A method and apparatus for providing stricter data types, and stricter data type checking, in a data flow diagram. For numeric data types, a unit data type is associated with the data type. Therefore, if a user desires to change the units from one compatible type to another, such as feet to meters, this can be done without recompilation. Unit type checking is also provided, whereby, as a block diagram is created, the block diagram editor continually checks each node or function for impermissible data unit operations. This provides a more powerful debugging environment than that previously known in the art. Polymorphic unit capabilities are also provided whereby a virtual instrument can perform calculations regardless of unit but also has unit type checking capabilities. A stricter data type referred to as the enumerated data type is also included which is a variation of an integer numeric data type that includes an enumerated list of mnemonics. Enumeration type checking is provided to prevent invalid accesses.