<html><body><div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: #000000"><div><br></div><div><br></div><hr id="zwchr" data-marker="__DIVIDER__"><div data-marker="__HEADERS__"><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><b>From: </b>"Brian Goetz" <brian.goetz@oracle.com><br><b>To: </b>"Remi Forax" <forax@univ-mlv.fr><br><b>Cc: </b>"valhalla-spec-experts" <valhalla-spec-experts@openjdk.java.net><br><b>Sent: </b>Wednesday, September 4, 2024 4:44:05 PM<br><b>Subject: </b>Re: Null-Restricted and Nullable Types<br></blockquote></div><div data-marker="__QUOTED_TEXT__"><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4" face="monospace">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. <br><br>
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.</font></blockquote><div><br></div><div>yes, this distinction is very important.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4" face="monospace">Let's assume we are a long
way from talking about what the surface syntax might look like.<br><br>
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.<br><br>
- T! -- the argument type, but null-restricted if it wasn't
already<br>
- T? -- the argument type, but explicitly null-allowing if it was
null-restricted<br>
- T* -- the argument type<br>
- T~ -- the argument type, but of *unknown* nullability<br><br>
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.<br><br>
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:<br><br>
- the client wants to use null-marked parameterizations
(Foo<String!>), but the class has not yet considered nullity
restrictions (legacy class)<br>
- the class wants to use null markings in its API, but the client
is not prepared to deal with nullity restrictions (legacy client)<br>
- both client and class are null-aware<br><br>
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".</font></blockquote><div><br></div><div>yes, and I think it can be useful to see T~ and String (raw) the same way, because it makes legacy class code and legacy client code the same thingy.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4" face="monospace"><br>
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.</font></blockquote><div><br></div><div>The drawback of letting too many possible migration paths to the user is that the majority of the users may never reach the maximal type checking.<br data-mce-bogus="1"></div><div>If you have several stages, it's not obvious that moving to stage 0 to stage 1, and from stage 1 to stage 2 is easier than moving from stage 0 to stage 2 in one step, this is a classical problem with gradual typing.</div><div><br data-mce-bogus="1"></div><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4" face="monospace">There are many tradeoffs here about "more restrictions but sounder
type checking" vs "lenient but more chances for runtime errors." </font></blockquote><div><br></div><div>For me, this is another axis. I think here, we have not a lot of leeway, given we want to support a world with both pre-nullity code and post-nullity code, we have to have null pollution warnings.<br></div><div>In most cases, this is okay because the pollution does not go far, it stops that the next function call (intra-function). Casting type variables or parametrized types sadly may have inter-functions side effects.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div>Rémi<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4" face="monospace"><br><br><br><br></font><br>
<div class="moz-cite-prefix">On 9/4/2024 10:06 AM, <a class="moz-txt-link-abbreviated" href="mailto:forax@univ-mlv.fr" target="_blank">forax@univ-mlv.fr</a>
wrote:<br>
</div>
<blockquote cite="mid:1750449966.63590551.1725458789773.JavaMail.zimbra@univ-eiffel.fr">
<div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: #000000">
<div><br>
</div>
<div><br>
</div>
<hr id="zwchr">
<div>
<blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><b>From:
</b>"Brian Goetz" <a class="moz-txt-link-rfc2396E" href="mailto:brian.goetz@oracle.com" target="_blank"><brian.goetz@oracle.com></a><br>
<b>To: </b>"Remi Forax" <a class="moz-txt-link-rfc2396E" href="mailto:forax@univ-mlv.fr" target="_blank"><forax@univ-mlv.fr></a><br>
<b>Cc: </b>"valhalla-spec-experts"
<a class="moz-txt-link-rfc2396E" href="mailto:valhalla-spec-experts@openjdk.java.net" target="_blank"><valhalla-spec-experts@openjdk.java.net></a><br>
<b>Sent: </b>Wednesday, September 4, 2024 3:26:01 PM<br>
<b>Subject: </b>Re: Null-Restricted and Nullable Types<br>
</blockquote>
</div>
<div>
<blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4" face="monospace">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.</font></blockquote>
<div><br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>Rémi<br>
</div>
<div><br>
</div>
<blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><br>
<br>
<div class="moz-cite-prefix">On 9/4/2024 8:36 AM, <a class="moz-txt-link-abbreviated moz-txt-link-freetext" href="mailto:forax@univ-mlv.fr" target="_blank">forax@univ-mlv.fr</a> wrote:<br>
</div>
<blockquote cite="mid:154950232.63481176.1725453401380.JavaMail.zimbra@univ-eiffel.fr">
<div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: #000000">
<div>Here are my questions:<br>
</div>
<div><br>
</div>
<div>1/ Nullness of type variable<br>
</div>
<div> "Like other types, a type-variable type (that is,
a <em>use</em> of a type variable) may express
nullness. <code>T!</code> is a null-restricted type,
and <code>T?</code> is a nullable type."<br>
</div>
<div> - what does 'T' exactly mean ?<br>
</div>
<div> 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.<br>
</div>
<div> During the JVMLS, Dan Smith references them as
'T' and 'T*'. </div>
<div><br>
</div>
<div>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<br>
</div>
<div> - model 1, we let the user annotate using '!',
'?' and '*'<br>
</div>
<div> - model 2, we have an opt-in mechanism that set
type as '!' by default and type variable as '*' by
default.<br>
</div>
<div> - 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<br>
(this is the semantics of jspecify).</div>
<div><br>
</div>
<div>
<div>3/ Nullness of wildcards<br>
</div>
<div> "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."<br>
</div>
<div><br>
</div>
<div> => I see two questions here :<br>
</div>
<div> - 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?>.</div>
<div> Do you agree ?</div>
<div> - 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,</div>
<div> so the content is neither a '!' nor a '?'.
Do you agree ?</div>
</div>
<div><br>
</div>
<div>Rémi<br>
</div>
<div><br>
</div>
<hr id="zwchr">
<div>
<blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><b>From:
</b>"Brian Goetz" <a class="moz-txt-link-rfc2396E" href="mailto:brian.goetz@oracle.com" target="_blank"><brian.goetz@oracle.com></a><br>
<b>To: </b>"Remi Forax" <a class="moz-txt-link-rfc2396E" href="mailto:forax@univ-mlv.fr" target="_blank"><forax@univ-mlv.fr></a>,
"valhalla-spec-experts" <a class="moz-txt-link-rfc2396E" href="mailto:valhalla-spec-experts@openjdk.java.net" target="_blank"><valhalla-spec-experts@openjdk.java.net></a><br>
<b>Sent: </b>Tuesday, September 3, 2024 10:44:03 PM<br>
<b>Subject: </b>Re: Null-Restricted and Nullable
Types<br>
</blockquote>
</div>
<div>
<blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4" face="monospace">There are many points
that are not clear to us as well, but we will try
to respond to your questions :)</font><br>
<br>
<div class="moz-cite-prefix">On 9/3/2024 4:41 PM,
Remi Forax wrote:<br>
</div>
<blockquote cite="mid:1949658111.62587211.1725396089114.JavaMail.zimbra@univ-eiffel.fr">
<pre class="moz-quote-pre">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] <a class="moz-txt-link-freetext" href="https://openjdk.org/jeps/8303099" target="_blank">https://openjdk.org/jeps/8303099</a>
</pre>
</blockquote>
<br>
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</div>
</blockquote>
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