On 13-3-2009 12:24, Invisible wrote:
>>> How about these?
>>>
>>> ABC+*DE+*F*
>>> ABC+DE+**F*
>>
>> such rewriting indeed works for any operator $ that satisfies 
>> (A$B)$C=A$(B$C) i.e. AB$C$=ABC$$ in RPN.
> 
> So... any associative operation then?

yes. because associativity is the property that you are using in the 
reordering.

> A quick check confirms that
> 
>   ABC*-DE*-F- ==> (A-(B*C))-(D*E)-F   ==> A - BC - DE - F
>   ABC*DE*--F- ==> (A-((B*C)-(D*E)))-F ==> A - BC + DE - F
> 
> which aren't the same.
> 
>> BTW Why did you choose this example with such a lot or irrelevant detail?
> 
> My brain is like mush this morning. This was the first correct example I 
> could find. It's about 10 years ago that I discovered this effect, so I 
> wasn't even sure if I was remembering right!

I think that as soon as you find an example you try to reduce it to the 
bare essentials and post that. The same actually as when posting a bug.

> (Currently attempting to build a Haskell parser, printer and type 
> interence engine. 

One word: why?

> This stuff is surprisingly non-trivial. Having an 
> argument about RPN in parallel with these activities is exceeding my 
> mental capacity!)

Post a reply to this message