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<font size="4"><font face="monospace">Its MatchException. Error
would not be appropriate, since this is the same exception that
gets used for remainder (e.g., Box(Box(var x)) against a target
of Box(null)).</font></font><br>
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<div class="moz-cite-prefix">On 11/14/2022 5:31 PM, Remi Forax
wrote:<br>
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<blockquote type="cite" cite="mid:842334026.45461578.1668465117769.JavaMail.zimbra@u-pem.fr">
<pre class="moz-quote-pre" wrap="">I'm confused, is it MatchError or MatchException ?
Because if it's an error instead of an exception, it may be less an issue in term of backward compatibility but it is not what is proposed, right ?
Rémi
----- Original Message -----
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<pre class="moz-quote-pre" wrap="">From: "John Rose" <a class="moz-txt-link-rfc2396E" href="mailto:john.r.rose@oracle.com"><john.r.rose@oracle.com></a>
To: "Alex Buckley" <a class="moz-txt-link-rfc2396E" href="mailto:alex.buckley@oracle.com"><alex.buckley@oracle.com></a>
Cc: "amber-spec-experts" <a class="moz-txt-link-rfc2396E" href="mailto:amber-spec-experts@openjdk.java.net"><amber-spec-experts@openjdk.java.net></a>
Sent: Monday, November 14, 2022 8:24:04 PM
Subject: Re: Late change to JEP 433
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<pre class="moz-quote-pre" wrap="">On 14 Nov 2022, at 10:56, Alex Buckley wrote:
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<pre class="moz-quote-pre" wrap="">… ICCE can hand over to MatchException …
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<pre class="moz-quote-pre" wrap="">
Precisely; I agree that it is time for this to happen. Thanks, Alex for
reminding us of the history and lineage of ICCE, and why it doesn’t make sense
for switch statements (not even classic switch-over-enum).
As a program linkage error, ICCE is necessarily uncommunicative when applied to
a misconfigured switch statement. Using a MatchError is on the other hand
highly informative: The user (and possibly try/catch logic surrounding the
failure) knows exactly what happened, that a set of matches expected to succeed
has failed.
So, here’s a possible knock-on advantage if we hand off from ICCE to ME: If
there is further development of the concept of “a set of matches which is
required to succeed”, the error processing can continue to be unified under the
ME. Brian’s ideas about “let-statements” entail a single pattern which is
required to match; ME is surely the right way to signal failure (if not
something more specific like CCE or NPE, which is an interesting
side-conversation). Or, if we ever did Haskell-style method overloads that
discriminate arguments by means of patterns, surely they would desugar to
omnibus methods that start with switches; once again ME surely makes sense as a
way to signal inapplicability of such match-based methods.
— John
P.S. More speculatively, and probably a bridge too far, would be to employ
MatchException as a part of a meta-language protocol that defines how sets of
patterns compose, and in particular how one part of a composite signals failure
to the whole composite. (Surely there is some future use for X in methods :
method handles :: patterns : X; that’s what I mean by a meta-language
protocol.) I say this is a bridge too far because Java exceptions are not a
very good tool for normally-frequent control flow, and also because ME, like
NPE or CCE, probably best signals a failure of the programmer’s settled
intentions about some code, rather than signaling an alternative control path
(like if/else). Still, I wanted to point this out because in other languages
exception-like concepts are used to convey backtracking out of composite
control flow patterns, and if we decided to try this out for Java,
MatchExpression would raise its hand and say “pick me!”
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