its a bit of a smell, but good elixir code doesn't have to worry about rebinding for bindings in the same scope.

Jose Valim goes on as to why he made that design decision from the following: https://groups.google.com/g/elixir-lang-talk/c/w83lKZs4YS8/m...

    I will post the same answer from before: I like the explicitness of pattern 
    matching. In Elixir, as soon as I see ^foo, I know I am matching. If there is 
    no ^, I know the previous value regardless if it there is one or not, will be 
    discarded. If pattern matching is not explicit, I always need to know if a 
    variable was previously defined or not to know what is going to happen. To me 
    this behaviour is non negotiable. In my experience, it is more likely to run 
    into accidental matches than into accidental rebindings.

    Another possible limitation to the suggestion above can be related to macros. 
    Let's suppose you have a macro that stores a value in a hygienic variable:

    defmacro do_something(a, b) do
      quote do
        var!(hello, Hygienic) = calculate_something(unquote(a), unquote(b))
      end
    end

    Someone may call this macro multiple times but we always care about the last 
    value 
    of hello. If we make a distinction in between being assigned once and then 
    multiple times, the macro simply won't work. Or the macro developer would need to 
    work around this behaviour by inspecting the environment or we would need to 
    provide some sort of functionality that does it for you. Maybe this behaviour 
    could be built-in in var! or maybe we'd need to introduce something like:

    defmacro do_something(a, b) do
      quote do
        set!(hello, Hygienic) = calculate_something(unquote(a), unquote(b))
      end
    end

    We can see how this is getting complex:

    1. Use = to define variables
    2. Use ^ for pattern matching
    3. Use := for rebinding
    4. Use set! in macros when you don't care if a variable was previously defined or not

    It is also interesting to point out that the := operator won't work for rebinding 
    inside clauses. For example, how would you make the rebinding below explicit?

    x = true
    case false do
      x -> :ok
    end

    Of course, there are interesting consequences for making a distinction in between 
    defining and rebinding a variable by introducing something like the := operator. 
    We could have better control of the scope to provide better warnings or even make 
    it easier to implement imperative for loops:

    x = 0

    for i <- 1..5 do
      x := x + i
    end

    x #=> 15

    But the fact Elixir have different ways for variables to be introduced in the 
    scope, adding more rules can make the overall system very complex.