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From Chiwan Park <chiwanp...@apache.org>
Subject Re: Why Scala Option is not a valid key?
Date Tue, 31 May 2016 02:10:31 GMT
I’ve merged a patch [1] for this issue. Now we can use Option as a key.

[1]: https://git-wip-us.apache.org/repos/asf?p=flink.git;a=commit;h=c60326f85faaa38bcc359d555cd2d2818ef2e4e7

Regards,
Chiwan Park

> On Apr 5, 2016, at 2:08 PM, Chiwan Park <chiwanpark@apache.org> wrote:
> 
> I just found that Timur created a JIRA issue for this (FLINK-3698).
> 
> Regards,
> Chiwan Park
> 
>> On Mar 31, 2016, at 7:27 PM, Till Rohrmann <trohrmann@apache.org> wrote:
>> 
>> Actually I think that it’s not correct that the OptionType cannot be used as a
key type. In fact it is similar to a composite type and should be usable as a key iff it’s
element can be used as a key. Then we only have to provide an OptionTypeComparator which will
compare the elements if they are set. If not, then the None element will be smaller, for example.
>> 
>> @Timur, if you want, then you can file a JIRA issue to add that.
>> 
>> Cheers,
>> Till
>> 
>> 
>> On Wed, Mar 30, 2016 at 7:17 PM, Timur Fayruzov <timur.fairuzov@gmail.com>
wrote:
>> Ok, I can't make Option comparable, so the only way that I see is to translate a
key to a Comparable data structure and use it (as it was alluded to in your example above).
Thank you for clarification!
>> 
>> Thanks,
>> Timur
>> 
>> On Wed, Mar 30, 2016 at 9:22 AM, Chiwan Park <chiwanpark@apache.org> wrote:
>> Hi Timur,
>> 
>> Sorry for confusing. I meant KeySelector.
>> 
>> `GenericType<T>` could be used as a key type if the `T` implements `Comparable`.
For example, `GenericType<Integer>` could be used as a key type but `GenericType<scala.Tuple2>`
could not.
>> 
>> About my example in previous mail, the type of key is `Int` because the return type
of KeySelector is `Int`. `TypeInformation<Int>` is not generic type.
>> 
>> Regards,
>> Chiwan Park
>> 
>>> On Mar 31, 2016, at 1:09 AM, Timur Fayruzov <timur.fairuzov@gmail.com>
wrote:
>>> 
>>> Thank you for your answers, Chiwan! That would mean that a generic type can't
be used as a key in general? This is a non-obvious limitation of Flink DSL that I didn't see
in documentation.
>>> 
>>> Could you please elaborate what you mean by KeyExtractor? I see that inside `where`
operator an instance of KeySelector is initialized, but I don't see how can I pass a custom
KeySelector in.
>>> 
>>> Thanks,
>>> Timur
>>> 
>>> On Wed, Mar 30, 2016 at 12:53 AM, Chiwan Park <chiwanpark@apache.org> wrote:
>>> Hi Timur,
>>> 
>>> Because Option[T] is not comparable type generally (if T is a POJO type), you
cannot use Option[T] as a key type. I think you have to implement KeyExtractor to compare
objects including Option[T]s.
>>> 
>>> ```
>>> case class MyKey(k1: Option[String], k2: Option[String])
>>> 
>>> val data1 = env.fromElements(MyKey(Some("a"), None), MyKey(Some("a"), Some("c")))
>>> val data2 = env.fromElements(MyKey(Some("b"), None), MyKey(Some("a"), Some("c")))
>>> 
>>> data1.join(data2)
>>>  .where(_.hashCode())
>>>  .equalTo(_.hashCode()).apply {
>>>    (left: MyKey, right: MyKey) => (left, right)
>>>  }.print()
>>> ```
>>> 
>>> Note that the approach in example (using hashCode()) cannot be applied to sort
task.
>>> 
>>> Regards,
>>> Chiwan Park
>>> 
>>>> On Mar 30, 2016, at 2:37 AM, Timur Fayruzov <timur.fairuzov@gmail.com>
wrote:
>>>> 
>>>> There is some more detail to this question that I missed initially. It turns
out that my key is a case class of a form MyKey(k1: Option[String], k2: Option[String]). Keys.SelectorFunctionKeys
is performing a recursive check whether every element of the MyKey class can be a key and
fails when encountering an Option.
>>>> 
>>>> Is it possible to work around this situation without giving up Options? Inability
to use Options in Domain objects could be really frustrating.
>>>> 
>>>> Thanks,
>>>> Timur
>>>> 
>>>> On Tue, Mar 29, 2016 at 10:19 AM, Timur Fayruzov <timur.fairuzov@gmail.com>
wrote:
>>>> Hello,
>>>> 
>>>> I'm evaluating Flink and one thing I noticed is Option[A] can't be used as
a key for coGroup (looking specifically here: https://github.com/apache/flink/blob/master/flink-scala/src/main/scala/org/apache/flink/api/scala/typeutils/OptionTypeInfo.scala#L39).
I'm not clear about the reason of this and appreciate if someone can explain.
>>>> 
>>>> Thanks,
>>>> Timur
>>>> 
>>> 
>>> 
>> 
>> 
>> 
> 


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