Well you need to define a new datatype to make a hertrogenous list, so I don't think there's any real way you can get around people doing that...
On Thu, Mar 18, 2010 at 1:27 PM, Darrin Chandler <dwchand...@stilyagin.com> wrote: > 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 > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
