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Re: Releasing several Holds simultaneously

  • To: mathgroup at smc.vnet.net
  • Subject: [mg82268] Re: Releasing several Holds simultaneously
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Wed, 17 Oct 2007 03:44:45 -0400 (EDT)
  • References: <ff1po5$90n$1@smc.vnet.net>

Andrew Moylan wrote:
> Hold[a := Hold[1]]
> 
> How can I release both of these Holds (and thus execute a:=1)
> simultaneously?
> 
> ReleaseHold[%] doesn't work; it evaluates a := Hold[1] before the other hold
> is removed.
> 

Hi Andrew,

How did you construct the expression Hold[a := Hold[1]]?  Perhaps there 
is a way to avoid the double Hold and construct Hold[a := 1] directly 
(even if there is an expression in place of '1' that must not be 
evaluated), using Unevaluated[].

> % /. Hold[x_]:>x does the same thing, because /. only matches once per part.
> 

If you use Replace instead of ReplaceAll, then you can specify the level 
at which the replacement should be done.

Replace[Hold[a := Hold[1]], Hold[x_] -> x, {2}]

-- 
Szabolcs


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