[Cython] Fused Types

[Dag Sverre Seljebotn]

On 05/04/2011 01:07 AM, Greg Ewing wrote:
> mark florisson wrote:
>
>> cdef func(floating x, floating y):
>> ...
>>
>> you get a "float, float" version, and a "double, double" version, but
>> not "float, double" or "double, float".
>
> It's hard to draw conclusions from this example because
> it's degenerate. You don't really need multiple versions of a
> function like that, because of float <-> double coercions.
>
> A more telling example might be
>
> cdef double dot_product(floating *u, floating *v, int length)
>
> By your current rules, this would give you one version that
> takes two float vectors, and another that takes two double
> vectors.
>
> But if you want to find the dot product of a float vector and
> a double vector, you're out of luck.

First, I'm open for your proposed syntax too...But in the interest of 
seeing how we got here:

The argument to the above goes that you *should* be out of luck. For 
instance, talking about dot products, BLAS itself has float-float and 
double-double, but not float-double AFAIK.

What you are saying that this does not have the full power of C++ 
templates. And the answer is that yes, this does not have the full power 
of C++ templates.

At the same time we discussed this, we also discussed better support for 
string-based templating languages (so that, e.g., compilation error 
messages could refer to the template file). The two are complementary.

Going back to Greg's syntax: What I don't like is that it makes the 
simple unambiguous cases, where this would actually be used in real 
life, less readable.

Would it be too complicated to have both? For instance;

  i) You are allowed to use a *single* fused_type on a *function* 
without declaration.

def f(floating x, floating *y): # ok

Turns into

def f[floating T](T x, T *y):

This is NOT ok:

def f(floating x, integral y):
# ERROR: Please explicitly declare fused types inside []

  ii) Using more than one fused type, or using it on a cdef class or 
struct, you need to use the [] declaration.


Finally: It is a bit uncomfortable that we seem to be hashing things out 
even as Mark is implementing this. Would it be feasible to have a Skype 
session sometimes this week where everybody interested in the outcome of 
this come together for an hour and actually decide on something?

Mark: How much does this discussion of syntax impact your development? 
Are you able to treat them just as polish on top and work on the 
"engine" undisturbed by this?

Dag Sverre