We introduce the abstract concept of \textit{rational type expression} and show its relationship to rational language theory. We further present a concrete syntax, regular type expression, and a Common Lisp implementation thereof which allows the programmer to declaratively express the types of heterogeneous sequences in a way which is natural in the Common Lisp language. The implementation uses techniques well known and well founded in rational language theory, in particular the use of the Brzozowski derivative and deterministic automata to reach a solution which can match a sequence in linear time. We illustrate the concept with several motivating examples, and finally explain many details of its implementation.