About Array Theory
Array theory is a onesorted theory of nested rectangular arrays that combines the
ZermeloFraenkel concept of nested sets with the vectors, matrices, and multiaxis arrays of
Iverson and Falkoff’s programming language APL. As every element of a set is again a set
in set theory, every item of an array is again an array in array theory. The consequent unbounded
depth of nesting ultimately terminates in arrays that have no axes and hold themselves as sole items.
Such selfcontaining “atoms” include numbers, truthvalues, characters, and indivisible
strings of characters called phrases and faults. For example, the number 2 is a 0axis array in
which the sole item is the number 2.
The flat arrays of APL, a language implemented and extensively applied in the late 1960s, had many
similarities to the flat, tensor representing, “nway matrices” that Gabriel Kron introduced into
electrical engineering in the 1930s and 1940s. On joining the APL Design Group at IBM’s Yorktown
Research Center in 1968, Trenchard More originated array theory by combining multiaxis arrays
with settheoretic nesting and Quine’s controversial concept of selfcontainment. The search for
a maximal number of universally valid equations while preserving consistency led More through
successive versions of the theory. Later versions benefited from implementations first by
Willard Bouricius at Yorktown, and then when More moved to the IBM Cambridge Scientific Center in 1974,
by Anthony Hassitt, Leonard Lyon, and Robert Creasy at the IBM Palo Alto Scientific Center.
Mike Jenkins, an IBM Visiting Scientist who had been actively studying treelike extensions to
APL at Queen’s University, joined More at the Cambridge Center in 1979. Over the next 5 years they
collaborated on the design of Nial and the refinement of array theory. The Q'Nial interpreter was
developed at Queen's during this time and released in 1983 as Version 4 of Q'Nial. To avoid contradiction,
the most controversial result was that there are as many different empty arrays of a given shape as
there are arrays themselves. Arrays of the same shape differ according to their virtual items.
A list of no 2s differs from a list of no 3s!
This initial version of Q'Nial reflected the desire to stay close to More’s theoretical ideas,
while providing procedural control, flexible file handling and interactive input/output. The intent
of the design was to follow More’s equations as well as those Jenkins developed to support the
implementation functionality.
After ten years of feedback from programmers using Q’Nial, Jenkins undertook a major redesign of the
language and its implementation. The resulting Version 6 of Q’Nial sacrificed some of the generality
of the functional model and modified More’s theory by requiring that there be only one empty array
of any given shape. One can now compare an abstract Version 4 of array theory with a more
conventional, conformal Version 6.
