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Re: Manipulate inside module

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  • Subject: [mg121351] Re: Manipulate inside module
  • From: John Fultz <jfultz at>
  • Date: Mon, 12 Sep 2011 04:19:56 -0400 (EDT)
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  • Reply-to: jfultz at

On Sat, 10 Sep 2011 07:30:40 -0400 (EDT), Charles Wells wrote:
> I expected the following expression to show the square of the current
> value of the slider, but it shows only a^2.  There is clearly something I
> don't understand about Module.  Any insights?
> Module[{b = a}, (Print[b]; Manipulate[b^2, {{a, 1}, 0, 5}])]

I could explain in some considerable detail exactly what's going on here (and it 
would be considerable), but it might be more useful and straightforward to give 
you a very simple rule...

Module variables should *never* appear inside Dynamics or Manipulates internal 
to that Module.

To be clear with some examples (all using Dynamic, but they could equally well 
use Manipulate, which is implemented using Dynamic)...

(* OK *) Dynamic[Module[{a}, a]]
(* OK *) Module[{a}, 
		(* expression involving a*);
		Dynamic[(* expression *not* involving a *)]
(* BAD *) Module[{a}, Dynamic[a]]

In fact, this rule is so straightforward and universal, there's no reason that 
we shouldn't have some advisory syntax highlighting which points out the 
badness...I intend to do this for a future release.


John Fultz
jfultz at
User Interface Group
Wolfram Research, Inc.

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