Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!udel!burdvax!emerald!dave
From: dave@emerald.PRC.Unisys.COM (David Lee Matuszek)
Newsgroups: comp.lang.misc
Subject: Re: Structured loops (WAS:Re: Turing programming language.)
Keywords: programming language
Message-ID: <9045@burdvax.PRC.Unisys.COM>
Date: 25 Jan 89 16:44:07 GMT
References: <11@euteal.UUCP> <89Jan20.111000est.4328@turing.toronto.edu> <12@euteal.UUCP> <7053@june.cs.washington.edu>
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Organization: Unisys Corporation, Paoli Research Center; Paoli, PA
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In article <7053@june.cs.washington.edu> pardo@cs.washington.edu (David Keppel) writes:

>There is a more general construct that I've not seen in any existing
>programming languages, although (a) doubtless it exists and (b) the
>idea is easily a decade old (ok, probably 30, but I *know* at least
>10) Substitute your favorite C/Pascal/Lisp/Smalltalk syntax.  In
>meta-Algol:
>
>	do ... while boolean ... od;

This is the same as the loop ... exit when <boolean> ... end loop;
except that the sense of the test is reversed.  This exists in Turing.
It also exists in an in-house language I designed and implemented a couple
of years ago.  I don't know of any other languages that use it, however.

I checked a little further and found that Knuth (Structured
Programming with go to Statements, ACM Computing Surveys, v6, n4,
December 1974, p. 261-300; see page 279) credits Ole-Johan Dahl with
"recently" inventing:  "loop; S; while ~B; T; repeat;", but he does
not cite a reference.  (He strongly implies that the negation on the B
is important, but when I read the paragraph carefully I found it very
ambiguous as to just what he thought was better.)
-- Dave Matuszek (dave@prc.unisys.com)
-- Unisys Corp. / Paoli Research Center / PO Box 517 / Paoli PA  19301
-- Standard disclaimer:  Any resemblance between my opinions and those of my
   employer is strictly coincidental.