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From: acha@CS.CMU.EDU (Anurag Acharya)
Newsgroups: comp.lang.functional
Subject: "Off-side rule"
Message-ID: <ACHA.91Jan11151418@DRAVIDO.CS.CMU.EDU>
Date: 11 Jan 91 20:14:18 GMT
References: <22307@rouge.usl.edu> <1991Jan10.111559.12440@odin.diku.dk>
	<27793.278c3bb0@kuhub.cc.ukans.edu>
	<1991Jan11.100048.3121@odin.diku.dk>
Sender: acha@dravido.soar.cs.cmu.edu
Organization: Carnegie Mellon University
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In-reply-to: sestoft@diku.dk's message of 11 Jan 91 10:00:48 GMT

In article <1991Jan11.100048.3121@odin.diku.dk> sestoft@diku.dk (Peter Sestoft) writes:
   The main difference between Miranda (TM) and Lazy ML is the syntax.
   In Miranda the program lay-out has semantics: the off-side rule
   determines where an expression ends, whereas (Lazy) ML has more
   Pascal-like conventions, using semicolons similar punctuation.  (The
   off-side rule was suggested by Peter Landin in the sixties).  

What is the justification for this "off-side" rule ? The idea of whitespace 
having semantics is a potential source of inscrutable bugs and, frankly 
speaking, seems to go against the grain of modern programming language
design. The concrete syntax of such a language would no longer be context-free,
let alone LR(1)/LL(1). In fact, I am hard pressed to conceptualize an
efficient tokenizing algorithm for such languages. 

Since this rule has also been incorporated into the design of Haskell, it isn't
just historical and I am sure the designers of Haskell must have had some
reason(s) for this decision. I would like to know what benefit(s) does this
rule provide a language deisgner to trade off against the parsing 
inconvenience and inelegance ? 


anurag