How to write programs in two easy steps

I’m bored, so I’m browsing the blog-o-tubes, and noticing the occasional comment wondering why functional languages like Haskell and Lisp are typically used for no more than implementing other programming languages. I won’t comment on the truth or otherwise of this claim, because I don’t think it’s a bad thing in any case. Here’s why — this is a technique I learned from Conor McBride (who learned it from his father Fred McBride), explaining how to write any computer program in two easy stages: