Like functional programming, which has been around in rudimentary form since the dawn of computing, virtualization only found wider commercial application more recently when people figured out how to make it work efficiently on x86 processors. So far virtualization means headless servers running on the cloud. XenClient is changing that. XenClient is a client-side virtual machine manager based on the Xen hypervisor. It targets laptop platforms, which means new challenges compared to servers: power management, 3D graphics, ubiquitous plug-in devices and worrying about adversaries tampering with the hardware. While still providing all the advantages of the cloud, like presenting a synchronized state to the user independent of the device they are using, or ease of provisioning and administration. Early XenClient borrowed heavily from XenServer, a server virtualisation platform that preceded XenClient by several years. As a major user of OCaml, XenServer had its share of talks here, too. Over time our code bases diverged. A more modular architecture allowed us to try different languages. Today we are the proud owners of more than ten thousand lines of Haskell employed in around ten daemons (and counting!) making up all the intelligence of XenClient, running reliably in embedded virtual machines without direct user intervention. This is the story of the second generation of functional programming at Citrix.

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