1. At a computer system including a processor and a memory, in a computer networking environment including a plurality of computing systems, a computer-implemented method for interpreting declarative program types at runtime without compiling, the method comprising: an act of accessing at least a portion of a declarative program, the declarative program including one or more fully modeled activity types; an act of dynamically constructing a dynamic activity type based on one of the one or more fully modeled activity types of the declarative program, the dynamic activity type being configured for interpretive execution without compilation; and an act of interpretively executing the dynamically constructed dynamic activity type such that the dynamic activity is executed without compilation.