On Wed, Apr 18, 2012 at 10:10 AM, umptious
One of the programming exercises I keep evolving as I learn Haskell is a toy 2D shape editor with four primitives:
data Shape = Circle {origin::Pt2, radius::Float} | Square {origin::Pt2, side ::Float} | Rect {origin::Pt2, other ::Pt2} | Composite {shapes::[Shape]} deriving (Show, Read)
The intent is Composites can contain Shapes of any kind, including other Composites so that you can apply transformations to a Composite and these will be applied to the contained Shapes recursively. So an arm might contain a hand which constains a dozen or so Rects. Transform the arm and the hand and rects should transform; transform the hand and its rects should transform but the not arm. Big deal.
..So what is the Haskell idiom for dealing with this??? In fact I suppose I'm asking two questions:
1. How do I re-design this program so it is safe (use class and instance maybe, abandoning use of a single data type? but I then have to have separate Lists for each type, even if they derived from a common class?)
2. How can one use compile time checking or (less good) coding practices to eliminate the possibilty of such runtime exceptions?
I think the usual answer is to use pattern matching. As in: myFunc :: Sahpe -> Foo myFunc (Composite xs) = ... myFunc (Circle o r) = ... etc. Does this do what you want? Antoine