On Thu, Mar 18, 2010 at 01:06:25PM +0800, Lyndon Maydwell wrote: > You could probably also use a typeclass for pointy things rather than > a data type, this would then require you to use existential > quantification to construct a hetrogenous list. > > For example: > > Class Point where > getCartesian :: ... > getPolar :: ... > > data Shape = Point p => ... | Polygon [p] > > Correct me if this is wrong though :-)
So in "normal" use Polygon list would be homogeneous, but could be made heterogeneous with effort? If I have that right it's closer, but I'd love to have the compiler whine if someone tried to mix them. > On Thu, Mar 18, 2010 at 12:56 PM, Alexander Solla <a...@2piix.com> wrote: > > I wrote this to Darrin, but didn't CC cafe: > > On Mar 17, 2010, at 9:20 PM, Darrin Chandler wrote: > > > > type Cartesian_coord = Float > > > > type Latitude = Float > > type Longitude = Float > > > > data Point = Cartesian (Cartesian_coord, Cartesian_coord) > > | Spherical (Latitude, Longitude) > > > > type Center = Point > > type Radius = Float > > > > data Shape = Circle Center Radius > > | Polygon [Point] > > > > This obviously stinks since a Polygon could contain mixed Cartesian and > > Spherical points. Polygon needs to be one or the other, but not mixed. > > > > My suggestion would be to use an alternate representation of "spherical" > > points in terms of polar coordinates, and then to normalize and mix at will: > > type Theta = Float > > type Radius = Float > > data Point = Cartesian (Cartesian_coord, Cartesian_coord) > > | Polar (Theta, Radius) > > normalize_point :: Point -> Point > > normalize_point Cartesian x y = Cartesian x y > > normalize_point Polar t r = Cartesian x y where x = r * cos t; y = r * sin > > t; > > It really depends on what you want to do with your points. If you want to > > do linear algebra, you might want your points to depend on a basis, for > > example. But your "spherical" points don't really form a basis in > > three-space, or even over all of two-space. > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Darrin Chandler | Phoenix BSD User Group | MetaBUG dwchand...@stilyagin.com | http://phxbug.org/ | http://metabug.org/ http://www.stilyagin.com/ | Daemons in the Desert | Global BUG Federation
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