merging cases in between values and types

[Gavin Bierman]

Hi Remi,

Sorry for the slow reply:

> On 8 Sep 2021, at 07:42, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> Hi all,
> the current spec support merging/fallthrough cases only when there is a case null,
> like
>  case null, case Integer i -> // i can use 'i' here
> 
> First, i'm not sure it's clearly written in the spec, i was not able to pinpoint the rule allowing it.

In 14.11.1 there is a bunch of well-formedness conditions listed for switch labels. The paragraph begins “For every switch label in a switch block, all of the following must be true, otherwise a compile-time error occurs…” What follows is a bunch of rules that spell out which switch labels are correctly formed, and which are not. The two rules that cover your question here are:


• If a switch label has a constant case label element then if the switch label also has other case element labels they must be either a constant case label element, the default case label element, or the null case label element.

and

• If a switch label has a null case label element then if the switch label also has any pattern case element labels, they must be type patterns (14.30.1).


So the first rule forbids you from writing case 42, Integer i. The second rule allows you to say case null, Integer i (and case Integer i, null actually.) 

So, yes, the null is treated specially. Indeed it’s behaviour is NOT about fall through really. When you write case null, Foo f you are not asking for the type pattern test to be carried out if the value is null (it might fail!). Nor are you asking for it to be ignored (cos then the pattern variable will not be initialised). You’re actually saying “if it’s null then it matches AND the variable i is initialised to null”. So it’s being treated like a special pattern label. If we were to give it an alternative syntax, we might write “case Foo? f” instead of “case null, Foo f”. 

Hope this helps,

Gavin


> Then i wonder if this special behavior is special because null is special or if it should work with any values of the type of the type pattern,
> i.e. it works because null is a possible value of Integer, in that case, it should also work with any other values like 42
> 
>  case 42, case Integer i ->  // i can use 'i' here
> 
> So is null a special value or does this rule also work with any values.
> 
> regards,
> Rémi
> 
>