We report on our experience using Haskell as an executable specification language in the formal verification of the seL4 microkernel. fication connects an abstract operational specification in the theorem prover Isabelle/HOL to a C implementation of the microkernel. We describe how this project differs from other efforts, and examine the effect of using Haskell in a large-scale formal verification. The kernel comprises 8,700 lines of C code; the verification more than 150,000 lines of proof script. Categories and Subject Descriptors D.2.4 [Software Engineer