Re: Jensen's device

The discussion about call-by-name and Jensen's device in Algol-60
seems to call for a few comments, based on the `Revised Report on the
Algorithmic Language Algol 60', Regnecentralen 1964, which claims to
present the modified text agreed upon in 1962 by the Algol committee
and approved by the Council of IFIP. The modifications listed in the
report do not include the sections mentioned below.


Section 5.4.2 contains an example declaration of a procedure called
Innerproduct. It runs as follows


procedure Innerproduct(a,b)Order:(k,p)Result:(y) ; value k;
integer k,p; real y,a,b ;
begin real s;
for p := 1 step 1 until k do s := s + a * b ;
y := s
end Innerproduct


Section 4.7.2 contains example calls with reference to those given
in Section 5.4.2. One example is


      Innerproduct(A[t,P,u],B[P],10,P,Y)


This illustrates that the committee knew reasonably well what they were
doing, do it seems wrong to claim that call-by-name was invented by
mistake and that Jensen's device was invented years after the language.


Jørn Jensen was a programmer at Regnecentralen and worked closely
together with Peter Naur during the design of Algol 60. Various stories
have been told and retold about it, and my recollection is that Peter
Naur was well aware what call-by-name implied, and that Jørn Jensen had
invented his device before the decisive meetings. Peter Naur has
written about the process of defining Algol 60 in SIGPlan Notices
sometime in the seventies, if I remember correctly. It was either a
reaction to other writings or stimulated a reply, but anyway it
demonstrated that not all of the involved had consistent memories.
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