[patterns] on treatment of null

[Maurizio Cimadamore]

On 07/07/17 15:13, Guy Steele wrote:
>> On Jul 7, 2017, at 8:56 AM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
>>
>> Hi,
>> over the last few weeks we've been exploring the twisted relationship between patterns and nulls. This document:
>>
>> http://cr.openjdk.java.net/~mcimadamore/nulls-patterns.html
>>
>> provides some (hopefully helpful) insights into what the design space looks like.
>>
>> tl;dr;
>>
>> Looks like trying to force the same rule on all patterns, regardless of where they appear, leads to problems. Distinguishing between toplevel and nested patterns provides a good basis to handle null in a more predictable/flexible fashion.
>>
> Nice write-up!  But there is one other direction that it does not seem to explore: extending the type system to have explicitly non-null reference types (an old idea, but perhaps the correct solution for patterns).
>
>     String     can be String or null
>     String!    cannot be null
>
>    if (o matches String! x) { … o cannot be null here … }
>
>    if (o matches LinkedList(Object! head, LinkedList tail)) { … head cannot be null here … }
>
> Not sure it’s where we want to go, but at least it should be explicitly considered, if only to explicitly reject it.
You are right  - there is a connection between the lack of denotability 
of non-nullnesss and the failure of some of the options considered in 
the document (esp. option 2).

One of the things that leaves a bit of a sour taste is that with '!' in 
place, you do have a difference between:

case List l:
case List(...) l:

The former means nullable, the latter means non-nullable (or at least, 
should, because you can dereference). So, to be fully consistent, should 
it be written as:

case List!(...) l:

?

But generally, yes, this is another piece of the puzzle (and one I 
should have called out more explicitly in the doc).

Maurizio
>
> —Guy
>