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From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.lang.misc
Subject: Block Closure
Message-ID: <5100@aw.sei.cmu.edu>
Date: 18 Apr 88 14:42:13 GMT
References: <2853@enea.se> <2400014@otter.hple.hp.com> <918@rlgvax.UUCP>
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Reply-To: firth@bd.sei.cmu.edu.UUCP (Robert Firth)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
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It seems clearly desirable that every control construct be able
to contain multiple statements, embedded within clear starting
and ending delimiters.

The Algol-68 style used the opening delimited written in reverse
(if...fi, case...esac, comment..tnemmoc [joke]).  The motivation
was decent, but the implementation, to put it mildly, was vile.

Ada is a little better, with  if ... end if,  case ... end case,
and so on.  Unfortunately, it is not consistent.  Moreover, because
the closing delimiter is two tokens, there is more chance of human
error and of bogus syntactic error recovery error.

CPL I think came closest to getting it right: every opening delimiter
can be given a mnemonic name, called a "tag", and the corresponsing
closing delimiter must repeat the tag.  For instance:

	IF p = NIL DO
	$(NullCase
	    ...
	    ...
	$)NullCase

The one mistake, I think, was to allow one tagged end delimiter to
close many open constructs:

	IF p = NIL THEN
	$(NullCase
	    IF freespaceptr = NIL THEN
	    $(NewBlock
		...
		...
	$)NullCase

This is a concession to laziness that loses far more than it gains.

Note that this still has the Algol-60 style: the control structure
really governs only one statement, so you have to add a pair of
delimiters after it. This is probably an obsolete notion.

A feasible design might be a combination of CPL tags and Modula syntax,
for instance:

	NullCase:
	IF p = NIL THEN
	    ...
	    ...
	END NullCase

though I'm unhappy with that layout.