Abstract : A lot of designers of functional languages have one dream: finding a killer application, outside of the world of symbolic programming (compilers, theorem provers, DSLs), that would make their language spread in the open-source community. One year ago, we tackled this problem, and decided to use to program a network application in the emerging world of peer-to-peer systems. The result of our work, , has superseded our hopes: it is currently the most popular peer-to-peer file-sharing client on the well-known freshmeat.net site, with about 10,000 daily users. Moreover, is the only client able to connect to several peer-to-peer networks, to download and share files. It works as a daemon, running unattended on the computer, and can be controlled remotely using three different kind of interfaces. In this paper, we present the lessons we learnt from its design and implementation.