[Haskell-cafe] Abstraction in data types
Darrin Chandler
dwchandler at stilyagin.com
Thu Mar 18 01:20:25 EDT 2010
On Wed, Mar 17, 2010 at 09:56:14PM -0700, Alexander Solla 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.
I see what you mean, but I don't think that's what I need. I want to
have keep Lat/Lon, as I may have large groups of shapes in Lat/Lon and
want to do things with them as is. And the same for cartesian coords.
Sometimes I will translate betweem lat/lon and cartesian, but many times
I will be doing calculations in "native" coordinates. But it's a nice
technique you show, and it will come in handy elsewhere.
--
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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