[Cython] How does a fused function differ from an overloaded function?

[mark florisson]

On 9 February 2013 03:44, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Hi,
>
> I noticed that Cython currently fails to do this:
>
>    cdef int (*int_abs)(int x)
>    cdef object py_abs
>    py_abs = int_abs = abs
>
> Here, abs() is an overloaded function with a couple of C signatures (fabs()
> and friends) and a Python signature (the builtin). While there is code in
> NameNode.coerce_to() that figures out that the RHS can be replaced by the
> Python builtin, the same is lacking for the general case of overloaded entries.
>
> While working on fixing this problem (and after turning ProxyNode into an
> actual node proxy when it comes to coercion), I thought it would be a good
> idea to make NameNode generally aware of alternative entries and just build
> a new NameNode with the right entry in its coerce_to() method. Then I
> noticed that the generic coerce_to() contains this code:
>
>     if src_type.is_fused or dst_type.is_fused:
>         # See if we are coercing a fused function to a pointer to a
>         # specialized function
>         if (src_type.is_cfunction and not dst_type.is_fused and
>                 dst_type.is_ptr and dst_type.base_type.is_cfunction):
>
>             dst_type = dst_type.base_type
>             for signature in src_type.get_all_specialized_function_types():
>                 if signature.same_as(dst_type):
>                     src.type = signature
>                     src.entry = src.type.entry
>                     src.entry.used = True
>                     return self
>
> This is essentially the same idea, just done a bit differently (with the
> drawback that it modifies the node in place, which coerce_to() must *never*
> do).
>
> So, two questions:
>
> 1) why is the above code in the generic coerce_to() method and not in
> NameNode? It doesn't seem to do anything sensible for most other nodes,
> potentially not even AttributeNode. And it might fail silently when working
> on things like CloneNode that don't care about entries. Are there other
> nodes where it does what it should?

I think it works for names and attributes, it allows you to retrieve a
specialized version of the fused c(p)def functions and methods.

> 2) couldn't fused functions be mapped to a set of overloaded functions
> (read: entries) before hand, instead of special casing both in places like
> this?

Quite possibly, although I'd have to dig in the codebase some more to
verify that. You can give it a try, it'd be nice to unify the
approaches under the same model.

> Stefan
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