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MathGroup Archive 2004

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Re: Do Modules Produce Side Effects?

  • To: mathgroup at
  • Subject: [mg47985] Re: Do Modules Produce Side Effects?
  • From: Peter Hahn <pg.hahn at>
  • Date: Tue, 4 May 2004 07:03:17 -0400 (EDT)
  • References: <c778ms$hf7$>
  • Sender: owner-wri-mathgroup at

Hi Harold,

you just have forgotten to declare m in the module as a local variable,

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

leaves the global m unchanged, as expected.


Harold Noffke wrote:

> $Version "5.0 for Microsoft Windows [2000] (November 18, 2003)"
> MathGroup:
> 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
> Modules?
> Regards,
> Harold

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