> De: "Brian Goetz" <brian.go...@oracle.com> > À: "Remi Forax" <fo...@univ-mlv.fr> > Cc: "Gavin Bierman" <gavin.bier...@oracle.com>, "amber-spec-experts" > <amber-spec-experts@openjdk.java.net> > Envoyé: Mardi 9 Avril 2019 21:14:25 > Objet: Re: switch statement and lambda
> I see why this is tempting, but I am going to suggest we wait. As part of > Valhalla, we would like for `void` to become a real type some day; that will > require evaluating all the places in the JLS where we treat statement and > expressions differently, or make exceptions like this. Until we’ve completed > this analysis, I’m reluctant to add more special cases. seems wise, ok ! Rémi >> On Apr 9, 2019, at 3:10 PM, [ mailto:fo...@univ-mlv.fr | fo...@univ-mlv.fr ] >> wrote: >>> De: "Gavin Bierman" < [ mailto:gavin.bier...@oracle.com | >>> gavin.bier...@oracle.com ] > >>> À: "Remi Forax" < [ mailto:fo...@univ-mlv.fr | fo...@univ-mlv.fr ] > >>> Cc: "amber-spec-experts" < [ mailto:amber-spec-experts@openjdk.java.net | >>> amber-spec-experts@openjdk.java.net ] > >>> Envoyé: Mardi 9 Avril 2019 19:28:57 >>> Objet: Re: switch statement and lambda >>>> On 6 Apr 2019, at 21:17, Remi Forax < [ mailto:fo...@univ-mlv.fr | >>>> fo...@univ-mlv.fr ] > wrote: >>>> Currently this code doesn't compile >>>> IntConsumer c = x -> switch(x) { default -> System.out.println(x); }; >>>> I believe it should because this is the basic pattern for supporting the >>>> actor >>>> model, >>>> you consume a message and do a side effect* depending on the type of the >>>> message, >>>> translated in Java, you want a lambda that takes a message as parameter, >>>> calls a >>>> switch to do the pattern matching and return void. >>> I understand, although this is actually to do with the way lambda >>> expressions >>> are typed, rather than the switch expression. In JLS 15.27.3 "Type of a >>> Lambda >>> Expression”, there is a special case: >>> • If the function type's result is void, the lambda body is either a >>> statement >>> expression (§14.8) or a void-compatible block. >>> Which means that the following code typechecks: >>> IntConsumer ic = x -> System.out.println(x); >>> but it breaks as soon as we nest the statement expression, e.g. >>> IntConsumer ic2 = x -> true ? System.out.println(x) : >>> System.out.println(x); // >>> Compilation error: target-type for conditional expression cannot be void >>> This is what is happening in your example. So to deal with this we’d either >>> have >>> to make typechecking of lambdas smarter - either by replacing the typing >>> rule >>> for lambdas above with something more compositional, or by making void a >>> first-class type, or we could perhaps add a pattern-matching form of lambda, >>> which has a void-aware typing rule. I’m not sure about any of these options >>> for >>> now. >> yes, >> i'm proposing to create a special case for a switch inside a lambda >> expression >> for the same reason we have a special treatment for methods. >> By example, this does not compile >> [ https://github.com/fora | https://github.com/fora ] >> x/loom-fiber/blob/cea7b86c26c2e86b00fb72e5098a37983e8b6441/src/main/java/fr.umlv.loom/fr/umlv/loom/actor/CounterStringActorExprSwitchDemo.java#L17 >> while this compiles >> [ >> https://github.com/forax/loom-fiber/blob/6d8a0a6ba870580d43988cd0d507d19d57653d62/src/main/java/fr.umlv.loom/fr/umlv/loom/actor/CounterStringActorExprSwitchDemo.java#L17 >> | >> https://github.com/forax/loom-fiber/blob/6d8a0a6ba870580d43988cd0d507d19d57653d62/src/main/java/fr.umlv.loom/fr/umlv/loom/actor/CounterStringActorExprSwitchDemo.java#L17 >> ] >>> Cheers, >>> Gavin >> cheers, >> Rémi
