Embodiments are directed to interpreting declarative program types at runtime without compiling and mapping between a declarative type and a dynamic runtime type. A computer system accesses a portion of a declarative program, where the declarative program includes fully modeled activity types. The computer system dynamically constructs a dynamic activity type based on one of the fully modeled activity types of the declarative program, where the dynamic activity type is configured for interpretive execution without compilation. The computer system also interpretively executes the dynamically constructed dynamic activity type such that the dynamic activity is executed without compilation.