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Upon reviewing the current documentation, I see that I misstated an
earlier point. StructuredTaskScope.join() returns a result of type R
(as produced by the configured Joiner), rather than returning null
unconditionally on success.<br>
<br>
That said, the outcome contract remains: join() either returns a
value or throws. After waiting for completion or cancellation, the
scope invokes Joiner.result(); if that method throws, join() throws
FailedException with the underlying exception as the cause (with
timeouts and cancellation also surfaced as exceptions).<br>
<br>
So while Joiners make join() result-producing and more configurable,
the failure channel is still exception-based. From that perspective,
I can still see value in a functional, value-oriented result
type—where success and failure are both represented explicitly as
values—coexisting alongside exceptions, rather than routing expected
failure exclusively through throws.<br>
<br>
Joiners improve policy flexibility, but they don’t quite address
that particular concern.<br>
<br>
Respectfully,<br>
<p>Eric Kolotyluk</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 2025-12-18 4:46 PM, Eric Kolotyluk
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:b309d805-d90b-4657-a5f7-2031ce26d879@kolotyluk.net">
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Respectfully, I think we’re talking past each other a bit.<br>
<br>
Calling Rust’s error handling “horrible” is a subjective judgment
about trade-offs, not an objective flaw. Rust’s Result<T, E>
is deliberately value-oriented and explicit; Java’s exception
model is deliberately stack-oriented and implicit. Each optimizes
for different things, and each has real costs.<br>
<br>
My appreciation of Java’s evolution is that it has consistently
expanded the set of available tools, rather than insisting on a
single paradigm. Generics, lambdas, streams, records, sealed
types, Optional, and now Loom itself all reflect that trajectory.
They didn’t replace older mechanisms; they complemented them.<br>
<br>
There has been sustained criticism in the Java community of both
null and over-reliance on exceptions, particularly where failure
is expected rather than exceptional. I’m not here to relitigate
either debate, nor to argue that exceptions should go away. My
point is simply that other options exist, and Java has
historically been at its best when APIs acknowledge and support
them.<br>
<br>
In that light, my concern with StructuredTaskScope.join() is not
that it uses exceptions at all, but that it offers only an
exception-based outcome model, with null representing success.
That feels like a missed opportunity in an otherwise
forward-looking API.<br>
<br>
I’m advocating for additional, not replacement, abstractions—ones
that allow structured concurrency outcomes to be expressed
explicitly when appropriate, while leaving exceptions fully
available for genuinely exceptional conditions.<br>
<br>
Respectfully,<br>
Eric Kolotyluk
<p><br>
</p>
<div class="moz-cite-prefix">On 2025-12-18 1:34 PM, Robert Engels
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:8C2482BF-515C-4A89-8E68-197399E89452@me.com">
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My two cents… Rust’s error handling is horrible - it is designed
to work in functional contexts, so like Java streams - the error
handling feels “random” (and finding out where the error
actually occurred is extremely difficult since it is a value
type).
<div><br>
</div>
<div>Java’s Exceptions are for ‘exceptional conditions’ and
should not be used for flow control (which I don’t think they
are in this case - they signify unexpected error conditions).</div>
<div><br>
</div>
<div>
<div><br>
<blockquote type="cite">
<div>On Dec 18, 2025, at 3:24 PM, Eric Kolotyluk <a
class="moz-txt-link-rfc2396E"
href="mailto:eric@kolotyluk.net"
moz-do-not-send="true"><eric@kolotyluk.net></a>
wrote:</div>
<br class="Apple-interchange-newline">
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<div> My $0.02<br>
<br>
Why are we still relying so heavily on exceptions as a
control-flow mechanism?<br>
<br>
Consider the current StructuredTaskScope design:<br>
<br>
The join() method waits for all subtasks to succeed or
any subtask to fail.<br>
The join() method returns null if all subtasks
complete successfully.<br>
It throws StructuredTaskScope.FailedException if any
subtask fails, with the exception from the first
subtask to fail as the cause.<br>
<br>
This design encodes normal outcomes as null and
expected failure modes as exceptions. That choice
forces callers into the least informative and least
composable error-handling model Java has.<br>
<br>
Returning null for success is especially problematic.
null conveys no semantic information, cannot carry
context, and pushes correctness checks to runtime. It
remains one of Java’s most damaging design decisions,
and Loom should not be perpetuating it.<br>
<br>
Optional<T> exists, but it is only a partial
solution and does not address error information. In
this context, even Optional<Void> would be an
improvement over null, but it still leaves failure
modeled exclusively as exceptional control flow.<br>
<br>
I also want to be clear that I am not confusing
try-with-resources with exceptions.
StructuredTaskScope being AutoCloseable is the right
design choice for lifetime management and
cancellation, and try blocks are the correct mechanism
for that. However, scope lifetime and outcome
reporting are separable concerns. The use of try does
not require that task outcomes be surfaced exclusively
via thrown exceptions.<br>
<br>
As a recent Rust convert, the contrast is stark.
