Programmers confront a minefield when they design interactive Web programs. Web interactions take place via Web browsers. With browsers, consumers can whimsically navigate among the various stages of a dialog and can thus confuse the most sophisticated corporate Web sites. In turn, Web services can fault in frustrating and inexplicable ways. The quickening transition from Web scripts to Web services lends these problems immediacy.

A language with a programmable type system is vital for the construction of an embedded domain specific language (EDSL). Driven by the requirements posed by the implementation of an EDSL for server-side Web scripting, we examine two major of extensions to the type system of the host language, Haskell. We show that a component that ensures the generation of correct HTML documents can take good advantage of type-level functions, as implemented using functional logic overloading. We further show that a function that ensures the consistency of data submitted to a Web script with the data expected by the script is less awkward to use in the presence of lambda expressions in the type language. In both cases we assess the guarantees obtained by the use of the typing and explore alternative solutions.