1. A method, implemented within a computing system that includes at least one processor, for integrating data type conversion into a programming language by using abstract data type schemas, implemented with a subset of the programming language, to map from an internal data type to an external data format, the method comprising: a computing system, which includes at least one processor, identifying internally-formatted data that is formatted in accordance with a first internal data type described by a first programming language of a first application; the computing system identifying a data model schema that defines an external data format described by a functional subset of the first programming language, and that defines a declarative mapping between the first internal data type and the external data format using the functional subset of the first programming language, the data model schema including: one or more data fields; and one or more value-based rules that define enforceable semantic rules about data exchanged in the one or more data fields and that provide automated argument validation when the data model schema is utilized; the computing system converting the internally-formatted data into externally-formatted data that is formatted in accordance with the external data format using the data model schema, wherein converting comprises: mapping the internally-formatted data to the one or more data fields; and automatically validating that the one or more data fields contain valid data by enforcing the one or more value-based rules on the one or more data fields; and the computing system exchanging the externally-formatted data with a second application that uses a second internal data type.