[eyelidlessness]

Object identity effectively implies mutability. Without that implication, two objects of the same structured value being non-equal doesn’t mean anything (and would probably be better classified as a mistake).

Another way to look at it is that property setters (or whatever mechanism is used to directly mutate an object’s sub-data) is not meaningfully different from a method doing the same. You could even call it syntax sugar for the same.

[odipar]

My take is that identity doesn't imply mutability, if you version objects. It could well be that your are looking at an old version of an object, using its unique identity combined with its version (number).

Objects refer to other objects using their (immutable) identity. In turn, resolving identities to objects requires (version) scope which can be in the past or present.

[eyelidlessness]

> My take is that identity doesn't imply mutability, if you version objects. It could well be that your are looking at an old version of an object, using its unique identity combined with its version (number).

Are you storing the version as part of the object? If so, they’re no longer equal values regardless of identity. If not, what purpose is there in versioning the same value? Even if there is a purpose, are same-value-different-version objects not otherwise interchangeable unless/until some value change does occur?

[odipar]

Identity doesn't imply 'value' equality, that's the whole point of mutability! Conversely, two objects can have the same 'value' while having different identities. Values and objects are different beasts.

[eyelidlessness]

> Identity doesn't imply 'value' equality, that's the whole point of mutability!

That’s exactly the point I started with!

> Conversely, two objects can have the same 'value' while having different identities. Values and objects are different beasts.

Agree completely. My point—my only point—was that identity implies mutability. Without mutability, identity distinct from value doesn’t mean anything.

[kaba0]

This point is actually emphasized strongly by the CS giant, Guy Steele himself.