Stretching the Glasgow Haskell Compiler: Nourishing GHC with Domain-Driven Design

Over the last decade Haskell has been productized; transitioning from a research language to an industrial strength language ready for large-scale systems. However, the literature on architecting such systems with a pure functional language is scarce. In this paper we contribute to that discourse, by using a large-scale system: the Glasgow Haskell Compiler (GHC), as a guide to more maintainable, flexible and effective, pure functional architectures. We describe, from experience, how GHC as a system, violates the desirable properties that make pure functional programming attractive: immutability, modularity, and composability. With these violations identified, we provide guidance for other functional system architectures; drawing heavily on Domain-Driven Design. We write from an engineering perspective, with the hope that our experience may provide insight into best practices for other pure functional software architects.