Koka: Programming with Row Polymorphic Effect Types

Daan Leijen
(Microsoft Research)

We propose a programming model where effects are treated in a disciplined way, and where the potential side-effects of a function are apparent in its type signature. The type and effect of expressions can also be inferred automatically, and we describe a polymorphic type inference system based on Hindley-Milner style inference. A novel feature is that we support polymorphic effects through row-polymorphism using duplicate labels. Moreover, we show that our effects are not just syntactic labels but have a deep semantic connection to the program. For example, if an expression can be typed without an _exn_ effect, then it will never throw an unhandled exception. Similar to Haskell's `runST` we show how we can safely encapsulate stateful operations. Through the state effect, we can also safely combine state with let-polymorphism without needing either imperative type variables or a syntactic value restriction. Finally, our system is implemented fully in a new language called Koka and has been used successfully on various small to medium-sized sample programs ranging from a Markdown processor to a tier-splitted chat application. You can try out Koka live at www.rise4fun.com/koka/tutorial.

In Paul Levy and Neel Krishnaswami: Proceedings 5th Workshop on Mathematically Structured Functional Programming (MSFP 2014), Grenoble, France, 12 April 2014, Electronic Proceedings in Theoretical Computer Science 153, pp. 100–126.
Published: 5th June 2014.

ArXived at: http://dx.doi.org/10.4204/EPTCS.153.8 bibtex PDF

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