On Nov 28, 2006, at 7:46 AM, Slavomir Kaslev wrote:
Hello,
I have to define a couple of float2, float3, float4 Cg, HLSL style
vectors in Haskell. At first I was tempted to make them instances of
Num, Floating, RealFrac, etc. but some of the functions defined in
those classes have no sense for vectors. One such example is signum
from class Num.
There are several workarounds for this. One may come up with some
meaning for vectors of such functions, for example:
instance Num Float3 where
.....
signum a | a == Float3 0 0 0 = 0
| otherwise = 1
This is silly. Other option, which I prefer, is to leave such
functions undefined (that is signum=undefined, not just not defining
them). Is this ok? Are there any other options?
This will work. So long as you don't call signum, all will be well.
Another bugging thing is that some of the functions do have meaning
for vectors but they need different signatures. For example (**) ::
Floating a => a -> a -> a, for vectors should be (**) :: (Floating a,
Vector v) => v -> a -> v, that is (**) applied for every component of
the vector. Any workarounds for that?
I know that I can scrap all those Num, Floating, RealFrac, etc.
classes and define class Vector from scratch, but I really don't want
to come up and use different names for +, -, etc. that will bloat the
code.
The inflexibility of the numeric classes is one of the well-known
problems with the definition of the Haskell prelude. As you say,
there are a number of things for which only a subset of the
operations make sense, or where more general types are needed for the
operations. There have been a couple of attempts to reformulate
these classes so that they are more flexible.
Here is one that I know of:
http://darcs.haskell.org/numericprelude/
I haven't used it, so I can't really comment, other than to say it
exists. I seem to recall that there were several other attempts in a
similar vein, but my brief google search didn't turn them up. Can
someone else fill in?
If you want to roll your own, you can still use the nice names if you
explicitly import the prelude and hide names. Eg,
import Prelude hiding ( (+), (-), .... etc .... )
Hope that helps.
Last question: Does haskell have something like C++ templates? For
example, some time in the future I may need types like int2, short3,
etc., that behave just like float2, float3, but use different types
for their components. I really, really wouldn't like to copy-paste the
definitions of floatn and manually change their types to intn
respectfully.
Cheers.
--
Slavomir Kaslev
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