How to make custom prompt take effect at iex start?

Question

I have a custom prompt but when I launch iex, a builtin prompt shows. After I hit enter my prompt takes effect. How can I make my prompt take effect when iex starts?

Here's what I'm seeing:

And this is my .iex.exs file:

IEx.configure(
  colors: [ enabled: true],
  default_prompt: [
      "\e[G",   # move to column 1
      "\e[35m", # magenta
      "MY %prefix(%counter)",
      ">",
      "\e[0m"   # reset
    ] |> IO.chardata_to_string
)

Answer 1

It's a bug in iex. I've tracked down and fixed it: https://github.com/elixir-lang/elixir/pull/4895