[Haskell-cafe] Abstraction in data types
Darrin Chandler
dwchandler at stilyagin.com
Thu Mar 18 01:27:33 EDT 2010
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 <ajs at 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.
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> >
> >
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--
Darrin Chandler | Phoenix BSD User Group | MetaBUG
dwchandler at stilyagin.com | http://phxbug.org/ | http://metabug.org/
http://www.stilyagin.com/ | Daemons in the Desert | Global BUG Federation
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