<div dir="auto"><div>Whilst I agree with Brian, is this not a smell though if it can't be restricted in this way, that the proposed additions aren't flexible enough to ensure program correctness in what's surely going to be a fairly common pattern?</div><div dir="auto"><br></div><div dir="auto">Maybe withers shouldn't just be a syntactic optimisation for exactly this reason.</div><div dir="auto"><div dir="auto"><br></div><div dir="auto">-- Ryan</div><br><br><div class="gmail_quote gmail_quote_container" dir="auto"><div dir="ltr" class="gmail_attr">On Mon, 26 Jan 2026, 5:40 am Brian Goetz, <<a href="mailto:brian.goetz@oracle.com">brian.goetz@oracle.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><u></u>
<div>
<font size="4" face="monospace">The important mental model here is
that a reconstruction (`with`) expression is "just" a syntactic
optimization for:<br>
<br>
- destructure with the canonical deconstruction pattern<br>
- mutate the components <br>
- reconstruct with the primary constructor<br>
<br>
So the root problem here is not the reconstruction expression; if
you can bork up your application state with a reconstruction
expression, you can bork it up without one. <br>
<br>
</font><font size="4" face="monospace">Primary constructors can
enforce invariants _on_ or _between_ components, such as:<br>
<br>
record Rational(int num, int denom) { <br>
Rational { if (denom == 0) throw ... }<br>
}<br>
<br>
or<br>
<br>
record Range(int lo, int hi) { <br>
Range { if (lo > hi) throw... }<br>
}<br>
<br>
What they can't do is express invariants between the record /
carrier state and "the rest of the system", because they are
supposed to be simple data carriers, not serialized references to
some external system. </font><font size="4" face="monospace">A
class that models a database row in this way is complecting entity
state with an external entity id. By modeling in this way, you
have explicitly declared that <br>
<br>
rec with { dbId++ }<br>
<br>
*is explicitly OK* in your system; that the components of the
record can be freely combined in any way (modulo enforced
cross-component invariants). And there are systems in which this
is fine! But you're imagining (correctly) that this modeling
technique will be used in systems in which this is not fine. <br>
</font><font size="4" face="monospace"><br>
The main challenge here is that developers will be so attracted to
the syntactic concision that they will willfully ignore the
semantic inconsistencies they are creating. <br>
<br>
<br>
</font><br>
<br>
<div>On 1/25/2026 1:37 PM, Andy Gegg wrote:<br>
</div>
<blockquote type="cite">
<font face="Times New Roman">Hello,<br>
I apologise for coming late to the party here - Records have
been of limited use to me but Mr Goetz's email on carrier
classes is something that would be very useful so I've been
thinking about the consequences.<br>
<br>
Since carrier classes and records are for data, in a database
application somewhere or other you're going to get database ids
in records:<br>
record MyRec(int dbId, String name,...)<br>
<br>
While everything is immutable this is fine but JEP 468 opens up
the possibility of mutation:<br>
<br>
</font><font face="Times New Roman">MyRec rec</font><font face="Times New Roman"> = readDatabase(...);<br>
rec = rec with {name="...";};<br>
writeDatabase(rec);<br>
<br>
which is absolutely fine and what an application wants to do.
But:<br>
</font><font face="Times New Roman">MyRec </font><font face="Times New Roman">rec = readDatabase(...);<br>
rec = rec with {dbId++;};<br>
writeDatabase(rec);<br>
<br>
is disastrous. There's no way the canonical constructor invoked
from 'with' can detect stupidity nor can whatever the database
access layer does.<br>
<br>
In the old days, the lack of a 'setter' would usually prevent
stupid code - the above could be achieved, obviously, but the
code is devious enough to make people stop and think (one
hopes).<br>
<br>
Here there is nothing to say "do not update this!!!" except code
comments, JavaDoc and naming conventions.<br>
<br>
It's not always obvious which fields may or may not be changed
in the application.<br>
<br>
record </font><font face="Times New Roman">MyRec(int dbId, int
fatherId,...)<br>
probably doesn't want<br>
rec = rec with { fatherId = ... }<br>
<br>
but a HR application will need to be able to do:<br>
<br>
record </font><font face="Times New Roman">MyRec(int dbId, int
departmentId, ...);<br>
...<br>
rec = rec with { </font><font face="Times New Roman">departmentId
= newDept; };<br>
<br>
Clearly, people can always write stupid code (guilty...) and the
current state of play obviously allows the possibility (rec =
new MyRec(rec.dbId++, ...);) which is enough to stop people
using records here but carrier classes will be very tempting and
that brings derived creation back to the fore.<br>
<br>
It's not just database ids which might need restricting from
update, e.g. timestamps (which are better done in the database
layer) and no doubt different applications will have their own
business case restrictions.<br>
<br>
Thank you for your time,<br>
Andy Gegg<br>
</font><br>
</blockquote>
<br>
</div>
</blockquote></div></div></div>