Recursive Data Structures in APL
A mathematical study of three approaches for defining nested
arrays in APL is presented.  Theorems exhibiting the relationships
between the definitional systems are given and illustrated through graph
representations.  One of the approaches is used to define an
APL array to be a recursive data structure equivalent to a tree structure
in which all data is stored at the leaves as homogeneous
arrays of numbers and characters.  An extension of APL is proposed that
includes new primitive functions to manipulate the nesting
level of arrays and new operators to assist in the
construction of data-driven algorithms.  
CACM February, 1979
Gull,W.E.
Jenkins, M.A.
