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	~~Let me first begin~~ by ~~introducing myself~~. ~~My title is~~ [http://~~www~~.~~aseandate~~.~~com~~/~~index~~.~~php~~?m=~~member_profile~~&p=~~profile~~&id=~~13345213 over~~ the ~~counter std test] Boyd Butts although~~ it is ~~at home std test not~~ the name ~~on my birth certificate~~. ~~Years~~ in the ~~past we moved~~ to ~~[http:~~//~~Hivtest~~.~~Cdc~~.~~gov~~/ ~~North Dakota~~]. ~~One~~ of the ~~extremely very best issues in~~ the ~~world~~ for ~~me is to do aerobics~~ and ~~now I~~'~~m trying~~ to ~~make money at home std testing with it~~. ~~She is a librarian but she~~'s ~~always needed her own company~~.<br><br>~~[http:~~//~~www~~.~~healthline~~.~~com~~/~~health~~/~~std~~/~~genital-herpes std testing~~] ~~at home~~ [http://~~wmazowiecku~~.pl/~~stay~~-~~yeast~~-~~infection~~-~~free~~-~~using-these-helpful-suggestions~~/ ~~over~~ the ~~counter std test~~] ~~My blog post; std home test~~ ([~~http:~~//~~www~~.~~eyelevelfederalway~~.~~com~~/xe/~~index~~.~~php?mid~~=~~Promotions&page~~=~~1&document_srl~~=~~571267 visit~~ the ~~following web page~~])		'''Jensen's Device''' is a computer programming technique that exploits [[call by name]]. It was devised by [[Denmark\|Danish]] computer scientist [[Jørn Jensen]], who worked with [[Peter Naur]] at [[Regnecentralen]]. They worked on the GIER Algol compiler, one of the earliest correct implementations of [[ALGOL 60]].<ref name="Gieralgol">[http://awards.acm.org/citation.cfm?id=1024454&srt=all&aw=140&ao=AMTURING Peter Naur's 2005 Turing Award citation] mentions his work with Jensen on GIER Algol</ref> ALGOL 60 used call by name.

			==Description==

			Jensen's device exploits [[call by name]] and side-effects. Call by name is an argument passing convention that delays the evaluation of an argument until it is actually used in the procedure (a consequence of the copy rule for procedures). Algol introduced call by name.

			A classic example of Jensen's device is a procedure that computes the sum of a series, <math>\Sigma_{k=l}^u a_k</math>:<ref>{{Citation \|last=MacLennan \|first=Bruce J. \|title=Principles of Programming Languages: Design, Evaluation, and Implementation \|edition=Second \|year=1987 \|publisher=Holt, Rinehart & Winston \|isbn=0-03-005163-0 \|doi= }}, pp 141–142</ref><ref>{{Citation \|last=Dijkstra \|first=E. W. \|authorlink=Edsger W. Dijkstra \|title=Defense of ALGOL 60 (Letter to the Editor) \|journal=Communications of the ACM \|volume=4 \|issue=11 \|pages=502–503 \|date=November 1961 \|doi= }}</ref><ref>{{Citation \|last=Knuth \|first=D. E. \|authorlink=Donald Knuth \|title=The Remaining Troublespots in ALGOL 60 \|journal=Communications of the ACM \|volume=10 \|issue=10 \|pages=611–617 \|date=October 1967 \|doi= }}</ref>

			<!-- ALGOL 60 source -->
			'''real procedure''' Sum(k, l, u, ak)
			'''value''' l, u;
			'''integer''' k, l, u;
			'''real''' ak;
			'''comment''' k and ak are passed by name;
			'''begin'''
			'''real''' s;
			s := 0;
			'''for''' k := l '''step''' 1 '''until''' u '''do'''
			s := s + ak;
			Sum := s
			'''end''';

			In the procedure, the index variable <code>k</code> and summation term <code>ak</code> are passed by name. Call by name enables the procedure to change the value of the index variable during execution of the <code>for</code> loop. Call by name also causes the <code>ak</code> argument to be reevaluated during each iteration of the loop. Typically, <code>ak</code> will depend upon the changing (side-effected) <code>k</code>.

			For example, code to compute the first 100 terms of a real array <code>V[]</code> would be:

			Sum(i, 1, 100, V[i]).

			During the execution of <code>Sum</code>, the actual argument <code>i</code> will increment during each step of the <code>for</code> loop, and each of the procedure's evaluations of <code>ak</code> will use the current value of <code>i</code> to access the successive array elements <code>V[i]</code>.

