Allen Wirfs-Brock wrote:
My concern is that the pattern
new T(x) //create a mutable instance:
T(x) //create an immutable instance
is a new one that we really don't reflect current usage in either the
specification or in the wild, eg RegExp, Date.
Those are definitely not value objects. More's the pity, perhaps, but
too late.
Also Map/Set as currently implemented in FF, but that is also a
seperate (but related) discussion.
Definitely not value objects.
I agree that int64(x) is nice for int64 (although I would expect such
scalar values to me immutable regardless of how you create them).
The intuition (supported by other languages) is that 'new'
heap-allocates something mutable by default. Stack allocation and
(implicit or not) coercion does not. C++ is not far from the mark here,
but IIRC C# is similar.
It is also appealing for structs but not necessarily for objects which
means it is hard to know when you see 'new Foo(x)' vs 'Foo(x)'
whether the mutable/immutable pattern applies or not for Foo.
Indeed, structs are new in this sense. I don't think T.value(x) helps,
though.
Presumably Structs are subclassable. But, where I see the real
problem is if we wanted to support class declarations that use Structs
as their private state. We would still want such classes to be
subclassable and super calls of the constructor would still be needed
for initialize both mutable and immutable subclasses.
That's a challenge to rise to ;-).
It seems like the real usability challenge is finding a scheme that is
pleasant for both scalars and potentially subclass able objects.
Here another stab at the int64 use case.
let int64 = (...args) => Object.freeze(new Builtins.Int64(...args));
// int64 is a non-constructable function. It is the normal way of
creating Int64 instances
// Builtins.Int64 is (at least conceptually) a normal constructor
that uses 'new' to create instances but would seldom be directly used
in that manner
// For Int64 the Object.freeze may actually be redundant but
wouldn't necessarily be so if the pattern was extended to structured data.
I think this goes in the wrong direction. 'new' implies heap allocation
and reference-type semantics. Value objects do not have either.
/be
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