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Re: Re: shadow warning with function parameters and local

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  • Subject: [mg105064] Re: [mg105029] Re: shadow warning with function parameters and local
  • From: brien colwell <xcolwell at>
  • Date: Thu, 19 Nov 2009 05:25:49 -0500 (EST)
  • References: <hdttip$hal$> <>

Thanks Albert and Bobby, the context behavior is clearer to me now.

But why does M have Module and Block? When is Module favorable? My 
current understanding is that both set up local contexts; however, 
unlike Block, Module touches or exports a symbol into the containing 

Albert Retey wrote:
> Hi,
>> I have a set of fairly complex packages. Frequently I use the same 
>> variable names in function definitions and local modules across many
>>  packages. Sometimes I see this type of warning:
>> <varname>::shdw : Symbol <varname> appears in multiple contexts {*};
>>  definitions in context * may shadow or be shadowed ...
>> This does not happen with symbols in the package context (already 
>> fixed); it happens only with function parameters and variables in
>> local modules. I can recreate the warning with the following code
>> snippet. It seems to me that the variable 'v' should be safely local
>> to each module. Is there really a symbol conflict to worry about
>> here?
> no, the variables v and t are safely localized by SetDelayed and Module,
> so everything would work as expected, on the other hand...
>> In[1]:= BeginPackage["a`"]; 
>> Test1[t_] := Module[{v = t + 1}, t + v]; 
>> EndPackage[]; 
>> BeginPackage["b`"];
>> Test2[t_] := Module[{v = t + 2}, t + v];
>> EndPackage[];
> ...the correct way to make the variable v and t private to the packages
> would be as follows:
> BeginPackage["a`"];
> Test1::usage="Test1[t] does something.";
> Begin["`Private`"]
> Test1[t_] := Module[{v = t + 1}, t + v];
> End[];
> EndPackage[];
> that is you would mention all symbols to be exported by your package
> between BeginPackage and Begin, then put the function definitions
> between Begin and End. Usually it is recommended to define the usage
> messages between BeginPackage and Begin, but actually it is good enough
> to just create the symbols there, so they are in the correct package...
> If you are interested, you might want to learn about how Mathematica
> handles name spaces (see Context, BeginPackage, Begin etc.) and
> localization via Module, Block and With (and of course some other
> functions).
> hth,
> albert

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