Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!yale!mintaka!snorkelwacker!spdcc!ima!esegue!compilers-sender
From: norvell@csri.toronto.edu (Theo Norvell)
Newsgroups: comp.compilers
Subject: Re: Low-Rent Syntax
Keywords: parse
Message-ID: <1990Aug12.135241.10330@esegue.segue.boston.ma.us>
Date: 12 Aug 90 13:52:41 GMT
References: <25630@cs.yale.edu> <58091@lanl.gov> <1990Jul26.024449.1777@esegue.segue.boston.ma.us> <1990Jul27.034115.8747@esegue.segue.boston.ma.us>
Sender: compilers-sender@esegue.segue.boston.ma.us
Reply-To: Theo Norvell <norvell@csri.toronto.edu>
Organization: CSRI, University of Toronto
Lines: 56
Approved: compilers@esegue.segue.boston.ma.us
In-Reply-To: <1990Aug09.180536.18782@esegue.segue.boston.ma.us>

In article <1990Aug09.180536.18782@esegue.segue.boston.ma.us> Donald Lindsay writes:
>In article <1990Jul27.034115.8747@esegue.segue.boston.ma.us> 
>	moss@cs.umass.edu (Eliot Moss) writes:
>>CLU was actually able to *eliminate* the ";" statement separator/terminator,
>>through *very* careful syntax design (and maybe it required more than one
>>token look-ahead, too; I don't recall clearly).
>
>The Icon language (Arizona) and the Turing language (Toronto) both
>have "low rent" syntax - that is, the ";" is only needed (as a
>separator) when one writes multiple statements on a single line.  In
>all other cases, it can be omitted.
>
>Is there now a "usual" way to implement this ?

In Turing, the semicolon is never needed.  The reason is careful syntax
design.  There is no trickery in the lexical analysis and the grammar
is LL(1).  Consider the following LL(1) grammar
	SS --> 
	     | S SS
	S --> var name : T
	    | procedure name A SS end name
	    | function name A : T SS end name
	    | E := E
	    | if E then SS [else SS] end if
	    | case E of {label E: SS} [label : SS] end case
	    | loop SS end loop
	    | for name : T SS end for
	    | exit [when E]
	E --> etc
	T --> etc
	A --> etc
No semicolons.  In Turing the second clause is really
	SS --> S [ ; ] SS
So you can sprinkle semicolons to taste.  Euclid had a similar syntax,
but a semicolon was still required in one obscure case.

In Icon, the newline takes the place of the semicolon when a semicolon
is syntactically allowed.  This means you have to write
	a := b + 
	     c
rather than
	a := b
	   + c
if you mean 
	a := b + c
The second is syntactically correct, but means something else.  Griswold has
a book on the implementation of Icon.  Perhaps it explains the
implementation of this rule.

I haven't a clue about CLU.

Theo Norvell
U of T
-- 
Send compilers articles to compilers@esegue.segue.boston.ma.us
{spdcc | ima | lotus| world}!esegue.  Meta-mail to compilers-request@esegue.