This thesis develops the core type system for a Haskell-like functional programming language that directly supports dynamic, active documents. The first part presents a system of type-indexed rows, that supports many aspects of XML's regular-expression types without abandoning the type features which make functional programming at tractive. In particular, type-indexed rows coexist cleanly with higher-order types and parametric polymorphism. The second part presents a system of staged computation, that allows server-side and client-side code to be cleanly separated