You want to emit the actual type notation, something like this:

@unquote(attr)(t() :: {unquote_splicing(types)})

You can’t mix bind_quoted with unquoutes.

Here you go:

defmodule Example do
  defmacro sample(ast, visibility) do
    # sample visibility handling
    attr =
      case visibility do
        :public -> :type
        :private -> :typep
      end

    # this optional line allows you to use sigils:
    # Example.sample(~w[a b c]a, :private)
    list = Macro.expand(ast, __CALLER__)

    # if unuoqte needs to be used which is the case here
    # then quote cannot have bind_quoted option set
    # as this option sets unquote option to false
    # making unquote/1 calls not work
    quote do
      # @ is a macro which means we need to use unquote here
      # to pass a raw data instead of ast not recognized by said macro
      @unquote(attr)(t() :: {unquote_splicing(list)})
    end
  end
end

Helpful resources:

1. @/1 macro
2. quote/2 special form