How do Rust's ownership semantics relate to uniqueness typing as found in Clean and Mercury?

Question

I noticed that in Rust moving is applied to lvalues, and it's statically enforced that moved-from objects are not used.

How do these semantics relate to uniqueness typing as found in Clean and Mercury? Are they the same concept? If not, how do they differ?

Tertiary

Answer 1

The concept of ownership in Rust is not the same as uniqueness in Mercury and Clean, although they are related in that they both aim to provide safety via static checking, and they are both defined in terms of the number of references within a scope. The key differences are:

• Uniqueness is a more abstract concept. While it can be interpreted as saying that a reference to a memory location is unique, like Rust's lvalues, it can also apply to abstract values such as the state of every object in the universe, to give an extreme but typical example. There is no pointer corresponding to such a value - it cannot be opened up and inspected within a debugger or anything like that - but it can be used through an interface just like any other abstract type. The aim is to give a value-oriented semantics that remains consistent in the presence of statefulness.

• In Mercury, at least (I can't speak for Clean), uniqueness is a more limited concept than ownership, in that there must be exactly one reference. You can't share several copies of a reference on the proviso that they will not be written to, as can be done in Rust. You also can't lend a reference for writing but get it back later after the borrower has finished with it.

• Declaring something unique in Mercury does not guarantee that writing to references will occur, just that the compiler will check that it would be safe to do so; it is still valid for an implementation to copy the contents of a unique reference rather than update in place. The compiler will arrange for the update in place if it deems it appropriate at its given optimization level. Alternatively, authors of abstract types may perform similar (or sometimes drastically better) optimizations manually, safe in the knowledge that users will be forced to use the abstract type in a way that is consistent with them. Ownership in Rust, on the other hand, is more directly connected to the memory model and gives stronger guarantees about behaviour.

Archbishopric