A direct function (dfn, pronounced "dee fun") is an alternative way to define a function and operator (a higher-order function) in the programming language APL. A direct operator can also be called a dop (pronounced "dee op"). They were invented by John Scholes in 1996. They are a unique combination of array programming, higher-order function, and functional programming, and are a major distinguishing advance of early 21st century APL over prior versions.
A dfn is a sequence of possibly guarded expressions (or just a guard) between { and }, separated by ⋄ or new-lines, wherein ⍺ denotes the left...