Linear Haskell: practical linearity in a higher-order polymorphic language

• DOI: 10.1145/3158093
• Publication Date: 2017-12-29T14:21:49Z
• Authors: Jean-Philippe Ber...; Mathieu Boespflug; Ryan R. Newton; Simon Peyton Jones; Arnaud Spiwack
• Keywords: FOS: Computer and information sciences; Computer Science - Programming Languages; Functional languages; linear types; 02 engineering and technology; polymorphism; [INFO.INFO-PL] Computer Science [cs]/Programming Languages [cs.PL]; Formal language definitions; laziness; Haskell; linear logic; 0202 electrical engineering, electronic engineering, information engineering; GHC; Programming Languages (cs.PL)

Linear type systems have a long and storied history, but not a clear path forward to integrate with existing languages such as OCaml or Haskell. In this paper, we study a linear type system designed with two crucial properties in mind: backwards-compatibility and code reuse across linear and non-linear users of a library. Only then can the benefits of linear types permeate conventional functional programming. Rather than bifurcate types into linear and non-linear counterparts, we instead attach linearity to function arrows . Linear functions can receive inputs from linearly-bound values, but can also operate over unrestricted, regular values.

To demonstrate the efficacy of our linear type system — both how easy it can be integrated in an existing language implementation and how streamlined it makes it to write programs with linear types — we implemented our type system in ghc, the leading Haskell compiler, and demonstrate two kinds of applications of linear types: mutable data with pure interfaces; and enforcing protocols in I/O-performing functions.