[Haskell-beginners] Pattern guard inside function

[Daniel Trstenjak]

Hi Nathan,

On Wed, Oct 31, 2012 at 03:02:45PM +0100, Nathan Hüsken wrote:
> collInfo' :: Maybe Rect -> Maybe Rect -> Maybe CollInfo
> collInfo' mr1 mr2 =
>   r1 <- mr1
>   r2 <- mr2
>   collInfo r1 r2

that doesn't seem to be that nice, because you could call it with
any two Rects and get a CollInfo, even if they don't collide:

collInfo' (Just $ Rect 0 0 1 1) (Just $ Rect 5 5 1 1)

Having Maybes as arguments just doesn't feel that right.


You could separate the collision test:

colliding :: Rect -> Rect -> Bool
colliding (Rect x1 y1 w1 h1) (Rect x2 y2 w2 h2) =
   not $ x1 + w1 < x2
            || x1 > x2 + w2
            || y1 + h1 < y2
            || y1 > y2 + h2


collInfo :: Rect -> Rect -> Maybe (Rect, Rect)
collInfo r1 r2
   | colliding r1 r2 = Just (r1, r2)
   | otherwise       = Nothing


Greetings,
Daniel