class, type, instance, object, value

[John Rose]

On Jul 12, 2017, at 6:20 AM, Karen Kinnear <karen.kinnear at oracle.com> wrote:
> 
> John,
> 
> Thank you for writing this up. Couple of questions/comments.
> 
> - The class of a boxed value is the value class.  (Thus each value class derives at least two types.)
> This one confuses me. I am not sure what you are referring to.
> 1) source: declaration - declared as a value class 
*If* the JLS user model of Valhalla values associates a derived "box" with a value,
something like Integer is associated with int, *then* a value class derives both
the plain value (the primary thing) and the box (a secondary thing).

We are *not* certain that boxes are necessary in L-world.  I think they are forced
on us in U-world.  This is an advantage of L-world, where L-types are at the top
and Q-types just "sneak in".

> 2) classfile - marked via ACC_VALUE, from could derive a value class and a boxed class
Yes, I meant that.
> 3) constant_pool entry - at which we want a way to distinguish the two types
Yes.  A common class with different kind/mode markings, Q vs. L vs. maybe U,
but all the same class.
> 4) vm internal metadata - again distinguish two types
Implementor's choice on that one.
> 5) java level use - at which I am assuming we will have a way to distinguish a value class and a boxed value
OK, I thought that's what you were asking in 1.

Key principle:  One source construct = one class.  (Inner classes are nested
source constructs which allow several classes to be defined by one file;
that's something the JVM can usually ignore, or model via nestmates.)
> I believe Dan’s use of the term value class is for the unboxed value.
There's one class, so if the box is defined from the same one central class,
then I'm OK with Dan's term "value class" for the *one class* that defines
*both* values and boxes.

(I'm discouraging us from using the word "class" to refer to individual values
of the object.  We don't say "print me a string class", we say "print me a
string value" or (maybe) "print me an string instance".  I don't think that's
how Dan was using his terms, but just in case…)
> I think we want to explicitly call out boxed value.
Indeed.  But there is no need to have multiple classes to describe both
values and their boxes; for any given value class it all flows from that
one single value class.  (I suspect we are in violent agreement here.)
> - A "java.lang.Class" is usually a reference to class metadata, but not necessarily unique.
>   There's wiggle room here for class-for-the-box vs. class-for-the-value, and int.class.
>   We don't allow java.lang.Class to constrain other uses of the term "class".
> - When clarity is at risk, we can say "class mirror" rather than just "Class".
> I’m not sure what you mean here by “not necessarily unique”. Perhaps that is because for me a java.lang.Class
> is always a mirror, i.e. a reflection of the vm’s internal representation for the unique class.

Not if we do what Brian calls "crasses".  For example, if we make a "crass" of java.util.List<int>,
then that reflects a "something" (which I call "species") that combines the *class* java.util.List
with the template argument "int".  The result of that *should not* be called a "class", because
that will cause confusion as folks scurry around trying to find the source code and/or classfile
for that "something", and find only plain old java.util.List.

So the term "class" should refer to "what comes out of a classfile", and/or an occurrence
in Java code of a class/interface/enum definition.

A "crass" is a java.lang.Class which refers not just to java.util.List (which is a perfectly
normal class mirror like today's) but to the *species* List<int>.  If we do "crasses" it
will be a carefully weighed decision to overload the legacy type java.lang.Class to
mirror not just true Java classes (and interfaces and enums) but *also* it will mirror
species which are derived from a template.  In this framework, a *box* looks like
another variation stamped out of the original class.

If we don't do "crasses" (the "r" stands for "runtime quasi-class type", approximately),
then we will need new kinds of mirrors, to mirror not the proper classes (and interfaces
and enums) but to mirror the species which are created by specializing templates,
and/or the boxes (if we do them) which represent the identity-laden variants of
identity-free values.

> Even if we use a name&mode to find a class-for-the-box vs. class-for-the-value, the java.lang.Class itself
> would be unique, wouldn’t it?

Hmmm… If we do both "crasses" and boxes then there could be a proper class
mirror for a real value class like java.math.FloatComplex, and then a "crass"
mirror for its identity-laden box, which is not Q-FloatComplex but L-FloatComplex.

If we do L-world (see forthcoming message) we can avoid the box, but then
we might template the complex numbers, and thus have many mirror "crasses"
for the various species Complex<float>, Complex<double>, etc.

