User defined data types in APL2

This paper is an investigation of possible definitions of new enclose-like functions, their impact on the APL 2 language, and their usefulness in applications. Several variants of enclose have been proposed over the years that range from data structure primitives to data type producing primitives. Operator analogues of enclose can be used to create functions that pervade structures and types in different ways. Defined operators can implement type hierarchy and respond to fill items in prototype execution by referring to tables of type characteristics.