Null-Restricted and Nullable Types

[Brian Goetz]

I appreciate the attempt to avoid the rabbit hole -- but there are just 
so many rabbits!  But let me try to separate the layers.  Each of these 
is a substantial discussion, and we're still gathering our thoughts on 
most of them.

We have to be very careful to separate the notations we use for 
discussing the semantics from notations that are being seriously 
proposed as part of the user model.  Let's assume we are a long way from 
talking about what the surface syntax might look like.

For a use of a type variable, there are four possible ways to combine a 
type variable with nullity information.  Let's call "the argument type" 
the thing the user put in the pointy brackets.

  - T! -- the argument type, but null-restricted if it wasn't already
  - T? -- the argument type, but explicitly null-allowing if it was 
null-restricted
  - T* -- the argument type
  - T~ -- the argument type, but of *unknown* nullability

I think we can agree we don't want to burden users with all of these 
fine distinctions; there is a discussion to be had about "what does bare 
T mean", but we should wait.

The reason we even talk about T* vs T~ is because of _migration_. And, 
unlike the migration from non-generic to generic, we have to deal with 
three separate migration scenarios:

  - the client wants to use null-marked parameterizations 
(Foo<String!>), but the class has not yet considered nullity 
restrictions (legacy class)
  - the class wants to use null markings in its API, but the client is 
not prepared to deal with nullity restrictions (legacy client)
  - both client and class are null-aware

So T~ is the type variable equivalent of "I have some old pre-nullity 
code, I have no idea what it thinks the nullity of this type is".

We of course want to get to the point where we get maximal type checking 
in the last stage, but we don't want to put impediments in people's way, 
and are aware that they may take either path. There are many tradeoffs 
here about "more restrictions but sounder type checking" vs "lenient but 
more chances for runtime errors." A strict "flag day" transition from 
"old world" to "new world" on both sides would have simple semantics, 
but would be a horrible experience for users, which would drastically 
slow the uptake of the feature.




On 9/4/2024 10:06 AM, forax at univ-mlv.fr wrote:
>
>
> ------------------------------------------------------------------------
>
>     *From: *"Brian Goetz" <brian.goetz at oracle.com>
>     *To: *"Remi Forax" <forax at univ-mlv.fr>
>     *Cc: *"valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
>     *Sent: *Wednesday, September 4, 2024 3:26:01 PM
>     *Subject: *Re: Null-Restricted and Nullable Types
>
>     Let's take a step back.  Are you looking for *answers* in the
>     short term (say, so you can answer someone elses questions), or
>     are you looking to open the dialog about how we will expose
>     nullness for purposes of generics migration?  Because there are
>     many, many pages of things to say about these questions, not all
>     of which we have answers to, and this will significantly affect
>     how we structure the discussion.
>
>
> I'm opening the discussion. Those questions are like anchors to me, to 
> try to avoid to go too deep into one of the rabbit holes.
>
> Rémi
>
>
>
>     On 9/4/2024 8:36 AM, forax at univ-mlv.fr wrote:
>
>         Here are my questions:
>
>         1/ Nullness of type variable
>           "Like other types, a type-variable type (that is, a /use/ of
>         a type variable) may express nullness. |T!| is a
>         null-restricted type, and |T?| is a nullable type."
>          - what does 'T' exactly mean ?
>            For me, there is the old 'T' which propagate the type but
>         not propagate the nullability and there is the new 'T' that
>         propagate both the type and the nullability of the type.
>            During the JVMLS, Dan Smith references them as 'T' and 'T*'.
>
>         2/ If we agree that there are 4 different kinds: String/T,
>         String?/T?, String!/T! and T*, we now have the choice of
>         several user facing models
>           - model 1, we let the user annotate using '!', '?' and '*'
>           - model 2, we have an opt-in mechanism that set type as '!'
>         by default and type variable as '*' by default.
>           - model 3, we only let user to annotate type variable
>         declaration, type of field, type of parameter types/return
>         type, parametrized type (inside the angle brackets) and cast,
>         the rest is inferred
>             (this is the semantics of jspecify).
>
>         3/ Nullness of wildcards
>           "A type variable declaration or wildcard may have nullness
>         markers on its bounds. A type may satisfy the bounds via
>         nullness conversion, though, so again these nullness markers
>         are not strongly enforced, but may cause warnings."
>
>           => I see two questions here :
>            - unbounded wildcards, they are special because they
>         represent a reified type, so ? is nullable in List<?> because
>         list.add(null) is valid at runtime, thus List<?> is equivalent
>         to List<? extends Object?>.
>              Do you agree ?
>            - super wildcard, the content of List<? super Foo!> may
>         accept null because Foo? is a super type of Foo!, so List<?
>         super Foo!> is maybe nullable or maybe not,
>              so the content is neither a '!' nor a '?'. Do you agree ?
>
>         Rémi
>
>         ------------------------------------------------------------------------
>
>             *From: *"Brian Goetz" <brian.goetz at oracle.com>
>             *To: *"Remi Forax" <forax at univ-mlv.fr>,
>             "valhalla-spec-experts"
>             <valhalla-spec-experts at openjdk.java.net>
>             *Sent: *Tuesday, September 3, 2024 10:44:03 PM
>             *Subject: *Re: Null-Restricted and Nullable Types
>
>             There are many points that are not clear to us as well,
>             but we will try to respond to your questions :)
>
>             On 9/3/2024 4:41 PM, Remi Forax wrote:
>
>                 Hello,
>                 if everybody is okay with that i would like to discuss about Null-Restricted and Nullable Types [1] tomorrow.
>
>                 There are several points that are not clear to me, i will try to come up with a list for tomorrow.
>
>                 Rémi
>
>                 [1]https://openjdk.org/jeps/8303099
>
>
>
>
>
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