A bit late to the party, but if you want to pattern match in a function definition, you can also capture the whole map:

defmodule Foo do
  def foo(map = %{"One" => one}) do
    rest = Map.delete(map, "One")
    IO.inspect(%{one: one, rest: rest})
  end
end

 Foo.foo(%{"One" => 1, "Three" => 3, "Two" => 2})
# > %{one: 1, rest: %{"Three" => 3, "Two" => 2}}

Note that you get that error because on %{"One" => one | tail} the | is used to update the key, in your case is like that you are trying to update the tail key but missing the value.