switch on Class ?

[forax at univ-mlv.fr]

> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>, "amber-spec-experts"
> <amber-spec-experts at openjdk.java.net>
> Envoyé: Dimanche 23 Août 2020 17:40:05
> Objet: Re: switch on Class ?

> It has indeed come up before. There is some overlap with pattern switch, and
> some non-overlap, but it's pretty clear the impact of pattern switch is much
> larger.

> I would much prefer to finish the discussions on the fundamentals first, which
> are actually blocking progress on a much-higher-priority feature. So let's come
> back to this later.

> I also have reason to believe that, if we do generalized patterns property, we
> won't need to do this as a language feature, we can do it as a library feature.
> So, let's come back to this later.
Exhaustiveness is hard to emulate in a library. 

Rémi 

> On 8/23/2020 11:25 AM, Remi Forax wrote:

>> There is a feature of Pizza (remember Generic Java ++) we have not discussed
>> yet,
>> being able to do a switch on Class.

>> public sealed interface Numeric<T extends Numeric<T>>
>>   permits Amount, Percentage, Quantity {

>>   private BigDecimal value() {
>>     return switch(this) {
>>       case Amount(value) -> value;
>>       case Percentage(value) -> value;
>>       case Quantity(value) -> value;
>>     };
>>   }

>>  private static <T extends Numeric<T>> T fromValue(Class<T> type, BigDecimal
>>   newValue) {
>>     return type.cast(switch(type) {
>>       case Amount.class -> new Amount(newValue);
>>       case Percentage.class -> new Percentage(newValue);
>>       case Quantity.class -> new Quantity(newValue);
>>     });
>>   }

>>  default T add(T numeric) { return fromValue(getClass(),
>>   value().add(numeric.value())); }
>> }

>> with Amount be declared like this
>>   record Amount(BigDecimal value) implements Numeric<Amount> { }


>> This kind of switch is interesting because it's also one that can be exhaustive,
>> like the switch on type or the switch on Enum.

>> In the method fromValue, type is typed as a Class<T> so a Class<? extends
>> Numeric<...>> and given that Numeric is a sealed class only permitting Amount,
>> Percentage and Quantity, the only possible Class for a switch(type) are
>> Amount.class, Percentage.class and Quantity.class.

>> I'm pretty sure the call fromValue(getClass(), ...) doesn't not compile because
>> the compiler has no idea that all subtypes of Numeric implements
>> Numeric<themselves> but you get the idea.

>> regards,
>> Rémi