[Haskell-beginners] Pattern guard inside function

[Bob Hutchison]

Hi,

On 2012-10-31, at 10:02 AM, Nathan Hüsken <nathan.huesken at posteo.de> wrote:

> Hey
> 
> I have a function which originaly locked like this:
> 
> collInfo :: Rect -> Rect -> Maybe CollInfo
> collInfo (Rect x1 y1 w1 h1) (Rect x2 y2 w2 h2)
>  | x1 + w1 < x2 = Nothing
>  | x1 > x2 + w2 = Nothing
>  | y1 + h1 < y2 = Nothing
>  | y1 > y2 + h2 = Nothing
>  | otherwiese = Just $ makeCollInfo r1 r2
> 
> Now I want to refactor it to take Maybe input values
> 
> collInfo' :: Maybe Rect -> Maybe Rect -> Maybe CollInfo
> collInfo' mr1 mr2 =
>  r1 <- mr1
>  r2 <- mr2
>  collInfo r1 r2
> 
> This is fine, but I would like to write it using only one function. Is
> there any possibility I can remove collInfo and merge its body into
> collInfo' and still somehow use the guards (and not a bunch of ifs)?

maybe like:

collInfo'' :: Maybe Rect -> Maybe Rect -> Maybe CollInfo
collInfo'' (Just r1@(Rect x1 y1 w1 h1)) (Just r2@(Rect x2 y2 w2 h2))
  | x1 + w1 < x2 = Nothing
  | x1 > x2 + w2 = Nothing
  | y1 + h1 < y2 = Nothing
  | y1 > y2 + h2 = Nothing
  | otherwise = Just $ makeCollInfo r1 r2
collInfo'' _ _ = Nothing

Cheers,
Bob

> 

> Thanks,
> Nathan
> 
> 
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