From jens at lidestrom.se  Sat Jul 25 19:51:37 2020
From: jens at lidestrom.se (=?UTF-8?Q?Jens_Lidestr=c3=b6m?=)
Date: Sat, 25 Jul 2020 21:51:37 +0200
Subject: Observation about nulls in type patterns
Message-ID: <99e2350d-8c38-9d72-22ed-ce5d4cff0849@lidestrom.se>

Brian Goetz posted a write-up of the feature Types patterns in switch in
amber-spec-experts on Jun 24:

https://mail.openjdk.java.net/pipermail/amber-spec-experts/2020-June/002235.html

I'd like to contribute an observation about the consequences of the
proposed mechanism for handling null values in switches:

If I understand the proposal correctly the behaviour of null values in a
switch will depend on the type of the expression being switch upon. This
is potentially non-local information, for example when the expression is a
field access or a method call.

To understand the behaviour of null values in a switch readers will have
to examine the source of the expression being switch upon. Also, if the
type of a method or a field is being narrowed or widened that change might
silently affect the behaviour of existing switches in other parts of the code.

Example:

switch (o.get1().get2()) {
??? case Integer i: ...
??? case Number n: ...
}

If get2 is some method that is declared to return Number then the second
pattern is total and will accept null; if get2 is declared to return
Object pattern the IS total and WILL accept null.

The expert group is probably aware of this fact already but I think it
deserves to be noted explicitly.

Best regards,
Jens Lidestr?m
Random Passer-by