Uniqueness type - Wikipedia

In computing, a unique type guarantees that an object is used in a single-threaded way, with at most a single reference to it. If a value has a unique type, a function applied to it can be optimized to update the value in-place in the object code. Such in-place updates improve the efficiency of functional languages while maintaining referential transparency. Unique types can also be used to integrate functional and imperative programming.

Uniqueness typing is best explained using an example. Consider a function readLine that reads the next line of text from a given file:

Now doImperativeReadLineSystemCall reads the next line from the file using an OS-level system call which has the side effect of changing the current position in the file. But this violates referential transparency because calling it multiple times with the same argument will return different results each time as the current position in the file gets moved. This in turn makes readLine violate referential transparency because it calls doImperativeReadLineSystemCall.

However, using uniqueness typing, we can construct a new version of readLine that is referentially transparent even though it's built on top of a function that's not referentially transparent:

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