Sentinels in collections, was RE: Primitive Queue<any T> considerations

[Vitaly Davidovich]

How do you imagine multiple sentinels would/could work in a generic
collection? Specialization for some known types? How would user defined
value types fit there?

sent from my phone
On Nov 18, 2015 8:15 PM, "Rezaei, Mohammad A." <Mohammad.Rezaei at gs.com>
wrote:

> Richard asked for examples of collections that had to have sentinels and I
> provided some, but realizing that these are internal ones, changed the
> subject.
>
>
>
> From a Valhalla perspective, the fact that one cannot manufacture a
> sentinel besides T.default is a real problem for collections that need more
> than one sentinel. The current types (primitives, references) don’t have
> this limitation. (This is also not a problem in languages like C where I
> can cast a struct to char* and fill it with arbitrary bit patterns).
>
>
>
> Thanks
>
> Moh
>
>
>
> *From:* Vitaly Davidovich [mailto:vitalyd at gmail.com]
> *Sent:* Wednesday, November 18, 2015 6:32 PM
> *To:* Richard Warburton
> *Cc:* valhalla-dev at openjdk.java.net; Rezaei, Mohammad A. [Tech]
> *Subject:* Re: Sentinels in collections, was RE: Primitive Queue<any T>
> considerations
>
>
>
> I'm still unclear on how we ended up discussing internal sentinels that
> are effectively implementation details.  I don't think there's any debate
> on this part.
>
> I do, however, disagree on sentinels that leak out of the implementation
> and require user to select a magic number.  And I say this as someone who
> constantly exploits domain specific constraints on values to optimize
> code.  In particular, in a world where optional<value_type> is cheap asking
> user to declare a magic number is unfortunate; this is required today, but
> it's a language limitation.  I'd hate to have that perpetuated once better
> language features are available.
>
> sent from my phone
>
> On Nov 18, 2015 6:19 PM, "Richard Warburton" <richard.warburton at gmail.com>
> wrote:
>
> Hi,
>
>
>
> Agreed, so maybe I misinterpreted Richard’s email (I tried to mitigate by
> changing the subject). Richard, which type of sentinel did you mean?
>
>
>
> Well I was originally thinking in terms of the Queue interface itself,
> which needed a sentinel taken out of the user's domain. I don't think its a
> problem to expand the discussion though and you're certainly right to
> observe that there are two different kinds of sentinel.
>
>
>
> regards,
>
>   Richard Warburton
>
>   http://insightfullogic.com
>
>   @RichardWarburto <http://twitter.com/richardwarburto>
>