrote:
> How do I get the x out of Just x?
> 
> Michael
> 
> =============
> 
> safeDivision :: Float -> Float -> Maybe Float 
> safeDivision x y = if y == 0 then Nothing else Just (x/y)
> 
> *Main Data.List> safeDivision 10 5
> Just 2.0
> *Main Data.List> 3 + (safeDivision 10 5)
> 

This would do:
case safeDivision 10 5 of
  Just x -> 3 + x
  Nothing -> Nothing

The Maybe-monad might make this a bit more convenient:
ghci> let plusthree a = do a' <- a; return (a'+3)
ghci> plusthree (Just 5)
Just 8