Eight years ago Compiling without continuations introduced the idea of so-called join points as a powerful optimisation tool in a functional language compiler. Since then join points have become more and more deeply entwined in GHC’s optimisation passes; for example they are treated specially by the Simplifier and its Occurrence Analyser, and a dedicated pass called Exitification makes a join-point-specific transformation

With the perspective of this experience, I am convinced that any serious functional compiler should use join points (or something equivalent): it’s a powerful and re-usable idea. Yet beyond the initial paper, none of what we have learned has appeared in print. In this talk I will share the lessons of the last eight years of using join points in practice in GHC.

Simon Peyton Jones, MA, MBCS, CEng, graduated from Trinity College Cambridge in 1980. After two years in industry, he spent seven years as a lecturer at University College London, and nine years as a professor at Glasgow University, before moving to Microsoft Research (Cambridge) in 1998. He became an Engineering Fellow at Epic Games in 2021.

His main research interest is in functional programming languages, their implementation, and their application. He has led a succession of research projects focused around the design and implementation of production-quality functional-language systems for both uniprocessors and parallel machines. He was a key contributor to the design of the now-standard functional language Haskell, and is the lead designer of the widely-used Glasgow Haskell Compiler (GHC). He has written two textbooks about the implementation of functional languages.

More generally, he is interested in language design, rich type systems, software component architectures, compiler technology, code generation, runtime systems, virtual machines, and garbage collection. He is particularly motivated by direct use of principled theory to practical language design and implementation – that’s one reason he loves functional programming so much.