Direct function: Difference between revisions

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name : expression0 : proposition : expression1

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Within a direct definition, {{code|⍺|apl}} denotes the left argument and {{code|⍵|apl}} the right argument. In the first instance, the result of {{code|expression|apl}} is the result of the function; in the second instance, the result of the function is that of {{code|expression0|apl}} if {{code|proposition|apl}} evaluates to 0, or {{code|expression1|apl}} if it evaluates to 1. Assignments within a direct definition are [[Scope (computer science)#Dynamic scoping|dynamically local]] (accessible within called functions). Examples of using direct definition are found in the 1979 [[Turing Award]] Lecture<ref name=TOT>{{cite journal|last=Iverson|first=Kenneth E.|title=Notation as a Tool of Thought|journal=Communications of the ACM|volume=23|number=8|date=August 1980|url=https://www.jsoftware.com/papers/tot.htm|access-date=8 April 2016|doi=10.1145/358896.358899|pages=444–465}}</ref> and in books and application papers.<ref name=Iverson1976>{{cite book|last=Iverson|first=Kenneth E.|title=Elementary Analysis|publisher=APL Press|date=1976}}</ref><ref name=Orth1976>{{cite book|last=Orth|first=D.L.|title=Calculus in a New Key|publisher=APL Press|date=1976}}</ref><ref name=Hui1987>{{cite journal|last=Hui|first=Roger|author-link=Roger Hui|title=Some Uses of { and }|journal=APL 87 Conference Proceedings|date=May 1987|url=http://www.jsoftware.com/papers/from.htm|access-date=15 April 2016}}</ref><ref name=McDonnell1987>{{citation|last=McDonnell|first=E.E.|title=Life: Nasty, Brutish, and Short|journal=APL 87 Conference Proceedings|date=May 1987|url=http://www.jsoftware/papers/EEM/life.htm|access-date=6 October 2019}}</ref><ref name=APL1978>{{citation|last1=Hui|first1=Roger|last2=Kromberg|first2=Morten|title=APL Since 1978|date=June 2020}}</ref>

Direct definition was too limited for use in larger systems. The ideas were further developed by multiple authors in multiple works<ref name=opfns>{{citation|last=Iverson|first=Kenneth E.|title=Operators and Functions|journal=Research Report Number #RC7091|date=26 April 1978|publisher=IBM Corporation|url=http://www.jsoftware.com/papers/opfns.htm|access-date=2019-09-19}}</ref>{{rp|§8}}<ref name=IversonWooster1981>{{citation journal|last1=Iverson|first1=Kenneth E.|last2=Wooster|first2=Peter|title=A Function Definition Operator|journal=APL81 Conference Proceedings, APL Quote Quad|volume=12|number=1|date=September 1981}}</ref><ref name=Cheney>{{citation|last=Cheney|first=Carl M.|title=APL*Plus Nested Array System Reference Manual|date=March 1981|publisher=STSC, Inc.|url=http://www.sudleyplace.com/APL/Nested%20Arrays%20System.pdf|access-date=18 September 2019}}</ref>{{rp|§4.17}}<ref name=ratapl>{{citation|last=Iverson|first=Kenneth E.|title=Rationalized APL|date=6 January 1983|publisher=I.P. Sharp Associates|url=http://www.jsoftware.com/papers/RationalizedAPL.htm|access-date=2019-09-19}}</ref><ref name=dictionary>{{citation journal|last=Iverson|first=Kenneth E.|title=A Dictionary of APL|journal=APL Quote Quad|volume=18|number=1|date=September 1987|url=http://www.jsoftware.com/papers/APLDictionary.htm|access-date=19 September 2019}}</ref><ref name=Bunda1987>{{citation journal|last=Bunda|first=John|title=APL Function Definition Notation|journal=APL87 Conference Proceedings, APL Quote Quad|volume=17|number=4|date=May 1987}}</ref><ref name=J>{{citation journal|last1=Hui|first1=Roger|display-authors=etal|title=APL\?|journal=APL90 Conference Proceedings, APL Quote Quad|volume=20|number=4|date=July 1990|url=http://www.jsoftware.com/papers/J1990.htm|access-date=2019-09-10}}</ref> but the results were unwieldy. Of these, the "alternative APL function definition" of Bunda in 1987<ref name=Bunda1987/> came closest to current facilities, but is flawed in conflicts with existing symbols and in error handling which would have caused practical difficulties, and was never implemented. The main distillates from the different proposals were that (a) the function being defined is anonymous, with subsequent naming (if required) being effected by assignment; (b) the function is denoted by a symbol and thereby enables [[anonymous recursion]].<ref name=APL1978/>