Dfn

A dfn (contraction of direct function or dynamic function, pronounced "dee fun") is an alternative way to define a function and operator, invented by John Scholes. A dfn operator can also be called a dop (pronounced "dee op").

John Scholes was responsible for numerous presentations and publications on and about dfns, and until his passing he maintained the dfns website.

As of 2020, dfns are fully implemented in Dyalog APL, NARS2000, ngn/apl, dzaima/APL, and partially in GNU APL, although not all dialects implement lexical scoping, in contrast to the dynamic scoping of tradfns. In other words, a dfn cannot "see" locals of its caller, but can see locals of its definer.

Wikipedia includes a thorough treatment of dfns.

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Direct function

Tutorials

APL Cultivation: Diving Into Functions in APL
Mastering Dyalog: page 174

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APL syntax [edit]
General	Comparison with traditional mathematics ∙ Precedence ∙ Tacit programming
Array	Numeric literal ∙ String ∙ Strand notation ∙ Object literal ∙ Array notation
Function	Argument ∙ Function valence ∙ Derived function ∙ Derived operator ∙ Niladic function ∙ Monadic function ∙ Dyadic function ∙ Ambivalent function ∙ Tradfn ∙ Dfn ∙ Function train
Operator	Operand ∙ Operator valence ∙ Tradop ∙ Dop ∙ Derived operator
Assignment	Multiple ∙ Indexed ∙ Selective ∙ Modified
Other	Function axis ∙ Branching ∙ System command ∙ User command ∙ Keyword ∙ Dot notation ∙ Function-operator overloading

Dfn

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