RE: Do Modules Produce Side Effects?

*To*: mathgroup at smc.vnet.net*Subject*: [mg47995] RE: [mg47964] Do Modules Produce Side Effects?*From*: "DrBob" <drbob at bigfoot.com>*Date*: Wed, 5 May 2004 08:11:01 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

The first argument of Module is a list of its local variables. If you use a variable not in that list, you're talking about a variable from the context outside the Module. DrBob www.eclecticdreams.net -----Original Message----- From: Harold Noffke [mailto:Harold.Noffke at wpafb.af.mil] To: mathgroup at smc.vnet.net Subject: [mg47995] [mg47964] Do Modules Produce Side Effects? $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