This paper argues that a pragmatic approach is needed for integrating design and formalisation of complex systems. We report on our approach to designing the seL4 operating system microkernel API and its formalisation in Isabelle/HOL. The formalisation consists of the systematic translation of significant parts of the functional programming language Haskell into Isabelle/HOL, including monadbased code. We give an account of the experience, decisions and outcomes in this translation as well as the technical problems we encountered together with our solutions. The longer-term goal is to demonstrate that formalisation and verification of a large, complex, OS-level code base is feasible with current tools and methods and is in the order of magnitude of traditional development cost.