Scope of variable

General Usage
7

The let and do here are red herrings. Here are some simpler questions to nudge you in the right direction.

Let’s say I have:

function foo()
  x = ...
end

This unambiguously creates a local variable named x inside foo(), right?

Now let’s say I have:

function foo()
  println(x)
end

This unambiguously refers to some (hopefully existing) global variable named x, right?

Now what if I have:

function foo()
  x = x + 1
end

What is x? If we follow the first example, it’s a local variable, but then the right-hand side makes no sense since that local variable doesn’t exist yet. If we follow the second example, then it’s a global variable, but now we are silently modifying a global variable, which the language does not allow. If we follow both examples, somehow x is both a local and a global variable at the same time, which is even worse. That’s why this situation is an error in Julia, since there’s no obviously right thing to do.

If you want x to refer to the global variable, then you need:

function foo()
  global x
  x = x + 1
end

That works with no issues. If you want x to be a local variable, then you can’t also have it refer to the global variable with the same name.

A let block creates a new local scope, so you can do:

julia> let 
         a = 1 
         function foo()
           a = a + 1  # updates `a` in the parent local scope. 
         end
         foo()
         println(a)
       end
2

as explained in Scope of Variables · The Julia Language

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