[Cython] New function (pointer) syntax.

[Robert Bradshaw]

On Thu, Nov 6, 2014 at 12:29 AM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Robert Bradshaw schrieb am 06.11.2014 um 08:34:
>> I'd like to propose a more pythonic way to declare function pointer
>> types, namelye
>>
>>     type0 (*[ident])(type1, type2, type3)
>>
>> would instead become
>>
>>     (type1, type2, type3) -> type0 [ident]
>
> Not convinced. Looks quite magic, very different from the C that people
> would know, and takes a while reading from left to right until you see the
> "->" which is essentially the only indication of what is happening here.

I suppose it depends on whether one knows just C, is familiar with a
wide variety of other (more functional) languages. I think it's a lot
easier for people to read who only know Python, or just a smattering
of C.

> Also, "->" has a widely known meaning in C/C++ which makes the above very
> confusing. I think in C when thinking about function pointers, and there is
> no "def ..." keyword indication here that would switch me back to Python
> (as for signature annotations).
>
> I do see the problem of "where did I have to put that '*' again in the C
> declaration?", but this looks like it will produce more confusion than it
> avoids.

Hmm, I hadn't thought of that meaning of ->. I don't think it would be
as confusing in context, which will be immediately following cdef or
in a function argument, e.g. one would write

    cdef (float -> float, float, float) -> float integrator =
get_integrator(...)

which doesn't to me look like member access, and is much clear than

    cdef float (*integrator)(float (*)(float), float, float) =
get_integrator(...)

This becomes especially clear for return types, e.g.

    cdef ((float -> float, float, float) -> float)
get_integrator(char* algorithm):
        if algorithm == ...

vs

    cdef float (*get_integrator(char* algorithm))(float (*)(float),
float, float):
        if algorithm == ...

> Many (or most?) function pointer type declarations will come from some C
> library's header file, so it means making it more difficult for users to
> copy over the declaration.

I really hope, in the long run, people won't have to manually do these
declarations.

As well as making things easier to read, part of the motivation is to
come up with a syntax for declaring this type without having to add
the notion of declarators (empty or not) to the grammar. Essentially
everyone thinks "cdef type var" even though that's not currently the
true grammar.

Another option that just occurred to me would be the syntax "lambda
type1, type2, type3: type0" so for this example one would have

    cdef lambda (lambda float: float), float, float: float integrator
= get_integrator(...)

doesn't read as nice to me, but thought I'd throw it out there.

- Robert