[Cython] Fused Types

Stefan Behnel stefan_ml at behnel.de
Sat Apr 30 16:24:38 CEST 2011

Robert Bradshaw, 29.04.2011 06:32:
> On Thu, Apr 28, 2011 at 7:09 PM, Stefan Behnel<stefan_ml at behnel.de>  wrote:
>> mark florisson, 28.04.2011 23:29:
>>> On 28 April 2011 22:31, mark florisson wrote:
>>>> On 28 April 2011 22:12, Robert Bradshaw wrote:
>>>>> On Thu, Apr 28, 2011 at 12:48 PM, mark florisson 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.
>> Well, the obvious place where this would happen for free would be constant
>> folding. But that runs *way* before type analysis, so all it sees is the
>> typeof() call, not the string.
> Actually, to support code like
> ctypedef cython.fused_type(object, double) my_fused_type
> cdef foo(my_fused_type x):
>      if my_fused_type is object:
>          print x.attr
>      else:
>          cdef my_fused_type *ptr =&x
> we need to resolve this branch and prune "dead code" before type
> analysis, so I'm thinking it may need to be a special phase, perhaps
> part of the fused-type-specialization phase. Here's another idea:
> cdef foo(numeric x):
>      if numeric in floating:
>          ...
>      elif numeric is long:
>          ...
>      else:
>          ...
>      print numeric is double   # after the specialization pass, this
> would be a literal True/False node.
> Again, this would all be resolved before type analysis.

Hmm, I wonder, if we ever want to start with whole program analysis, and we 
find code like this:

     def func(arg):

     a = [1,2,3]

we could infer that "arg" is actually a "fused_type(object, list)". So, I 
think, what we eventually want is an intermingling of type analysis and 
specialisation. Meaning, we'd find a new type during type analysis or 
inference, split off a new version of the function and then analyse it. But 
maybe that's just a pie in the sky for now.


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