[Cython] Fused Types

[Greg Ewing]

Dag Sverre Seljebotn wrote:

> ctypedef fused_type(float, double) speed_t
> ctypedef fused_type(float, double) acceleration_t
> 
> then you get 4 specializations.
> 
> ctypedef speed_t acceleration_t
> 
> I guess only 2 specializations.
> 
> Treating the typedefs in this way is slightly fishy of course.

Indeed. This whole business seems rather too implicit to
me. I think I'd rather have explicit type parameters in
some form. Maybe

   cdef func2[floating F](F x, F y):
     # 2 specialisations

   cdef func4[floating F, floating G](F x, G y):
     # 4 specialisations

This also makes it clear how to refer to particular
specialisations: func2[float] or func4[float, double].

Pointers are handled in a natural way:

   cdef funcfp[floating F](F x, F *y):
     # 2 specialisations

It also extends naturally to fused types used in other
contexts:

   cdef struct Vector[floating F]:
     F x, y, z

-- 
Greg