[Haskell-cafe] Typeclasses vs simple functions?

[Olex P]

Sure! I completely forgot about Maybe. The only one question is is it good
from the point of view of ordinary user who doesn't know about such things
like functional programming, monads etc. Imagine average user who is looking
into a spreadsheet and sees values 0.1, 1.4, Nothing... From other side it
seems to be logical. Why not.
Thanks for the idea :)

On Tue, Sep 15, 2009 at 12:05 PM, Lyndon Maydwell <maydwell at gmail.com>wrote:

> I think the problem is that you want to compose a list with no
> indication of weather one member can have a perimeter or not. I'm not
> sure if this is a good solution or not, but I immediately think to
> make all Geometry objects instances of a class that return a Maybe
> value for the perimeter:
>
> e.g.
>
> ---
>
> import Data.Maybe
>
> data Geometry = Sphere Position Radius | Circle Position Radius deriving
> (Show)
>
> type Position = (Double, Double)
> type Radius = Double
> type Height = Double
>
> class Encircled x where
>        perimeter :: x -> Maybe Double
>
> instance Encircled Geometry where
>        perimeter (Sphere _ r) = Nothing
>        perimeter (Circle _ r) = Just $ 2.0 * pi * r
>
> list = [Sphere (1,1) 1, Circle (2,2) 2]
>
> main = (print . catMaybes . map perimeter) list
>
> --- [12.566370614359172]
>
> On Tue, Sep 15, 2009 at 6:29 PM, Olex P <hoknamahn at gmail.com> wrote:
> > Hey guys,
> >
> > It's a dumb question but I'd like to know a right answer...
> > Let's say we have some geometry data that can be Sphere, Cylinder, Circle
> > and so on. We can implement it as new data type plus a bunch of functions
> > that work on this data:
> >
> > data Geometry = Sphere Position Radius
> >                         | Cylinder Position Radius Height
> >                         | Circle Position Radius
> >                         deriving (Show)
> >
> > perimeter (Sphere _ r) = 0.0
> > perimeter (Cylinder _ r h) = 0.0
> > perimeter (Circle _ r) = 2.0 * pi * r
> >
> > Perimeter doesn't make sense for Sphere or Cylinder. So we could define a
> > type class for objects that have perimeter and make an instance of it
> only
> > for Circle (data Circle = Circle Position Radius). Make sense. But these
> > three functions above have desired behaviour. If user has a list of
> objects
> > like [Sphere, Circle, Circle, Cylinder] he would like to calculate
> > perimeters of each object using map perimerer list (in this case we also
> > have to modify Geometry data type).
> > So we could make instances of "perimeter" type class for all objects and
> > return zero in case if perimeter doesn't make sense.
> > Same as previous version but with typeclasses and with additional
> > constructors (constructors for each type of object + constructors in
> > Geometry data). Looks a bit overcomplicated.
> > Any reasons to use type classes in this case? Maybe there is something
> I'm
> > missing?
> >
> > Cheers,
> > -O
> >
> > _______________________________________________
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> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
>
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