26-27 July 1976

aßW«kdfe«BS»* "-d by NNeewclI and Sirllon

wiN^HU^ng.anT

Porl,

'1 "d Pi«c - gO^ Ollfl V < 1967) ; similar, but not for&&£&sm^ p^W^^■gg^** ri:r:g

isintendedtodra K!s»dsiiPisggg £ssiasS SSSBS;?T I#flffiffiysasfi -^on in th:d;s^;;;: ss;rc

°«-*£sIZSS^ BrUSSeIS> °M f",

C

°m,mUnity' and which was re- , , *' |s^ec^^t°isap0

of1'!!r Tf?

hno,^

f'>' S, y lndePende"tly suggested by <BUarded com""<<> »= I«1?«te;S Edsger Diikst". Much can be said , <gUard) "* *««*«« «*>SFSWn^^^Wa^SE ab°Ut the "eedfor and value of cos T"? "~ <boo,ea"«p--)aßEfflsa^K^MSi s?ation between diverse st) := is;rcnt> ,JaZ*MT5 6 ' f arfas: ,ts vir t«es are commonly (guarded command^ >J I

1L60670. Na<">-> »«■. Chic W . P" iced, and for quite goodreasons. (D<guardedcommand)}- Ac\Y%tuV?76 » ■However, cross fertilization can be Alternative construct) "~r^^Bn"-"^ especiai,y va,L!able when the di-rs IF

*««« se[> FI333-9452.P-a m- D—■ co 802oi ; 303 areas share a fundamental concern (repetitive """D "=_ T^ISy^o'^STAT svn, p„s ium *. ** "^ now between Pro- D0Warded command set) OD

f ra"m"lg Lan^Uages a» d Artificial <S,ateme'll) ::= 7S^Cl,i"' Harnac^ra^^^Kj; Intelligence. Both areas are vitally (repetitive construct) I■ am4

7eP'ember i"6 concerned with the development of "otherstatements"

SKW&IffiLHIS^ P^erful useful methods of specify- The relevant portions of thefea&^l^*ffiMS£: FU

grtrp e

hx ,mforr tion processi NeweJl and simon £*«foLw

n-15 October 1976 ' mer' X be,,eve that we are on the ,„„,,,„,■ C iWareffirtSFsSTpS Confer;,,ce °" s°'<-

Verge°f a rev°'i<tion in programmingon-»> ::= (production) I

20-22 October 1976

„„,,.„,

■ ;. to " P0 ;i kc > (action-sequence)cLmSMS Houston. Texas. f

"

° reV°'"tiOn. What alge- ::= (action, Imen, a"«es did for calculation of <action);(ac«ioiCe

nC

c)■ n" J CXpCCtPotion system <«*»> ::= "other statement"oESf^SSSSS^SSk^ c»SSSSfSSBiSS: Pr°S' amm,ng and related concepts Th*S5, S'SLrirS? rf d° fof ,08iCa' fl °W °f

C

°"tro'-^ acte " w^S:heSTSUbStUUte Char

"

rSStfW ACM;?pS socles oTIScT the rem ainder of this letter J will "„ £ , , he °bvious meanings,ssas»#-^ rpt to sh°w thc «*x bS£ ortofthr ewci,andsim-ni-^n p.o *SS«bS?W?Sff iJSf- Dtjkstra's "guarded command" and 2d h ° h" .^ a,ternath

'«-M

AmVr/crarffi7„,a,,cal

soeie,

a "Produc^n" of Newell et al tTi c,,l'Sed ° ther statement" ingSS&As^ SS»p4S 3nd n,ention so-e of the uses and rf

,h

Md Sim°" Sy"tax in P,ace" l'an.ary-2 Februa„ 1977 ' char^tenstics of production system spiritDiit' f? "!;

"

" thC SamC", tegTsS* "" Attarna, Programming. J". that Dj Jkitra used it m his syn- ,;,SdKS^S?^^"^^ '"his article in the August 1975 Th, « „

30332i„slltu ,e ot TechnoloBy. Atlanta> GA issue of Caww|f|l&^^ >* 5 - The guarded command and the- aMirShn 7

slraulaHon s,mpo . introduces what he calK a «fu Id 1Ct,0,1lCt,0,1

C

'early identica,> as

f^fes* SSSSSFr =a.SS=« ;

-?-:^=x I,05 ' a gUarded command: the action- :

;

