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.