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Summarizing my takeaways from talking over these use cases today:<br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Sep 7, 2022, at 12:56 AM, <a href="mailto:forax@univ-mlv.fr" class="">
forax@univ-mlv.fr</a> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<blockquote type="cite" class="">Here is a list of such value types:<br class="">
- unit types, value types like by example Nothing (which mean that a method<br class="">
never returns) with no fields.<br class="">
Because creating a ref on it creates something :)<br class="">
</blockquote>
<br class="">
If you truly mean for such a class to have no instances, that's not something a<br class="">
class with a value type can assert—the default instance always exists. I can<br class="">
see how it would be nice, for example, to have a type like Void that is also<br class="">
non-nullable, but value types are not the feature to accomplish that.<br class="">
</blockquote>
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">no,
such classes have instances but they are empty.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">By
example, an array of Nothing stores just its length in memory. Accessing a cell return the default value, Nothing.default which is equivalent to new Nothing().</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">And
it can be used in generics too, a HashMap<Integer.val, Nothing> is more or less a set of ints.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
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</blockquote>
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</div>
<div>Clarifying: we're not talking about Nothing types, we're talking about Singleton types.</div>
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</div>
<div>The main concern is that an array of Singleton.ref must have an O(n) size, while an array of Singleton.val *could* have an O(1) size (if this optimization were implemented—it's not something we've prototyped so far). In a sense, that's a much worse penalty
for forgetting ".val" than the up to 2x storage penalty you might typically pay for a null flag.</div>
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</div>
<div>Is this practical? Singleton[] is probably not something you would write directly, but it could make sense in a generics context (something like the HashMap Rémi referenced above). Not in the mainstream of our use cases, but a use case to keep in mind.</div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<blockquote type="cite" class="">- wrappers/monads that modify the semantics, by example a generic value class<br class="">
Atomic that plays the same role as an AtomicReference, AtomicInteger, etc<br class="">
the problem here is that the default semantics is not the semantics the user<br class="">
want.<br class="">
</blockquote>
<br class="">
Okay, so say we have value class Atomic<T>, and we're in a future where this<br class="">
gets specialized. I think you're saying it will be important to say<br class="">
'Atomic.val<Foo>' at all uses rather than 'Atomic<Foo>'. But I'm not clear on<br class="">
the argument for why that would be. Can you elaborate?<br class="">
</blockquote>
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">If
Atomic is declared as a ref-by-default value class, Atomic<Foo> behave as a ref an not as a Foo + some atomic magic.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">So
it is always wrong to use it.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">As
the author of Atomic, i don't want my users to have a way to shoot themselves into the foot.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">Here,
Atomic should be flat-by-default and also the ref variant should not exist, so there is no way to misuse the Atomic API.<span class="Apple-converted-space"> </span></span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
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</blockquote>
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<div>(Point of confusion for me: no, we're not talking whether the *field* of Atomic is flattened. This discussion is about the treatment of the Atomic instance, the *wrapper* around the field.)</div>
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</div>
<div>The argument in this case is that the class Atomic is, essentially, a user-defined variable modifier. (If it could be spelled 'atomic Foo x', that would be great, but user-defined modifiers are not a feature of Java.) It doesn't need to create an extra
layer of wrapping around whatever it's storing; it just needs to add some custom behavior to reads/writes. In standard usage, it should never be null (and should probably always be final...).</div>
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</div>
<div>Except... (sorry, just thinking about this now, didn't raise it earlier): how can a variable wrapper be a value class? The whole point is to control *mutation* of the payload. Value class fields are final.</div>
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<div>So I'll tentatively say that this isn't a category of use cases Valhalla is going to address. You can't have mutable fields without object pointers to get to them.</div>
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</div>
<blockquote type="cite" class="">
<div class="">
<blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<blockquote type="cite" class="">- SIMD vectors, if those are nullable, the VM/JIT will insert implicit null<br class="">
checks which are not usually a problem apart in thigh loop like users write<br class="">
with SIMD vectors.<br class="">
</blockquote>
<br class="">
The *storage* should definitely use a value type, so in this sort of application<br class="">
we'd encourage value-typed array allocations (and value-typed type arguments<br class="">
for wrapping data structures).<br class="">
<br class="">
In a loop over a flat array, I would expect it to be okay to talk about the<br class="">
reference type in source, and have the JIT generate optimal code to work with<br class="">
the underlying flat storage, without any allocations or null checks. My sense<br class="">
is that we suspect that this can work reliably, but it could use more targeted<br class="">
performance testing to confirm.<br class="">
</blockquote>
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">Sorry
for not being clear, i'm more worry about people using IntVector instead of IntVector.val on stack and the VM having to emit a nullcheck inside a loop, by example if there is a call to a method that returns an IntVector is not inlined. For me, the vector API
represents operation so a ref-by-default IntVector is a kind of useless, worst it will work but performance will suffer with no simple way to fix that apart taking a look to the generated assembly code.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
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<div>In theory, there's no problem here: value class reference types can be scalarized everywhere on stack, and null checks/flags can often be optimized away. (And even if they can't—if a loop body can't be inlined and so contains method calls that do null
checks, say—the overhead of null checks is probably not a significant extra cost.)</div>
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</div>
<div>But in practice, totally possible that there are some holes. Insert open invitation here for people to provide performance test cases where on-stack usage of reference types is noticeably more expensive than the equivalent code using value types. That
would really help to ground this conversation (is it something we can optimize away, or an inherent limitation? is it a serious problem for reference-default, or a corner case where we can encourage sprinkling in ".val" as a workaround?)</div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<blockquote type="cite" class="">- existing value classes in Scala or Kotlin, those are not nullable by default<br class="">
but in the current design, getClass() will happily reflect them with a nullable<br class="">
class making Scala/Kotlin second class citizens of the Java platform.<br class="">
</blockquote>
<br class="">
Is the problem here that Scala/Kotlin will want reference-default interpretation<br class="">
of names in Java source? (If so, <shrug>, if you want a good user experience<br class="">
with Kotlin types, write Kotlin source.)<br class="">
<br class="">
</blockquote>
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">I
don't know the answer to that question but i disagree with your conclusion, interrop with Java, so with the Java ecosystem is important.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
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<div>Answer: no, we're not talking about Java source using Kotlin types. It's reflection, discussed below.</div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
Or is the problem that they will want the reflection API to behave differently,<br class="">
making Foo.val.class the "primary" class object, not a secondary one? (If so,<br class="">
<shrug> again, the Java reflection API is Java-oriented, interop is not a major<br class="">
factor in its design.)<br class="">
<br class="">
Maybe I'm missing your point on this one?<br class="">
</blockquote>
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">The
problem is that we are forcing the companion class design to other languages than Java.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">C#
has done the same mistake with their generics being too specific to C# so other languages on the .Net platform all suffer.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">Other
languages than Java can not makes Foo.val.class their primary class object because the companion class design is bolt into the VM.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
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Right: other languages might have value classes that are a good match for value-default—in particular they would be happiest to see the value type when they call 'getClass'. (Perhaps doing something else would be a behavioral incompatibility, should they choose
to migrate their implementation from roll-your-own to native JVM features.)
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<div class="">On this one, I think we agreed that this is a "nice to have", if it works out in a way that is convenient for other languages, but this isn't something that would drive our design decisions.</div>
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