3
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Here is an example code:

a = 1;
b = 1;
c = a + b;

f[x_, y_] := Module[{a = 3, b = 2, c}, c = (x + y)*(a + b)];

a,b,c are used outside of the function f and also inside f.

How can I ensure that all used variables in a function are local WITHOUT specifing all of them in Module[{local variables},...]. This can be extremely annoying when I use many variables.

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  • $\begingroup$ Why not just create unique variables then? $\endgroup$ – Feyre May 5 '17 at 11:49
  • $\begingroup$ @Feyre: I want to reuse some SAME variable names outside and inside the function. $\endgroup$ – lio May 5 '17 at 12:32
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I guess this is a duplicate but I can't find it. Here's something you may find useful:

myModule // ClearAll
myModule // Attributes = {HoldAll}
myModule[expr_] := Module[
    { spec = (
          Union[ Join @@ Cases[
              Hold[expr]
           ,  s_Symbol /; Context[Unevaluated[s]] === "Global`" :> Hold[s]
           , {-1}
           , Heads -> False
          ]]
      ) /. Hold[spec__] :> Hold[{spec}]
    }
  , Module @@ (Hold[Evaluate[Unevaluated @@ spec], expr])
]


a = 1;
b = 1;
c = a + b;

f[x_, y_] := myModule[a = 3; b = 2; c = (x + y)*(a + b)];


f[1, 2]
a
b

15

1

1

Short explanation

  • Cases extracts all symbols from Global` context, could be not from System` but then Package symbols would be used too. I know Global` isn't always the $Context so maybe $Context would be better? Don't know, up to you.

    It ignores head so from f[g,h] only g and h will be extracted. Do you want that? Again, don't know, feel free to change it.

    At the end it returns {Hold[g], Hold[h],...

  • then we transform this result to Hold[{g,h,...}

  • Module assembling

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  • $\begingroup$ This is exactly what I was looking for. Perfect. But, I am not able to understand what the code is doing. Could you please shortly describe in words what you main idea was. Thank you. $\endgroup$ – lio May 5 '17 at 13:01
  • $\begingroup$ @lio let me know if that is enough $\endgroup$ – Kuba♦ May 5 '17 at 13:16
  • $\begingroup$ Thank you for the explanation. This is a very important solution for all newbies who are coming from other languages to Mathematica. Am I really the first who is asking this question, I cannot believe it? $\endgroup$ – lio May 5 '17 at 13:36
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As described in Modules and Local Variables, there is no other way of making variable literally local, than specifying it in the Module statement. You may consider declaring list as a local variable: Module[{locVars={1, 3, 4}}... and referring its elements or introduce an wolfram language package with its own private symbols: Setting Up Wolfram Language Packages.

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