Web sites are modeled using mappings from paths to expressions. A mapping data structure of expressions represents a web site's hierarchical structure, and an enumerator, an array of characters, or an array of bytes represents the web site's content. Expressions are written in a declarative programming language, and are lazily evaluated to provide results used in HTTP responses. A mapping may be recursive. An expression may contain a function call which returns contents of a file named in the HTTP request path, or the expression may evaluate to a text string, for example. Expressions may call functions which produce side-effects. An expression may recursively map a directory name to a mapping of a file name. Scalability is promoted by concurrently evaluating expression(s) on different machines in a stateless middle tier, and by streaming results. An arbitrarily large web site may be implemented in this manner.