When a list is not a list

Question

When I provide a list of local variables to Module, it complains that it is not a list:
v = {x, y};
Module[v, x = 2]
... Module: Local variable specification v is not a List.
So my question is how do I achieve the above (with list v specified outside of Module) without error?

Nam Nguyen · Answer

Module[{x},expr] specifies that occurrences of the symbols x in expr should be treated as local.
When you forget to put x in {}, you will get this
In[1]:= Module[x, y]

During evaluation of In[1]:= Module::lvlist: Local variable specification x is not a List.

Out[1]= Module[x, y]

The correct way is:
In[2]:= Module[{x}, y]

Out[2]= y

When a list is not a list

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