Rust’s Result<T, E> treats failure as a
first-class, explicit outcome, enforced by the type
system. Java doesn’t need to abandon exceptions—but it
does need to support alternate paradigms where failure
is expected, structured, and composable.<br>
<br>
APIs like join() should envision a future beyond
“success = null, failure = throw”. Even a simple
structured outcome type—success or failure—would be a
step forward. Exceptions could remain available for
truly exceptional conditions, not routine concurrency
outcomes.<br>
<br>
Loom is a rare opportunity to modernize not just how
Java runs concurrent code, but how Java models
correctness and failure. Re-entrenching null and
exception-only outcomes misses that opportunity.<br>
<br>
I’ll stop bloviating now.<br>
<br>
Sincerely,<br>
Eric Kolotyluk<br>
<br>
<p><br>
</p>
<p>On 2025-12-18 1:00 PM, David Alayachew wrote:</p>
<blockquote type="cite"
cite="mid:CAA9v-_MxzoznSe6EFJ8P44RTQVm6SmVTiuVuQ-nN4xPPZppodw@mail.gmail.com">
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content="text/html; charset=UTF-8">
<div dir="auto">
<div>For 1, the javadoc absolutely does help you.
Please read for open.
<div dir="auto"><br>
</div>
<div dir="auto"><a
href="https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/StructuredTaskScope.html#open()"
rel="noreferrer noreferrer" target="_blank"
moz-do-not-send="true"
class="moz-txt-link-freetext">https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/StructuredTaskScope.html#open()</a></div>
<div dir="auto"><br>
</div>
<div dir="auto">As for verbose, can you go into
more detail? This is a traditional builder
pattern addition, so it is literally 1 static
method call.</div>
<div dir="auto"><br>
</div>
<div dir="auto">That said, if you dislike a 0
parameter call being forced into being a 2
paramefer call when you need to add timeout,
then sure, I think adding an overload for that
static method that takes in the configFunction
is reasonable. I'd support that.</div>
<br>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, Dec
18, 2025, 3:46 PM Holo The Sage Wolf <<a
href="mailto:holo3146@gmail.com"
rel="noreferrer noreferrer"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">holo3146@gmail.com</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">
<div>Hello Loom devs,</div>
<div>Few years ago I experimented in a
personal PoC project with
StructuredConcurrency in Java 19 and I
had to stop working on it for personal
reasons.</div>
<div><br>
</div>
<div>Recently I came back to the project
and updated it to Java 25 and had to
change my code to the new way the API is
built and while doing that I noticed a
couple of stuff I want to point out:</div>
<div><br>
</div>
<div>1. The default Joiner method can't
receive timeout</div>
<div>Obviously that is wrong, but the API
and JavaDoc don't actually help you. Say
you start with:</div>
<div> ```java</div>
<div>try (var scope =
StructuredTaskScope.open()) {</div>
<div> ...</div>
<div>}</div>
<div>```</div>
<div>And I want to evolve the code to add
timeout, I look at
the StructuredTaskScope static methods,
and won't see any way to do that. After
reading a bit
what StructuredTaskScope.open(Joiner, configFunction)
does, I will realise that I can set the
timeout using the configFunction.</div>
<div>But then I will encounter the problem
that I need to provide a Joiner,
currently the only way to actually get
the "no args method"-joiner is to look
at the source code of the method, see
which Joiner it uses and copy that into
my method to get:</div>
<div>
<div> ```java</div>
<div>try (var scope =
StructuredTaskScope.open(Joiner.awaitAllSuccessfulOrThrow(), (conf)
-> ...)) {</div>
<div> ...</div>
<div>}</div>
<div>```</div>
<div>Not only is this a lot of work to
do something very simple, there is a
high chance that people who start
learning concurrency will want to use
timeout before they even know what the
Joiner object is.</div>
<div><br>
</div>
<div>2. Changing only the timeout is
"verbose".</div>
I can only talk from my experience, so I
may have the wrong impression, but I
feel like setting timeout is orders of
magnitude more common than changing the
default ThreadFactory (especially when
using virtual threads) or setting a
name.</div>
<div>I feel like adding a couple of
overloads of the open method that takes
only an extra parameter of duration will
be convenient:</div>
<div>> StructuredTaskScope.open()</div>
<div>>
StructuredTaskScope.open(Duration
timeout)
<div>>
StructuredTaskScope.open(Joiner
joiner)</div>
<div>>
StructuredTaskScope.open(Joiner
joiner, Duration timeout) </div>
<div>>
StructuredTaskScope.open(Joiner
joiner, Function<Configuration,
Configuration> configFunction) </div>
<br>
</div>
</div>
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