Dfn

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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.

Examples

      {⍵*0.5} 16        ⍝ square root
4
      3 {⍵*÷⍺} 27       ⍝ ⍺th root
3
      ⍝ Multiline dfn with optional left parameter and conditional result
      root←{
          ⍺←2           ⍝ square root by default
          ⍺=0:0         ⍝ return zero if zeroth root
          ⍵*÷⍺          ⍝ result
      }

External links

Tutorials

Documentation


APL syntax [edit]
General Comparison with traditional mathematics ∙ Precedence ∙ Tacit programming (Train, Hook, Split composition)
Array Numeric literal ∙ String ∙ Strand notation ∙ Object literal ∙ Array notation (design considerations)
Function Argument ∙ Function valence ∙ Derived function ∙ Derived operator ∙ Niladic function ∙ Monadic function ∙ Dyadic function ∙ Ambivalent function ∙ Defined function (traditional) ∙ Dfn ∙ Function train
Operator Operand ∙ Operator valence ∙ Tradop ∙ Dop ∙ Derived operator
Assignment Multiple ∙ Indexed ∙ Selective ∙ Modified
Other Function axis ∙ Bracket indexing ∙ Branch ∙ Statement separator ∙ Quad name ∙ System command ∙ User command ∙ Keyword ∙ Dot notation ∙ Function-operator overloading ∙ Control structure ∙ Comment
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