Editing POV-Ray SDL project (section)

== The problems ==

Well... almost. There are one or two little glitches.

First of all, every object conceptually has a 'shape' property. This could be a plane, sphere, cylinder, etc. Do we make 'shape' a type or a class?

If we make it a class, then we instantly have problems. A sphere and a cylinder both have a 'radius'. Unless you're going to start writing ugly and tedious constructions like 'cylRadius' and 'sphRadius', you're going to hit a name-clash problem. On top of that, there are situations where you might very well want to process a ''list'' of shapes. If 'shape' is a class, all the elements of the list must be the exact same shape type.

Alternatively, we can make 'shape' a type. Then we avoid the name clash problem, and 'radius' will work as expected. Also, we can now construct a list of shapes containing all different kinds of shape. The problem now is the reverse: we might want to demand that a list contains ''only'' spheres, for example. There is now no way to force this though the type system.

On balance, shape = type seems to work best.

There's still a slight problem though. Some shapes have a 'radius'. However, spotlights also have a 'radius'. Some shapes have a 'normal'. But so too does the 'gradient' texture type. There isn't an obvious way to fix this. (Aside from making a class 'HasRadius' with a single method 'radius'. And likewise for every other clashing name...)

Also, in SDL you add things to the scene by just writing them out. To make the SDL into valid Haskell you would have to assign them to some kind of top-level identifier or some such. A minor issue...

Next, Haskell's named-field syntax very closely matches POV-Ray's SDL (though not exactly), but there are still problems. Declaring a field named <hask>abc</hask> neatly defines a new query function <hask>abc :: TypeX -> TypeY</hask>. However, it does not define any update function. The only way to update is by the curiose syntactical construct <hask>my_x {abc = foo}</hask>. Last time I checked, that's not a function, and you can't pass it around the way the <hask>abc</hask> function can be passed.

I did experiment with a construction such as
<haskell>
type Property a b = (a -> b, b -> a -> b)

get = fst
set = snd

radius = (get_radius, set_radius) :: Property
</haskell>
With this done, <hask>get radius</hask> fetches the radius and <hask>set radius</hask> sets it. It proved to be a little unwieldy though.