Mode declarations can also specify so-called “unique modes”. Mercury's unique modes are similar to “linear types” in some functional programming languages such as Clean. They allow you to specify when there is only one reference to a particular value, and when there will be no more references to that value. If the compiler knows there will be no more references to a value, it can perform “compile-time garbage collection” by automatically inserting code to deallocate the storage associated with that value. Even more importantly, the compiler can also simply reuse the storage immediately, for example by destructively updating one element of an array rather than making a new copy of the entire array in order to change one element. Unique modes are also the mechanism Mercury uses to provide declarative I/O.
We have not yet implemented unique modes fully, and the details are still in a state of flux. So the following should be considered tentative.