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From: jack@cs.glasgow.ac.uk (Jack Campin)
Newsgroups: comp.lang.misc
Subject: Re: Structured loops (WAS:Re: Turing programming language.)
Message-ID: <2294@crete.cs.glasgow.ac.uk>
Date: 27 Jan 89 11:34:08 GMT
References: <11@euteal.UUCP> <89Jan20.111000est.4328@turing.toronto.edu> <12@euteal.UUCP> <7053@june.cs.washington.edu>
Reply-To: jack@cs.glasgow.ac.uk (Jack Campin)
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pardo@cs.washington.edu (David Keppel) wrote:
  >>[Ongoing discussion of Turing loop structure]
  
>  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 construct exists in S- (and PS-) algol, and I find it one of the less
lovable features of those languages.  The actual syntax is

	repeat	<void statement>
	while	<boolean valued statement>
	do	<void statement>

where either the repeat-part or the do-part may be omitted.

The problem is that if you're reading a piece of alien code and you see a
"while <boolean>", you have no idea whether it's the beginning, middle, or end
of a control construct without scanning a LOT of surrounding context.  The
construct itself is fine; economizing on keywords like this in the concrete
syntax isn't.

-- 
Jack Campin  *  Computing Science Department, Glasgow University, 17 Lilybank
Gardens, Glasgow G12 8QQ, SCOTLAND.    041 339 8855 x6045 wk  041 556 1878 ho
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