Final variables without explicit type: Intersection types issue.

[Marek Kozieł]

2009/3/21 Reinier Zwitserloot <reinier at zwitserloot.com>:
> Intersection types are annoying, to say the least. Possibly hand-wave
> it away by stating that you can't use those expressions in this new
> language construct.
>
> Or, a slight adaptation to that: If all but 1 of the intersection
> types are subtypes of the other one, and this can be said for only 1
> type, then that type wins. Thus:
>
> final foo = someBoolean ? Arrays.asList("foo", "bar") : new
> ArrayList<String>();
>
> foo's type would be List<String>, because that's the type of one of
> the intersections (List<String>), and all other types of that
> intersection (ArrayList<String>) are a subtype of this, and there's no
> other type for which this can be said. However, writing up the
> specifics of this sounds difficult to say the least, and it can always
> be added in java8 if it causes an inordinate amount of whining.
>
> Either way, this:
>
> final foo = someBoolean ? new LinkedList<String>() : new
> ArrayList<String>();
>
> would be a compiler error. What should foo be? List<String>?
> AbstractList<String>? Serializable? Cloneable? Object? They're all
> common supertypes, and there's no clear winner in the set; attempting
> to use the nearest common parent still gives you 2 winning options:
> Serializable, and Cloneable, while in actual fact you were probably
> shooting for List<String>, which isn't even close to the winner here,
> what with AbstractList in the way, and AbstractList itself being 2
> removed from LinkedList, which extends AbstractSequentialList, which
> extends AbstractList.
>
>
> I think it'll be okay to just compiler-error on those, because I
> expect the majority usage would be something akin to:
>
> final foo = methodCall();
>
>  --Reinier Zwitserloot
>
>
>
> On Mar 21, 2009, at 22:46, Florian Weimer wrote:
>
>> * John Rose:
>>
>>> On Mar 21, 2009, at 12:52 PM, Marek Kozieł wrote:
>>>
>>>> Allow final variables and final Fields (except blank final), to not
>>>> having explicit Type.
>>>
>>> Yes.  Someone should work exactly this (and no more) into a separate
>>> proposal, if it hasn't been done already.
>>
>> What should be the inferred type of an expression with an intersection
>> type?  Is there an answer which is acceptable in the COIN context?
>>
>
>
>


Most natural be for me would be (autoboxing is forbidden):

If at least one of intersected types is Object then returned type is
Object and no error occurs.
Else If only one type (except Object) is effect of intersection then
it is the type and no warning nor error should occur.
Else If both types have common ancestor (except Object) then ancestor
is the type and no warning should occurs.
Else error should occurs.

-- 
Pozdrowionka. / Regards.
Lasu aka Marek Kozieł

http://lasu2string.blogspot.com/