gistfile1.txt

defmodule MyApp.Plug.Mobile do
  import Plug.Conn

  def init(options), do: options

  @default_layout "application"
  def call(conn, opts) do
    layout = case get_req_header(conn,"user-agent") do
       [ua|_] ->
          cond do
             String.contains?(ua,"Android") -> "mobile"
             true -> @default_layout
          end
       _ -> @default_layout
    end
    assign(conn,:layout,layout)
  end
end

Proof-of-concept of Plug (see https://github.com/elixir-lang/plug) detecting mobile clients.