Koka-inspired local mutable variables for cleaner comprehensions

[tmbb]

@benwilson512

A key part of the proposal is that it would be syntactic sugar over normal immutable operations.

Ok, so that part is important to you. Ok, in that case my solution is defiinitely unacceptable. Also, my solutino also deviates regarding this:

@josevalim

Local mutable variables can only be mutated within their scope. You can mutate them inside if/unless, for comprehensions, cond, receive… but that’s all. So you could write code like this, if you want to:

mut list = []

if some_value? do
  list = ["prefix" | list]
end

But you cannot write code like this, since anonymous functions escape the local function:

mut sum = 0

Enum.map([1, 2, 3], fn x ->
  sum = x + sum
end)

In fact, the code above would fail to compile.

I my implementation, the code above works:

def factorial_with_map(n) do
    mutable! result: 1 do
      # Update the product inside a map function
      Enum.map(1..n, fn i -> result <<~ result * i end)

      # Return the mutable variable
      result
    end
  end

and in my opinion it should work. If we want mutable variables, then they better be real mutable variables instead of a very artificial thing that works in for loops and not in map functions. But that is just my personal opinion, of course.