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<div class="moz-cite-prefix">On 08/05/2024 22:56, Konstantin Triger
wrote:<br>
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<blockquote type="cite" cite="mid:CAGKqGxLtdnM7MXY4YcAMx-JiUb3C5J46Buq9ogTajTsa0gKVTA@mail.gmail.com">
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<div dir="ltr">Hi Maurizio,
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<div>Looking on this, I'm wondering how important is this
case: <span style="color:rgb(51,51,51);font-family:Consolas,Inconsolata,Courier,monospace;font-size:11.05px;background-color:rgb(248,248,248)">Quoted<Function<?,
?>> f = (Integer i) -> </span><span style="font-family:Consolas,Inconsolata,Courier,monospace;font-size:11.05px;color:rgb(221,17,68)">""</span><span style="color:rgb(51,51,51);font-family:Consolas,Inconsolata,Courier,monospace;font-size:11.05px;background-color:rgb(248,248,248)">;</span></div>
<div>The "practical" case is something like: <T, R>
RETURN_TYPE method(Quoted<Function<T, R>>
f).</div>
<div>Which should compile well because of inference.</div>
<div>And if someone insists on working with wildcards, so
he has an option.</div>
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<p>The specific case per se is not important. But if Babylon needs
more help from the language, I think whatever feature we add needs
to slot it correctly into the existing type system/inference
machinery. The Quoted<T> idea at the moment seems more a
case of a square peg in a round hole - but that's not to say that,
maybe, we'll come up with some clever idea to rescue that
approach.</p>
<p>Maurizio<br>
</p>
<blockquote type="cite" cite="mid:CAGKqGxLtdnM7MXY4YcAMx-JiUb3C5J46Buq9ogTajTsa0gKVTA@mail.gmail.com">
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<div>Kosta</div>
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<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, May 8, 2024 at 3:19 PM
Maurizio Cimadamore <<a href="mailto:maurizio.cimadamore@oracle.com" moz-do-not-send="true" class="moz-txt-link-freetext">maurizio.cimadamore@oracle.com</a>>
wrote:<br>
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<p style="margin:0px 0px 1.2em">On 06/05/2024 18:50, Paul
Sandoz wrote:</p>
<div>
<blockquote type="cite">
<pre><blockquote type="cite" style="color:rgb(140,140,140)"><pre>Just wanted to mention that there might be alternative implementations addressing this requirement.
1. Structural quoting (a la C#) - derives from the variable type, e.g. Quoted<T>. So, the user understands that the lambda body will be accessible.
</pre></blockquote><pre>There are some challenges with Java’s type system, which is why we embedded up where we are with the current approaches. Maurizio is better able than I to describe these challenges.</pre></pre>
</blockquote>
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<p style="margin:0px 0px 1.2em">Hi,<br>
we have explored approaches such as <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Quoted<T></code>
(where <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T</code>
is a functional interface) early on. And found them a
bad fit for Java. The main issue we faced is that being
a functional interface, as per today’s JLS, is a
predicate on class declarations not types (there was
some intense discussion on this point during Java 8).
But here we have something that can be treated as a
functional interface or not, precisely depending on its
type (e.g. the type parameter). E.g. a <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Quotable<T></code>
is a functional interface or not depending on <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T</code>.
This is a pretty deep big departure from how the spec
currently deal with functional interfaces.</p>
<p style="margin:0px 0px 1.2em">One possibility would be
for the compiler/spec to ignore the <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Quoted<T></code>,
and just focus on the inner <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T</code>,
and then add the quoting back, after type-checking. But
doing so leads to (bad) edge cases. For instance, Java 8
supports a form of inference of the target type from the
explicit lambda parameter types, in this case:</p>
<pre style="font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em;color:rgb(51,51,51);background:rgb(248,248,248);display:block">Function<?, ?> f = (Integer i) -> <span style="color:rgb(221,17,68)">""</span>;
</code></pre>
<p style="margin:0px 0px 1.2em">In this case, the type of
the lambda expression is not just <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Function<?, ?></code>
(the target type), but <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Function<Integer,Object></code>.
That is, the compiler tries to get rid of the wildcards
before attempting to type-check the lambda expression
(this step is performed to avoid wildcards popping up
when checking the lambda body). But with quoting, this
won’t compose well - e.g.:</p>
<pre style="font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em;color:rgb(51,51,51);background:rgb(248,248,248);display:block">Quoted<Function<?, ?>> f = (Integer i) -> <span style="color:rgb(221,17,68)">""</span>;
</code></pre>
<p style="margin:0px 0px 1.2em">Will give an error:</p>
<pre style="font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;background-color:rgb(248,248,248);white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block">error: incompatible types: Quoted<Function<Integer, Object>> cannot be converted to Quoted<Function<?, ?>>
Quoted<Function<?, ?>> f = (Integer i) -> “”;
^
</code></pre>
<p style="margin:0px 0px 1.2em">That is, the functional
interface type inferred by the compiler (by dropping
wildcards) is now incompatible with the target type
(because of the way in which generics type-argument
containment rules work). To make things work, the client
would need at least to do this:</p>
<pre style="font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;background-color:rgb(248,248,248);white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block">Quoted<? extends Function<?, ?>> f = (Integer i) -> “”;
</code></pre>
<p style="margin:0px 0px 1.2em">Overall it felt like we
were pushing against the physics of the language here.
Serializable lambdas (with all their limitations) were a
strong precedent to build on top of, which didn’t suffer
from any of the limitations described above.</p>
<p style="margin:0px 0px 1.2em">Maurizio</p>
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<span class="gmail_signature_prefix">-- </span><br>
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<div dir="ltr">Regards,<br>
Konstantin Triger<br>
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