On 17/05/18 23:22, John Rose wrote:
A Java constructor in a value class will internally use withfield
to translate any assignment of the form "this.x = y", and instead
of the blank instance being an incoming reference in L[0], the
constructor builds a blank value instances out of thin air using
vdefault.
So, if I understand correctly, a classic Java constructor is a
void-returning instance method; in the model you propose a value class
constructor would be more similar to a V-returning static method (where
V is the value to be constructed).
This is all and well, but I feel that this pushes the problem under the
(assignment) rug. E.g. I believe that reinterpreting the meaning of
'this.x = y' inside a value constructor to mean "get a brand new value
and stick y into x" would be very confusing, as semantically, there's no
assignment taking place. And, semantically, it doesn't even make sense
to think about a 'this' (after all this is more like a static factory?).
Of course you can spin this as reinterpreting the meaning of the word
'this' inside a value constructor - e.g. the new meaning being "the
opaque value being constructed"; but that is likely to clash with other
utterances of 'this' in the same value class (e.g. in other instance
methods - where 'this' would simply mean 'this value').
Language-wise (and I repeat, it might well be too soon to dive into
this), it feels like we're missing a way to express a new kind of a
primitive operation (the wither). Without that, I'm a bit skeptical on
our ability to be able to express value type constructors in a good way.
Maurizio
