Iverson has greatly enlarged the mathematical notion of function composition and made it available to computer programmers. This paper explains the concept, and uses practical examples to show how concise, powerful programs can be written and read. Alternative forms are given, and ways of overcoming initial difficulties are explained. The systematic use of composition extends the wellknown advantages of APL; namely conciseness, consistency, and generality. Keywords executable mathematical notation, function composition, composite verbs, adverbs, conjunctions, trains, forks, hooks, gerunds, geometry, Iverson, APL, J
Donald B. McIntyre