Some applications are most easily expressed in a programming language that supports concurrency, notably interactive and distributed systems. We propose extensions to the purely-functional language Haskell that allow it to express explicitly concurrent applications; we call the resulting language Concurrent Haskell. The resulting system appears to be both expressive and efficient, and we give a number of examples of useful abstractions that can be built from our primitives. We have developed a freely-available implementation of Concurrent Haskell, and are now using it as a substrate for a graphical user interface toolkit. This paper appears in the Proceedings of the 23rd ACM Symposium on Principles of Programming Languages (POPL '96), St Petersburg Beach, Florida, Jan 1996. 1 Introduction Concurrent Haskell is a concurrent extension to the lazy functional language Haskell. Our principal motivation is to provide a more expressive substrate upon which to build sophisticated I/O-perform...