The REPL is obscuring things somewhat - interpolate runs during compilation and creates an AST which is then evaluated. bar has a value during the second part of that process.

Here’s a revised version that handles the example from your post:

defmodule Foo do
  defmodule Column do
    defstruct [:name]
  end

  defmacro select(columns) do
    interpolate(columns)
  end

  def interpolate(q) do
    IO.inspect(q, label: :q)

    ast = Macro.postwalk(q, fn
      {:^, _, [{name, _, _} = v]} when is_atom(name) ->
        quote do
          atom_to_column(var!(unquote(v)))
        end
      a when is_atom(a) ->
        quote do
          atom_to_column(unquote(a))
        end
      x -> x
    end)

    IO.inspect(ast, label: :ast)

    ast
  end

  def atom_to_column(a) when is_atom(a) do
    %Column{name: a}
  end
end

Running this produces output like:

iex(32)> select([:foo, :bar])
q: [:foo, :bar]
ast: [
  {:atom_to_column, [context: Foo, import: Foo], [:foo]},
  {:atom_to_column, [context: Foo, import: Foo], [:bar]}
]
[%Foo.Column{name: :foo}, %Foo.Column{name: :bar}]
iex(33)> wat = :foobar
:foobar
iex(34)> select([:foo, ^wat])
q: [:foo, {:^, [line: 34], [{:wat, [line: 34], nil}]}]
ast: [
  {:atom_to_column, [context: Foo, import: Foo], [:foo]},
  {:atom_to_column, [context: Foo, import: Foo],
   [{:var!, [context: Foo, import: Kernel], [{:wat, [line: 34], nil}]}]}
]
[%Foo.Column{name: :foo}, %Foo.Column{name: :foobar}]