[Haskell-cafe] Typeclasses vs simple functions?

[Tom Nielsen]

I think you are in trouble because you have mixed 2D and 3D shapes in
one data type.

--not checked for typos, syntax, idiocy etc.
{-# LANGUAGE GADTs #-}

data Z
data S n

type Two = S (S Z)
type Three = S Two

data Geometry dims where
    Sphere :: Position -> Radius -> Geometry Three
    Cylinder :: Position -> Radius -> Height -> Geometry Three
    Circle :: Position -> Radius -> Geometry Two

    Postcard :: Position -> Orientation -> Geometry Two -> Geometry Three

perimeter :: Geometry Two -> Double
perimeter (Circle _ r) = 2*pi*r

Tom

On Tue, Sep 15, 2009 at 11:29 AM, 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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