[HIW'22] Compiling Mu with GHC: Halfway Down the Rabbit Hole

Gergo Erdi shares an experience report on compiling Mu, a strict Haskell dialect, by customizing and patching the Glasgow Haskell Compiler.

Compiling Mu with GHC: Halfway Down the Rabbit Hole Gergo Erdi (Standard Chartered Bank) Mu and Cortex are, respectively, a dialect of Haskell and a runtime system used at Standard Chartered Bank for financial modeling and analytics. The Mu compiler is a proprietary implementation in use since 2009. In 2021 we started work on a new implementation based on the Glasgow Haskell Compiler. Mu and Cortex differ from GHC’s assumptions about its source language and compilation target in interesting ways: among others, Mu uses strict evaluation, and Cortex supports polymorphic definitions but doesn’t adhere to parametricity. We address some of these challenges with creative use of the public API of GHC, and others by patching GHC itself. This talk is an experience report of this work-in-progress effort, explaining the important aspects of Mu and Cortex, the compilation pipeline of the original Mu compiler, and our design for a new compilation pipeline using GHC as the frontend. We also present the customizations we apply to GHC, some of which may turn out to be of general interest for upstream GHC development.