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Re: Need a new type of Hold

• To: mathgroup at smc.vnet.net
• Subject: [mg7538] Re: Need a new type of Hold
• From: "David B. Wagner" <dbwagner at princon.com>
• Date: Fri, 13 Jun 1997 03:08:53 -0400 (EDT)
• Organization: Principia Consulting
• Sender: owner-wri-mathgroup at wolfram.com

Ersek_Ted%PAX1A at mr.nawcad.navy.mil wrote:
> 
> I could use a new type of Hold that is similar to HoldForm.
> The difference is I don't want the new Hold to show up in
> the FullForm.
> 
> Lets call this new feature HoldTemporary.
> The following shows the way I want it to work.
> 
> (*-----------------------------------------*)
> 
> In[1]:=  foo=HoldTemporary[z+2+d+a+3]
> Out[1]=  z+2+d+a+3
> 
> In[2]:= foo-d+4
> Out[2]= 9+a+z
> 

At first I thought that what you really want to do is define your
own addition operator, but then I decided thathat you are trying to
do is fundamentally inconsistent with the way
Mathematica works (I won't say "impossible", since everytime I say
that some smart-aleck figures out a way to do it.) 

Here's why:  
in your first output, the 2 and the 3 don't combine.  Yet, in the second
output, they have combined with the 9.  What should happen if I type
"foo + b"?  Should the result be "z+2+d+a+3+b" or should it be
"5+a+d+a+b".  Or should it be something else?  The fact is, if you want
the 2 and 3 to combine, then they should have combined in the first
output; if you want the order of the terms to be strictly maintained,
then the second output should have been "z+2+d+a+7" (let's assume that
it's okay to combine adjacent terms).  One or the other, you can't
have it both ways.

By the way, if it's a non-associative addition operator that you're
after, I don't recommend removing the Orderless attribute from Plus,
or you'll break a lot of other things.

-- 
		Dave Wagner
		Principia Consulting
		http://www.princon.com/princon/
		Voice: (500) PRN-CPIA or (303) 258-9178
		Fax:   (303) 258-3859