1. A computer-readable storage medium that is not a signal and that comprises computer-executable instructions that when executed by one or more processors of a computing environment cause the computing environment to: receive input specifying a name for a first modeled type, an underlying type for the first modeled type, at least one attribute of the first modeled type, and programming logic to be executed on a program element of the first modeled type during execution of an application to validate the program element against a range of valid values of the program element; save, in a first library, the first modeled type and the programming logic to be executed on the program element of the first modeled type during execution of the application; receive input specifying a second modeled type based on the first modeled type and a name for the second modeled type and providing a description of how to determine an explicit value for the at least one attribute during execution of the application; and save, in a second library, the second modeled type including the description of how to determine the explicit value for the at least one attribute during execution of the application, the second library being different from the first library in which the first modeled type is saved.