			Jensen's device is general. A double summation can be done as:

			Sum(i, l, m, Sum(j, l, n, A[i,j]))

			The <code>Sum</code> function can be employed for arbitrary functions merely by employing the appropriate expressions. If a sum of integers were desired the expression would be just <code>Sum(i,1,100,i);</code>, if a sum of squares of integers, then <code>Sum(i,1,100,i*i);</code>, and so on.<ref><code>Sum</code> requires a <code>real</code> argument for the term, so type conversion is assumed.</ref> A slight variation would be suitable for initiating a numerical integration of an expression by a method very similar to that of <code>Sum</code>.

			The evaluation of <code>ak</code> is implemented with a [[Thunk (functional programming)\|thunk]], which is essentially a subroutine with an environment. The thunk is a [[Closure (computer science)\|closure]] with no arguments. Each time a procedure needs the value of its formal argument, it simply calls the thunk. The thunk evaluates the actual argument in the scope of the calling code (not the scope of the procedure).

			In the absence of this pass-by-name facility, it would be necessary to define functions embodying those expressions to be passed according to the protocols of the computer language, or to create a compendium function along with some arrangement to select the desired expression for each usage.

			===GPS===

			Another example is GPS(General Problem Solver), described in D. E. Knuth and J. N. Merner's ''ALGOL 60 confidential''.<ref>Donald E. Knuth and Jack N. Merner. 1961. ALGOL 60 confidential. Commun. ACM 4, 6 (June 1961), 268-272. DOI=10.1145/366573.366599 http://doi.acm.org/10.1145/366573.366599</ref>

			<!-- ALGOL 60 source -->
			'''real''' '''procedure''' GPS(I, N, Z, V);
			'''real''' I, N, Z, V;
			'''begin''' '''for''' I := 1 '''step''' 1 '''until''' N '''do''' Z := V;
			GPS := 1
			'''end'''

			==Criticism==

			Jensen's device relies on call by name, but call by name is subtle and has some problems. Consequently, call by name is not available in most languages. Knuth comments that ALGOL 60 cannot express an <code>increment(n)</code> procedure that increases its argument by one; the call <code>increment(A[i])</code> does not do the expected action if <code>i</code> is a functional that changes with each access.<ref>{{Harvnb\|Knuth\|1967\|p=613}}. For example, <code>increment(A[increment(j)])</code> will increment <code>j</code> twice.</ref> Knuth says, "The use of 'macro' definition facilities to extend language, instead of relying solely on procedures for this purpose, results in a more satisfactory running program."

			Others point out that a call by name procedure that swaps its argument can have subtle problems.<ref>{{Harvnb\|MacLennan\|1987}}</ref> An obvious swapping procedure is:
			procedure swap(a, b)
			'''integer a, b;
			'''begin'''
			'''integer temp;'''
			temp := a;
			a := b;
			b := temp;
			'''end;'''
			The procedure does the right thing for many arguments, but the invocation of <code>swap(i,A[i])</code> is problematic. Using the Copy Rule leads to the assignments:
			temp := i;
			i := A[i];
			A[i] := temp;
			The problem is the second assignment changes <code>i</code>, so the <code>A[i]</code> in the third assignment probably will not be the same array element as at the start. If on the other hand the procedure were to be coded the other way around (with ''b'' being saved to ''temp'' instead of ''a'') then the desired action would result, unless it were invoked as <code>swap(A[i],i)</code>

			==See also==
			* [[Call stack]] – stack frame, static link, and display (closure including environment link)
			* [[Funarg problem]] – closures can be complicated
			* [[Man or boy test]] – environment test

			==References==
			{{reflist\|30em}}

			==External links==
			* https://www.cs.indiana.edu/l/www/pub/techreports/TR186.pdf

			[[Category:Articles with example ALGOL 60 code]]
			[[Category:Programming language design]]

en>ClausiusMcKnight: Patterson was a visiting researcher, and not visiting a researcher...

2012-06-25T19:45:16Z

Patterson was a visiting researcher, and not visiting a researcher...

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Patterson function - Revision history

en>Addbot: Bot: Migrating 5 interwiki links, now provided by Wikidata on d:q905709 (Report Errors)

en>ClausiusMcKnight: Patterson was a visiting researcher, and not visiting a researcher...