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From: jonab@sdcrdcf.UUCP (Jonathan Biggar)
Newsgroups: net.lang
Subject: Re: declarations and nested blocks
Message-ID: <749@sdcrdcf.UUCP>
Date: Tue, 20-Dec-83 11:51:33 EST
Article-I.D.: sdcrdcf.749
Posted: Tue Dec 20 11:51:33 1983
Date-Received: Fri, 23-Dec-83 01:28:56 EST
References: <1072@mit-eddie.UUCP>
Reply-To: jonab@sdcrdcf.UUCP (Jonathan Biggar)
Organization: System Development Corporation, Santa Monica
Lines: 46

In article <1072@mit-eddie.UUCP> barmar@mit-eddie.UUCP (Barry Margolin)
writes:
>
>There is a feature of PL/I and Algol which is used in most of their
>descendents which is both useful and painful.  I am referring to the
>fact that the scope of variables includes nested procedures.  It allows
>one to write simpler (though less clear) code, but it also permits one
>to write incorrect code pretty easily.  This problem is exemplified in
>the following fragment:
>
>	do index = 1 to 10;
>	     call subr (array (index));
>	end;
>
>	subr: procedure (thing);
>	      ...
>	      do index = 1 to 5;
>	      ...
>	      end;
>	end subr;
>
>Since the programmer forgot to declare a local "index" variable in subr
>he ends up with an infinite loop, since index will never reach 10; it
>will be set to 5 after each iteration.
>
>I'm surprised that Ada(tm) does not require some explicit statement to
>allow you to inherit from an outer block.  Why is this feature
>propogated to all new languages, with hardly any change?

Ada(tm) does not have this problem, because the index variable in a
for loop is implicitly declared to be the type of the range it varies
over:

	loop1: for i in 1..10 loop
		loop2: for i in 1..5 loop
			...
		end loop loop2;
	end loop loop1;

loop1.i is not the same variable as loop2.i, therefore there can
be no infinite loop.  (Note that the code inside both loops can
reference both variables as loop1.i and loop2.i.)
-- 
Jon Biggar
{allegra,burdvax,cbosgd,hplabs,ihnp4,sdccsu3,trw-unix}!sdcrdcf!jonab