> 
> thanks,
> Karen
> 
> 
>> On Jun 25, 2017, at 10:17 PM, John Rose <john.r.rose at oracle.com <mailto:john.r.rose at oracle.com>> wrote:
>> 
>> So, I'm writing more and more documentation that discusses
>> objects and primitives while bringing values into the mix.
>> 
>> What seems right to me is that we allow the terms "class", "type",
>> and "instance" to symmetrically cover both legacy object types
>> and new value types.  We should continue to use the word "value"
>> but be careful about distinguishing its overloadings, especially
>> its role as an absolute noun vs. its role as an adjective.
>> 
>> We should tolerate asymmetries that arise from the reference vs.
>> value distinction, and from box types which arise from value classes.
>> 
>> Summary:
>> 
>> Classes = Object Classes <+> Value Classes
>> Instances = Object Instances <+> Value Instances
>> Object Instances = instances of Object Classes <+> boxes of Value Instances
>> Reference Values = Object Instances  <+> null
>> Values (noun) = Reference Values <+> Value Instances <+> Primitives
>> 
>> (…where <+> denotes disjoint union)
>> 
>> Details:
>> 
>> - A "class" is at root metadata describing a type or implementation.
>>  (It has API surface and/or implementation:  super types, methods, fields, etc.)
>> - An "instance" is derived from a class and/or conforms to that class's API.
>> - An instance of an "object class" (or "object type") is an "object instance" (or just "object").
>> - An instance of an "value class" (or "object value") is a "value instance" (or just "value" if context allows).
>> - When clarity is at risk, we can call a value class or value instance a "non-object class" or "non-object value".
>> - Because object instances are referred to by reference, a variable bound to one is a "reference".
>>  (References can be to object instances, to boxes of value instances, or to the unique reference null.)
>> - A reference can also take "null" ("the null reference") as a value.
>> - References, primitives, and value instances are all "values" since they are passed by value.
>> - An instance of a "value class" is a "value instance" or (when clarity is not at risk) just a "value".
>> - Because value instances are referred to "by value", a variable bound to one is just a "value".
>>  (When clarity is at risk, such a variable can be called a "pure value" or  "non-reference".)
>> 
>> Ambiguity:
>> 
>> - The term "value" used as a noun can refer to the contents of a variable: reference, primitive, or value instance.
>> - The term "value" used as an adjective distinguishes a class, type, or instance from the "object" version.
>> - The term "value" can abbreviate "value instance"; context must clear this usage from ambiguity.
>> - Thus, the term "value" must always be used in a context which resolves it ambiguity.
>>  (We could coin a new term to avoid ambiguity, but the meaning of "value" perfect, so let's keep it.)
>> - Sometimes when we say "value type" we really mean "non-object type", and expect primitives to be included.
>> - As part of fit-and-finish of Value Types we will give the primitives a comfortable seat at the table.
>>  (Perhaps we can cleverly ret-con primitive types as value types, and their wrappers as boxes thereof.)
>> 
>> Boxing, buffering, identity:
>> 
>> - A value can be "boxed" into an object.  Such an object can be "unboxed" back into its value.
>> - Boxed values are true objects, with object type.
>> - The class of a boxed value is the value class.  (Thus each value class derives at least two types.)
>> - A value (of any sort) does not have identity, only the object instance under a non-null reference does.
>> - A "boxed value" (or "value  is an object  has identity, since it is an object instance.
>> - If an implementation uses pointer indirection to access a value, we say it is stored in a "buffer".
>>  (This avoids confusion, since boxes are objects but buffers are not necessarily objects.)
>> - Buffers are invisible to the user, except perhaps via performance effects, or trusted APIs like Unsafe.
>> - Buffers can be on the stack, the heap, or anywhere else in memory.
>> - Boxes can secretly serve as buffers.
>> 
>> False friends:
>> 
>> - A "java.lang.Class" is usually a reference to class metadata, but not necessarily unique.
>>  There's wiggle room here for class-for-the-box vs. class-for-the-value, and int.class.
>>  We don't allow java.lang.Class to constrain other uses of the term "class".
>> - When clarity is at risk, we can say "class mirror" rather than just "Class".
>> - Similar point for "CONSTANT_Class" in the constant pool schema. Relatively few folks
>>  are conscious of this term anyway.
>> - "Object-oriented" programming usually refers to some combination of
>>  classes with reference-based polymorphism.  Value types are object
>>  oriented, even though their reference-based polymorphism is limited to
>>  interfaces.  Also they are squarely based on classes.
>> 
>> Glossary:
>> 
>> value type: a type which may be used without an accompanying reference (i.e., no intrinsic reference identity or aliasing)
>> value class:  a code entity which defines a value type
>> value instance: a possible value (at runtime) of a variable of value type, derived from a value class
>> value field: (ambig.) field whose type is a value type (in any kind of class) OR a field in a value class (of any type)
>> value parameter: (ambig.) a parameter whose type is a value type OR a parameter, with emphasis on by-value transmission
>> value: (ambig.) a reference, value instance, or primitive OR context-dependent ellipsis for value type/class/instance
>> 
>> object type: a type without references (i.e., no reference identity, no aliasing)
>> object class:  a code entity which defines an object type
>> object instance: a possible value (at runtime) of a non-null variable of reference type
>> object field/parameter: (ambig., see above)
>> object: (ambig.) a reference to an object instance OR context-dependent ellipsis for value type/class/instance
>> 
>> box type: an object type derived from a value class
>> box instance: a possible value (at runtime) of a non-null variable of box type
>> box: (ambig., see above)
>> 
>> instance/class/type: (ambig., see above)
>> 
>