Maude-help mailing list

[Steven Eker <eker AT csl.sri.com>]

_+_ is assoc and therefore doesn't really need to associate left or right. 
However the MSCP10 parser that Maude uses doesn't understand associative 
symbols so the system has to force an associativity to avoid trivial 
ambiguities. By default it uses gather to force right associativity since 
this 
is the convention for dealing with associative lists and the code is 
optimized 
(linear vs quadratic time) to deal with long right associative lists.

As these two examples show, the default assoc gather is wrong for _+_

I think this can be fixed by putting gather (E e) for _+_ in the prelude.

Steven

On Monday 30 May 2011 08:40:28 am Chucky Ellison wrote:
> Worse yet, 1 - 2 + 3 actually fails to parse.  It isn't even considered
> ambiguous.
> 
> Maude> rew 1 - 2 + 3 .
> Warning: <standard input>, line 3: didn't expect token +:
> 1 - 2 + <---*HERE*
> Warning: <standard input>, line 3: no parse for term.
> 
> -Chucky
> 
> On Mon, May 30, 2011 at 10:24 AM, Ziad H. Najem 
> <najem AT acm.kuniv.edu>
>  wrote:
> > Core Maude 2.6 reports the expression (1+2-3) is ambiguous (as shown
> > below)
> >
> > Shouldn't the gather rules (associativity) resolve the ambiguity?
> >
> >  - Ziad
> >
> >
> > -------
> > Maude> red in INT : 1 + 2 - 3 .
> > Warning: <standard input>, line 2: ambiguous term, two parses are:
> > 1 + (2 - 3)
> > -versus-
> > (1 + 2) - 3
> >
> > Arbitrarily taking the first as correct.
> > reduce in INT : 1 + (2 - 3) .
> > rewrites: 2 in 0ms cpu (0ms real) (~ rewrites/second)
> > result Zero: 0
> > ------
> >
> >
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