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A '''dfn''' (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").
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").


As of 2020, dfns are fully implemented in [[Dyalog APL]], [[NARS2000]], [[ngn/apl]], [[dzaima/APL]], and partially in [[GNU APL]].
John Scholes was responsible for numerous presentations and publications on and about dfns, and until his passing he maintained the [https://dfns.dyalog.com dfns website].


Wikipedia has a full treatment of dfns.
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 [[wikipedia:lexical scoping|lexical scoping]], in contrast to the dynamic scoping of [[tradfn]]s. In other words, a dfn cannot "see" locals of its caller, but can see locals of its definer.
== Documentation ==
 
Wikipedia includes a thorough treatment of [[Wikipedia:dfns|dfns]].
== External links ==
* [[wikipedia:Direct function|Direct function]]
=== Tutorials ===
* APL Cultivation: [https://chat.stackexchange.com/rooms/52405/conversation/lesson-2-diving-into-functions-in-apl Diving Into Functions in APL]
* Mastering Dyalog: [https://www.dyalog.com/uploads/documents/MasteringDyalogAPL.pdf#page=174 page 174]
=== Documentation ===
* [http://help.dyalog.com/latest/#Language/Defined%20Functions%20and%20Operators/DynamicFunctions/Dynamic%20Functions%20and%20Operators.htm Dyalog]
* [http://help.dyalog.com/latest/#Language/Defined%20Functions%20and%20Operators/DynamicFunctions/Dynamic%20Functions%20and%20Operators.htm Dyalog]
== External links ==
* [https://www.gnu.org/software/apl/apl.html#Section-3_002e7 GNU APL]
* [[Wikipedia: Direct function]]
* [[Dyalog Ltd.]]'s [[Official website|dfns website]] (created and maintained by John Scholes until his passing)


{{APL syntax}}
{{APL syntax}}[[Category:Kinds of functions]][[Category:Defining functions]]
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