Path: csiph.com!usenet.blueworldhosting.com!feeder01.blueworldhosting.com!peer02.iad.highwinds-media.com!news.highwinds-media.com!feed-me.highwinds-media.com!border1.nntp.dca1.giganews.com!border2.nntp.dca1.giganews.com!nntp.giganews.com!news.iecc.com!.POSTED.news.iecc.com!nerds-end
From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.compilers
Subject: Re: precedences vs. hierarchy
Date: Wed, 08 Jun 2016 09:48:49 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
Lines: 21
Sender: news@iecc.com
Approved: comp.compilers@iecc.com
Message-ID: <16-06-010@comp.compilers>
References: <16-06-001@comp.compilers> <16-06-004@comp.compilers> <16-06-008@comp.compilers>
Injection-Info: miucha.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970"; logging-data="72777"; mail-complaints-to="abuse@iecc.com"
Keywords: parse
Posted-Date: 10 Jun 2016 22:23:21 EDT
X-submission-address: compilers@iecc.com
X-moderator-address: compilers-request@iecc.com
X-FAQ-and-archives: http://compilers.iecc.com
X-Received-Bytes: 1674
X-Received-Body-CRC: 84645730
Xref: csiph.com comp.compilers:1716

Kaz Kylheku <545-066-4921@kylheku.com> writes:
>You need the concept of associativity to tell whether
>A + B + C  is  (A + B) + C  or  A + (B + C).

But if + is really associative (e.g., not FP addition), these two are
equivalent.

>In table shift-reduce parsers (Yacc, and such), associativity rules
>resolve conflicts.

Of course, you can also write the grammar in an unambiguous way, then
you don't need associativity rules to resolve the ambiguity; a
disadvantage of precedence and associativity declarations is that they
can hide more ambiguities than you intended.

- anton
--
M. Anton Ertl
anton@mips.complang.tuwien.ac.at
http://www.complang.tuwien.ac.at/anton/