Uniqueness type

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.

Introduction

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

 function readLine(File f) returns String
     return line where
         String line = doImperativeReadLineSystemCall(f)
     end
 end

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:

 function readLine2(unique File f) returns (unique File, String)
     return (differentF, line) where
         String line = doImperativeReadLineSystemCall(f)
         File differentF = newFileFromExistingFile(f)
     end
 end

The unique declaration specifies that the type of f is unique; that is to say that f may never be referred to again by the caller of readLine2 after readLine2 returns, and this restriction is enforced by the type system. And since readLine2 does not return f itself but rather a new, different file object differentF, this means that it's impossible for readLine2 to be called with f as an argument ever again, thus preserving referential transparency while allowing for side effects to occur.

Programming languages

Uniqueness types are implemented in the functional programming languages Clean, Mercury and Idris. They are sometimes used for doing I/O operations in functional languages in lieu of monads.

A compiler extension has been developed for the Scala programming language which uses annotations to handle uniqueness in the context of message passing between actors.

Relationship to linear typing

The term is often used interchangeably with linear type, although often what is being discussed is technically uniqueness typing, as actual linear typing allows a non-linear value to be "cast" to a linear form, while still retaining multiple references to it. Uniqueness guarantees that a value has no other references to it, while linearity guarantees that no more references can be made to a value.

See also

• Linear type
• Linear logic

External links

• Bibliography on Linear Logic
• Uniqueness Typing Simplified
• Philip Wadler's writings on linear logic

Discussions of uniqueness typing in programming languages

• Lively Linear Lisp -- 'Look Ma, No Garbage!'
• Linear Logic and Permutation Stacks--The Forth Shall Be First
• Minimizing Reference Count Updating with Deferred and Anchored Pointers for Functional Data Structures
• 'Use-Once' Variables and Linear Objects -- Storage Management, Reflection and Multi-Threading

Experiments with uniqueness typing (from a performance perspective)

• A "Linear Logic" Quicksort
• The Boyer Benchmark Meets Linear Logic
• Sparse Polynomials and Linear Logic