I was thinking earlier today that it'd be nice if felix had strong
typedefs, or typedefs that create a whole new type instead of just an
alias. My canonical reasoning for this is trying to implement a vector
math library. It makes sense to build on top of felix arrays, but some
times you want operations that have the same name performing over the
same fundamental structure, that do different things. For instance,
multiplying vectors. Say we implement mul to equal the cross product:
typedef rowvec4[T] = array[T,4];
typedef colvec4[T] = array[T,4];
fun mul[T](x:rowvec4[T], y:colvec4[T]) =>
[|x.[0]*y.[0],x.[1]*y.[1],x.[2]*y.[2],x.[3]*y.[3]|];
Now, if we want to implement quaternions, we'd have:
typedef quat[T] = array[T,4];
fun mul[T])(x:quat[T], y:quat[T]) =>
[|
x.[3]*y.[0] + x.[0]*y.[3] + x.[1]*y.[2] - x.[2]*y.[1],
x.[3]*y.[1] + x.[1]*y.[3] + x.[2]*y.[0] - x.[0]*x.[2],
x.[3]*y.[2] + x.[2]*y.[3] + x.[0]*y.[1] - x.[1]*y.[0],
x.[3]*y.[3] - x.[0]*y.[0] - x.[1]*y.[1] - x.[2]*y.[2],
|]
;
But this will conflict with the first definition. The traditional answer
would be to put both in their own respective modules, but this won't
help the case when we want the modules opened up. Or, if we had strong
typedefs, this could be handled transparently, as these two functions
would be operating over two separate types and thus are overloadable. If
you ever wanted to use a regular array, you could just cast it in using
the "x:rowvec4" casting.
Of course, having type aliases is also quite useful as well. Since the
word "type" is already a keyword, and "type foo = int" doesn't currently
work, what if we used that for this purpose? Then we'd have
typedef quat[T] = array[T,4];
Be the type that requires explicit constructors. The quaternion
multiplication could then use the _ctor and be used like this:
fun mul[T])(x:quat[T], y:quat[T]) =>
quat[T](
x.[3]*y.[0] + x.[0]*y.[3] + x.[1]*y.[2] - x.[2]*y.[1],
x.[3]*y.[1] + x.[1]*y.[3] + x.[2]*y.[0] - x.[0]*x.[2],
x.[3]*y.[2] + x.[2]*y.[3] + x.[0]*y.[1] - x.[1]*y.[0],
x.[3]*y.[3] - x.[0]*y.[0] - x.[1]*y.[1] - x.[2]*y.[2],
)
;
What do you all think?
-e
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