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MathGroup Archive 2004

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Re: Do Modules Produce Side Effects?

  • To: mathgroup at
  • Subject: [mg48002] Re: Do Modules Produce Side Effects?
  • From: Bill Rowe <readnewsciv at>
  • Date: Wed, 5 May 2004 08:11:07 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

On 5/4/04 at 1:08 AM, Harold.Noffke at (Harold Noffke)

>The MathBook definition of Module tells me, "Module creates new
>symbols to represent each of its local variables every time it is
>called."  I am led by this, and other Module descriptions, to
>conclude Modules do not produce side effects, like Blocks do. 
>However, we have ....

>In[1]:= m=i^2
>Out[1]= i^2

>In[2]:= Module[ {}, m=4; 2*m ] 
>Out[2]= 8

>In[3]:= m Out[3]= 4

>I expected m to remain unchanged from its original i^2.  But Module
>changed m to 4, just as I would expect a Block to do.

>Am I misunderstanding something about the "side effect safety" of

I think you are misunderstanding the syntax of Module. To get be able to use m as a local variable and preserve global values of m, you need to specify m to be a local variable. That is you need to use the following to get the behavior you want

Module[{m}, m=4; 2 m]
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