Reviewing feedback on patterns in switch

[Remi Forax]

> From: "Brian Goetz" <brian.goetz at oracle.com>
> To: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Sent: Tuesday, February 15, 2022 7:50:06 PM
> Subject: Re: Reviewing feedback on patterns in switch

> We're preparing a third preview of type patterns in switch. Normally we would
> release after a second preview, but (a) we're about to get record patterns,
> which may disclose additional issues with switch, so best to keep it open for
> at least another round, and (b) we're proposing some nontrivial changes which
> deserve another preview.

> Here's where we are on these.

>> 1.  Treatment of total patterns in switch / instanceof

> Quite honestly, in hindsight, I don't know why we didn't see this sooner; the
> incremental evolution proposed here is more principled than where we were in
> the previous round; now the construct (instanceof, switch, etc) *always* gets
> first crack at enforcing its nullity (and exception) opinions, and *then*
> delegates to the matching semantics of the pattern if it decides to do so. This
> fully separates pattern semantics from conditional construct semantics, rather
> than complecting them (which in turn deprived users of seeing the model more
> clearly.) In hindsight, this is a no-brainer (which is why we preview things.)
> We'll be addressing this in the 3rd preview.
Not sure it's a no-brainer. 
The question is more a question of consistency. There are two consistencies and we have to choose one, either switch never allows null by default and users have to opt-in with case null or we want patterns to behave the same way if they are declared at top-level or if they are nested. I would say that the semantics you propose is more like the current Java and the other semantics is more like the Java of a future (if we choose the second option). 

I think we should try the semantics you propose and see if people agree or not. 

>> 2.  Positioning of guards

> Making guards part of switch also feels like a better factoring than making them
> part of patterns; it simplifies patterns and totality, and puts switch on a
> more equal footing with our other conditional constructs. We did go back and
> forth a few times on this, but having given this a few weeks to settle, I'm
> pretty convinced we'd regret going the other way.

> There were two sub-points here: (a) is the guard part of the pattern or part of
> switch, and (b) the syntax. There was general agreement on (a), but some had
> preference for && on (b). I spent some more time thinking about this choice,
> and have come down firmly on the `when` side of the house as a result for a
> number of reasons.
Still agree on (a) 

> - Possibility for ambiguity. If switching over booleans (which we will surely
> eventually be forced into), locutions like `case false && false` will be very
> confusing; it's pure puzzler territory.
> - && has a stronger precedence than keyword-based operators like `instanceof`';
> we want guards to be weakest here.
I don't understand your point, we want instanceof pattern && expression to be equivalent to instanceof type && expression + cast, so the fact that && has a stronger precedence makes that possible so it's not an issue. 

> - Using && will confuse users about whether it is part of the expression, or
> part of the switch statement. If we're deciding it is part of the switch, this
> should be clear, and a `when` clause makes that clear.
I don't think it's that important, apart if we start to also want to combine patterns with && 

> - There are future constructs that may take patterns, and may (or may not) want
> to express guard-like behavior, such as `let` statements (e.g., let .. when ..
> else.) Expressing guards here with && is even less evocative of "guard
> condition" than it is with switches.
It's not clear to me how to use "let when else". Is it more like a ?: in C than the let in in Caml ? 

> - Users coming from other languages will find `case...when` quite clear.
> - We've talked about "targetless" switches as a possible future feature, which
> express multi-way conditionals:

> switch {
> case when (today() == TUESDAY): ...
> case when (location() == GREENLAND): ...
> ...
> }

> This would look quite silly with &&.
For me, this is like cond in Lisp but more verbose. using "case" and "when" here is sillly. 

> Similarly, one could mix guards with a targeted switch:

> switch (x) {
> case Time t: ...
> case Place p: ...
> default when (today() == TUESDAY): ... tuesday-specific default
> default: ... regular default ...
default && today() == TUESDAY is fine for me. 

> Expressing guards that are the whole condition with `when` is much more natural
> than with &&.
For me, && is more natural than "when" because i've written more switch that uses && than "when". 
And don't forget that unlike most of the code, with pattern matching the number of characters does matter, this is more similar to lambdas, if what you write is too verbose, you will not write it. 

> tl;dr: inventing a `when` modifier on switch now will save us from having to
> invent something else in the future; choosing && will not.
I want to be convinced, but i'm not, the argument about the precedence is exactly why we have chosen && in the first place. 

Rémi 
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