Assignment and mutation

The simplest version of this Stephan Karpinksy can come up with is this:

Assignment changes which object a name refers to: x = ex causes the name x to refer to the value resulting from the evaluation of the expression ex. Assignment never changes the values of any objects.
Mutation changes the value of an object: x[i] = ex and x.f = ex both mutate the object referred to by x changing a value at index or a property with a name, respectively. Mutation never changes what objects any names in any scope refer to.

Perhaps the confusion comes from the fact that these all use the = in their syntax? They’re really totally unrelated. It’s also possible that people think of assignment as setting a named property on some implicit “scope object”. That’s probably a view that can be worked out coherently, in which case having a clear notion of what all the different “scope objects” are would be crucial but I’m not entirely sure if that’s a helpful way to think about the matter or not.

In summary:

x=[2]  # x points to memory location m1
y=x    # y points to memory location m1
x=[3]  # x points to memory location m2, y still points to m1
z=x    # z points to memory location m2
x[1]=4 # m2 changes value in place, affecting all variables that point there

julia> println(x,y,z)
[4][2][4]