§Communicationsof February 1976the ACM Volume 19

C"

t-; plastlcArt. t

only eligible for execution when its there » cl& rf m curren

condition (guard) is satisfied. Just as between 4^ Puctiom debate on proper structuring of

guarded commands are not them- whirf «"*JJ^ other> and computer programs appears to be

Lives executable statements, but and used by sustained by two combustibles: a

must be used within other constructs, theV*«*^"£ tend t0 be realization that programming may be

L production is also not an execut- JjPJ^^S^ with an art, and a conviction that some

able statement in the usual sense. programmed. ways of practicing art are better than

The production system is an execut- prndnction SYStem programming others,

able construct, and is quite similar to J o uct on ys P^Diikstra's repetitive construct. has alsc revta i 4 he jnterest in them indicates that

The procedure for executing chine l""^"^^P«>- programmers recognize programs to

Dijk.tra'

S

Prepetitive construct is: (1) be more useful as they are more com-

SeLet a guarded command G from duction due. prehendable, programmers see them-

fhe ouarded command set such that terns in the Lives as creative agents, and they

he guard of G evaluates to true; if tion and then q{ g_none! exit. (2) Execute the state- ment by their fellow artaan,To-

ments in guardedlist of G; go to 1. successfully appassociation gether these attitudes say that the

Di kstra specifies that the selec- tasks nonsense. sj 1 able ass gprogreSSive, challenging, and

tion b nondeterministic: more than and dtscnmina on ks, and P^ iQ

Z 'guarded command may be cli- in . complain about influences which ap-

gible? and no rule is given to deter- Di ; derivation rf pear to have suchdes.rabl c «s_minewhich to select. and applies the concepts However, as art and ennusm

The usual interpretation of pro- programs anai app.^ the professlorl) they

■

duction-system execution is similar toc^c^^^t0 see CSSCn- as aready oxidizer for a third tnnu-

fo the above procedure, except that ft surfacing in ence, the field's habitual identifica-he nondeterminacy is removed by a ttalljr the : "e concp

g mat

conflict resolution rule that specifies so man '^^ this concept ai s. On this identificat.on, consider,

how to select one of several eligible tha produc. the field's technical jargon includesproductions in a production system. is structured prog" g P

The usual rule is that thefirst eligible tion^ system program gparflgrapfo, to****!* Usts

production in the production system «^W top do.^

is selected. P JL Cven in a suitable /to, documents, and to get a httlc

flections and Advantages. P^.^dtob remarkably clear farfetched, symbols and characters.

Production systems and similar con- -tat on tend to X r „ot"cons«ds-ructs have been found to have a and co .

ogramming is are mitten, and they are not written

number of advantages over other P"*^2to has come. to be "understood" so much as tobeuroSamtning concepts for the devel- an idea whose time i^pis just about the only

onment of knowledge-based pro- GregGibbons word we havefor what it is that pro-arCs such as MYCIN and DEN- ComputerScience Group are made of The identificationDRAI Some of the advantages ex- Naval Postgraduate School * long_standing and pervasive

nerienced are- (1) each production is Monterey,Calif. there can be said to exist within the

an intelligible piece of information, references c A-, and sri£ field almost a confusion of P">6ratn -

Sknowledge, whoserole and impact £«£&s[VS^^ST*** ming with writing. Given the devel-op the operation of the system as a ffiC^A^^ o. Press, E«h .^^fa art and style this

whole is relatively easy to under- bias deserves attention. In combma-Stand- (2) the fIOW Of Control IS gOV-

nacy

and formal denvan^ P for art and

e,Td by the state of the program » , -^|K|f- style it flames especially strong and

variables which are tested by the &£=Robert^^XM"»>S}- specific Specific, that is, in that it

condtt on parts of the productions, «. AsjSyffi^U«- cuts. nx. .P^ programmers t0 judge pro-

and no flow charts, branches or GO- iffe, Dj^-J^wgjSaSSK grams and program goodness speci-

TO statements are required; (3) S^ggrW&i-w R—ft 6 ltinclines thefield toconsiderUrge >- as a result of item (2), produc- good programs, not as vaguely artful

system programs tend to be easy S^ff^;±L—. constructions, but as^to modify, and the effects of mod.fi- wj^mjfitt^TfJUS„

g **,

p^p" good ivr//,ng. I o put it exactl) ,cations are easier to anticipate than 285. Came

g

ie-Mei.on

u„

19,

February 1976Communications Volume 19

106 of Number 2the ACM