[Cython] Fused Types

mark florisson markflorisson88 at gmail.com
Tue Apr 26 17:18:50 CEST 2011

On 26 April 2011 16:43, Stefan Behnel <stefan_ml at behnel.de> wrote:
> mark florisson, 26.04.2011 16:23:
>> I've been working a bit on fused types
>> (http://wiki.cython.org/enhancements/fusedtypes), and I've got it to
>> generate code for every permutation of specific types. Currently it
>> only works for cdef functions. Now in SimpleCallNode I'm trying to use
>> PyrexTypes.best_match() to find the function with the best signature,
>> but it doesn't seem to take care of coercions, e.g. it calls
>> 'assignable_from' on the dst_type (e.g. char *), but for
>> BuiltinObjectType (a str) this returns false.
> Which is correct. "char*" cannot coerce from/to "str". It can coerce to
> "bytes", though.
> http://wiki.cython.org/enhancements/stringliterals

Right, I see, so the thing is that I was using string literals which
should be inferred as the bytes type when they are assigned to a char
*. The thing is that because the type is fused, the argument may be
anything, so I was hoping best_match would figure it out for me.
Apparently it doesn't, and indeed, this example doesn't work:

cdef extern from "Foo.h":
    cdef cppclass Foo:
        Foo(char *)

cdef char *foo = "foo"
cdef Foo* foo = new Foo("foo") # <- this doesn't work ("no suitable
method found")
cdef Foo* bar = new Foo(foo)   # <- this works

So that's pretty lame, I think I should fix that.

>> Why is this the case,
>> don't calls to overloaded C++ methods need to dispatch properly here
>> also?
> If this doesn't work, I assume it just isn't implemented.
>> Other issues are public and api declarations. So should we support
>> that at all? We could define a macro that calls a function that does
>> the dispatch. So e.g. for this
>> ctypedef cython.fused_type(typeA, typeB) dtype
>> cdef func(dtype x):
>>     ...
>> we would get two generated functions, say, __pyx_typeA_func and
>> __pyx_typeB_func. So we could have a macro get_func(dtype) or
>> something that then substitutes __pyx_get_func(#dtype), where
>> __pyx_get_func returns the pointer to the right function based on the
>> type names. I'm not sure we should support it, right now I just put
>> the mangled names in the header. At least the cdef functions will be
>> sharable between Cython implementation files.
> I'm fine with explicitly forbidding this for now. It may eventually work for
> Python object types where we can properly dispatch, but it won't easily work
> for C types. It may work in C++, though.

Ok, will do.

>> I also noticed that for cdef functions with optional argument it gets
>> a struct as argument, but this struct doesn't seem to end up in the
>> header file when the function is declared public. I believe that also
>> the typedefs for ctypedef-ed things in the .pyx file don't make it
>> there when used to type a cdef function's arguments. Should that be
>> fixed?
> No, I think this should also become a compiler error. These functions are
> not meant to be called from C code. It's a Cython convenience feature. As
> long as it's not correctly supported on both ends of the publicly exported
> C-API, it's best to keep users from using it at all.

Ok, I'll try to make Cython issue an error for these cases.

> Stefan
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