Late change to JEP 433

[John Rose]

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!”