[Cython] Fused Types

[mark florisson]

On 29 April 2011 12:28, Pauli Virtanen <pav at iki.fi> wrote:
> Fri, 29 Apr 2011 11:30:19 +0200, mark florisson wrote:
>> On 29 April 2011 11:03, Pauli Virtanen <pav at iki.fi> wrote:
> [clip]
>>> Are you planning to special-case the "real_t complex" syntax? Shooting
>>> from the sidelines, one more generic solution might be, e.g.,
>>
>> I'm sorry, I'm not sure what syntax you are referring to. Are you
>> talking about actual complex numbers?
>
> This:
>
> On 28 April 2011 23:30, Robert Bradshaw <robertwb at math.washington.edu>
> wrote:
>> OK, I take back what I said, I was looking at the RHS, not the LHS. If
>> one needs to specialize in this manner, explicitly creating two
>> branches should typically be enough. The same for casting. The one
>> exception (perhaps) is "my_fused_type complex." Otherwise it's
>> starting to feel too much like C++ template magic and complexity for
>> little additional benefit.
>
> That is, declaring a complex type matching a real one.

Ah, I see what you mean now.

>>>        ctypedef cython.fused_type(A, B) struct_t
>>>        ctypedef cython.fused_type(float, double, paired=struct_t) real_t
>>>        ctypedef cython.fused_type(int_t, string_t, paired=struct_t) var_t
>>>
>>> and just restrict the specialization to cases that make sense.
>>
>> The paired means you're declaring types of attributes?
>
> No, just that real_t is specialized to float whenever struct_t is specialized
> to A and to double when B. Or a more realistic example,
>
>        ctypedef cython.fused_type(float, double) real_t
>        ctypedef cython.fused_type(float complex, double complex) complex_t
>
>        cdef real_plus_one(complex_t a):
>            real_t b = a.real
>            return b + 1
>
> which I suppose would not be a very unusual thing in numerical codes.
> This would also allow writing the case you had earlier as
>
>        cdef cython.fused_type(string_t, int, paired=struct_t) attr_t
>
>        cdef func(struct_t mystruct, int i):
>            cdef attr_t var
>
>            if typeof(mystruct) is typeof(int):
>                var = mystruct.attrib + i
>                ...
>            else:
>                var = mystruct.attrib + i
>                ...
>
> Things would need to be done explicitly instead of implicitly, though,
> but it would remove the need for any special handling of
> the "complex" keyword.

I see, so it's like a mapping. So, I didn't realize that you can't do this:

def func(arbitrary_type complex x):
    ...

But if we just allow that for fused types, then couldn't we simply do

ctypedef cython.fused_type(float, double) real_t

cdef real_plus_one(real_t complex a):
    real_t b = a.real
    return b + 1

? Then you don't need to pair anything. Perhaps we could introduce
real_t as a type, just like numeric and floating. So I guess
special-casing complex sounds fine with me. Perhaps real_t should be
builtin (not as an attribute of the Cython module), so the parser can
just recognize it immediately?

> --
> Pauli Virtanen
>
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