1. At a computer system including a processor and a memory, a computer-implemented method for interpreting declarative program types at runtime without compiling, the method comprising: an act of the computer system accessing at least a portion of a declarative program that defines a parent activity using a declarative language, the parent activity including a fully modeled activity type that includes one or more members which define a data flow for the fully modeled activity type, each member including a name and a type of input for the member, wherein the parent activity also includes an identification of a schema to be used to determine how the fully modeled activity type is to be interpreted at runtime by a continuation-based runtime; an act of the computer system dynamically constructing a dynamic activity type based on the fully modeled activity type of the declarative program, the dynamic activity type being configured for interpretive execution by the continuation-based runtime without compilation, dynamically constructing the dynamic activity type comprising: generating, based on the schema defined in the declarative program, a type in memory without creating a common language runtime type to represent the fully modeled activity type, and mapping, based on the schema defined in the declarative program, each member of the fully modeled activity type to a property of the in-memory type; and an act of the computer system interpretively executing the dynamically constructed dynamic activity type within the continuation-based runtime at a first process, such that the dynamic activity type is executed without compilation, and during execution of the dynamically constructed dynamic activity type: pausing execution of the dynamically constructed dynamic activity type at the first process; and resuming execution of the dynamically constructed dynamic activity type at a second, different process.