Reviewing feedback on patterns in switch

Wed Feb 16 02:55:37 UTC 2022

On Feb 15, 2022, at 5:58 PM, Remi Forax <forax at univ-mlv.fr<mailto:forax at univ-mlv.fr>> wrote:

________________________________
From: "Brian Goetz" <brian.goetz at oracle.com<mailto:brian.goetz at oracle.com>>
To: "amber-spec-experts" <amber-spec-experts at openjdk.java.net<mailto: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.

And I agree we should try these semantics.

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 ?

That is what I understood the implication to be: something like

let User(var firstname, var lastName) = x when firstName.length() > 8 in
System.out.printf(“User with long first name”);
else System.out.printf(“Not a user, or user with a short first name”);

although this particular example could also be framed as

if (x instanceof User(var firstname, var lastName) && firstName.length() > 8)
System.out.printf(“User with long first name”);
else System.out.printf(“Not a user, or user with a short first name”);

so maybe I am misunderstanding something here, or have misremembered the proposal.

 - 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.

At the risk of premature bikeshedding, have we already discussed and discarded the idea of spelling “when” as “if”? It’s been a long time, and I forget.

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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