sentinels & who owns the value space Re: valhalla-dev Digest, Vol 17, Issue 10

Vitaly Davidovich vitalyd at
Sat Dec 19 06:16:44 UTC 2015

I also fail to see how Moh's example generalizes over arbitrary user
types.  That technique works well for integrals and in particular for
sets.  If anything, this is an example of where specialization would be

How would T.sentinel work? I'm not quite getting it.  What's the difference
between it and T.default? How do I create multiple sentinels? Can you
sketch it out a bit more?

On Saturday, December 19, 2015, Thomas W < at> wrote:

> Great example. Thanks for being specific, Mohammad.
> Your example illustrates the use of additional storage (the
> *zeroToThirtyOne* field, etc) as an auxilliary to provide sufficient
> representation to distinguish known sentinels from valid client values.
> I would agree that this design might be harder to apply generically to
> arbitrary value types, but I'd presume it works for any integral types
> having >2 values (it doesn't exactly fail for boolean, representation
> become entirely auxilliary though).
> My thinking on Valhalla has emphasized "abstracting" the difference
> between types (reftypes, primitives & value types) and allowing existing
> idiom & usage to be expressed in an abstract way;  my feeling being that
> building capable abstractions over 'any' type, would give a stronger
> foundation that needing to lean on programmatic specialization.
> I previously proposed having the language offer both "T.default" and
> "T.sentinel" as distinct values -- to represent the two distinct uses of
> null better in eg. integer space. Null can be a default and 0 is a good
> default;  null can also be a sentinel, but Integer.MIN_VALUE is a much
> better sentinel than 0 :)
> I guess what I'm working to is, that having the language provide two
> abstracted values -- 'default' and 'sentinel' -- which are distinct, could
> be used as the two cell sentinels for your hashset & special-cased in the
> add/ contains check etc.
> This would be extensible to value-types by the component fields being set
> to default/ or sentinel appropriately. The only problem is ref-types, which
> in my proposal would have both RefType.default == null and RefType.sentinel
> == null; and would thus not be amenable to providing your two separate cell
> markers.
> Thanks for your thoughts.
> Regards,
> Thomas

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