Late change to JEP 433

[Brian Goetz]

I win my bet!  I had my money on "If you send this, Remi will surely 
attempt to reopen his pet NPE issue."  We have been through this all 
before, so unless you have something dramatically new and compelling, 
we're not reopening this issue now.

As a reminder, not all causes of MatchException come from separate 
compilation anomalies.  Some come from match remainder, which is just an 
ordinary domain error, and separating the two is only possible in the 
easy cases.

I am not sure you fully understand how this works, otherwise you 
wouldn't keep raising the same issue after getting the same 
explanation.  (For example, the "NPE can be thrown organically" claim 
you make here, which you've made before, is wrong; it is possible a 
later pattern can match Box(null), and we do not throw ME until all 
choices are exhausted.)






On 11/14/2022 6:23 PM, forax at univ-mlv.fr wrote:
>
>
> ------------------------------------------------------------------------
>
>     *From: *"Brian Goetz" <brian.goetz at oracle.com>
>     *To: *"Remi Forax" <forax at univ-mlv.fr>, "John Rose"
>     <john.r.rose at oracle.com>
>     *Cc: *"Alex Buckley" <alex.buckley at oracle.com>,
>     "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
>     *Sent: *Monday, November 14, 2022 11:40:27 PM
>     *Subject: *Re: Late change to JEP 433
>
>     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)).
>
>
> I still think we should throw a NPE when a destructuring something 
> which is null.
>
> It will make the semantics of MatchException very similar to 
> LambdaConversionException, i.e. a lot of users will never see it 
> because it can be thrown only when there is a separate compilation 
> issue or a bug in the deconstructor.
>
> If you take a look to the implementation, the NPEs can be thrown 
> organically as the result of calling the deconstructor (or the 
> accessor methods) on null, instead of adding a nullcheck that goto to 
> the location where the MatchException is thrown.
>
> Rémi
>
>
>
>     On 11/14/2022 5:31 PM, Remi Forax wrote:
>
>         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 -----
>
>             From: "John Rose"<john.r.rose at oracle.com>
>             To: "Alex Buckley"<alex.buckley at oracle.com>
>             Cc: "amber-spec-experts"<amber-spec-experts at openjdk.java.net>
>             Sent: Monday, November 14, 2022 8:24:04 PM
>             Subject: Re: Late change to JEP 433
>
>             On 14 Nov 2022, at 10:56, Alex Buckley wrote:
>
>                 … ICCE can hand over to MatchException …
>
>             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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