Empty value types

[Gavin Bierman]

Greetings from Northern Australia! Just saw this - this is a well known pattern in functional programming called phantom types (due to our friend Erik Meijer)

Gavin 

Sent from my iPhone

> On 2 Aug 2014, at 02:35, Paul Govereau <paul.govereau at oracle.com> wrote:
> 
> I don't think there is anything wrong with it. I can even think of a use case: phantom types.
> 
> final __ByValue class ReadWrite {}
> final __ByValue class ReadOnly {}
> final __ByValue class File<T> { ... }
> 
> File<ReadWrite> openForWrite(String file) { ... }
> File<ReadOnly>  openForRead(String file) { ... }
> 
> void write(File<ReadWrite> file, ...) { ... }
> 
> However, I think we need to add checks to detect construction of empty value types. I am not sure what to do about methods defined inside of an empty type? Maybe only static methods are OK?
> 
> Paul
> 
>> On 08/01/2014 12:50 PM, Brian Goetz wrote:
>> While an empty value is silly, is there something actually wrong with it?
>> 
>> Sent from my iPhone
>> 
>>> On Aug 1, 2014, at 9:47 AM, Paul Govereau <paul.govereau at oracle.com> wrote:
>>> 
>>> What are we going to do with empty value types?
>>> 
>>> The most sensible thing seems to allow them to be declared but not constructed. e.g.
>>> 
>>> final __ByValue class Void {}   // <- ok
>>> 
>>> Void v = __Make Void();   // <- error cannot construct empty type.
>>> 
>>> Paul