[Cython] Fused Types

[mark florisson]

On 28 April 2011 22:31, mark florisson <markflorisson88 at gmail.com> wrote:
> On 28 April 2011 22:12, Robert Bradshaw <robertwb at math.washington.edu> wrote:
>> On Thu, Apr 28, 2011 at 12:48 PM, mark florisson
>> <markflorisson88 at gmail.com> wrote:
>>
>>> So I fixed all that, but I'm currently wondering about the proposed
>>> cython.typeof(). I believe it currently returns a string with the type
>>> name, and not the type itself.
>>
>> Yes. This is just because there's not really anything better to return
>> at this point. We should "fix" this at some point in the future.
>>
>>> So I think it would be inconsistent to
>>> suddenly start allowing comparison with 'is' and 'isinstance' and
>>> such.
>>
>> I'm open to other suggestions, but would like an expression that
>> resolves at compile time to true/false (and we need to do branch
>> pruning on it). Note that type() is not good enough, because it has
>> different semantics, i.e.
>>
>>    cdef object o = []
>>    typeof(o), type(o)
>>
>> so lets not touch that one.
>
> Right, so for fused types I don't mind string comparison with
> cython.typeof(), but retrieval of the actual type for casts and
> declaration remains open. I'd be fine with something like
> cython.gettype().

It seems that this isn't optimized yet, but it looks to me like it
wouldn't be very hard to do so. At least == and != could be resolved
at compile time if the other operand is a string literal.

> On the other hand, the user could already do this thing by manually
> declaring another fused type that would perhaps be resolved based on
> its usage. e.g. for my original example:
>
> ctypedef char *string_t
> ctypedef cython.fused_type(int, string_t) attrib_t
>
> cdef func(struct_t mystruct, int i):
>    cdef attrib_t var = mystruct.attrib + i
>
> Is this something that should be supported anyway? Currently fused
> types can only be used when appearing as argument types (in the
> function body and as return value).
>
>>> I'm also wondering if it would be useful to allow actual type
>>> retrieval, which could be used in declarations and casts. For instance
>>> consider fusing two structs with the same attribute name but different
>>> attribute types. Perhaps in your code you want to introduce a variable
>>> compatible with such a type, e.g. consider this:
>>>
>>> ctypdef struct A:
>>>    int attrib
>>>
>>> ctypedef struct B:
>>>    char *attrib
>>>
>>> ctypedef cython.fused_type(A, B) struct_t
>>>
>>> cdef func(struct_t mystruct, int i):
>>>    cdef cython.gettype(mystruct.attrib) var = mystruct.attrib + i
>>>    ...
>>>
>>> What do you think?
>>
>> Yes, I was thinking that would be supported as well.
>>
>> - Robert
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>