use gen_tcp to connect to postgresql

pg.ex

Mix.install([
  {:binpp, "~> 1.1"}
])

# version_number taken from docs:
# https://postgrespro.com/docs/postgresql/9.4/protocol-message-formats
version_number = 196_608
version = <<version_number::signed-32>>

whoami = System.cmd("whoami", []) |> elem(0) |> String.trim()

params = [
  {"user", whoami}
]

message =
  for {key, value} <- params do
    [key, <<0>>, value, <<0>>]
  end
  |> List.flatten()
  |> List.insert_at(-1, <<0>>)

length =
  <<byte_size(version) + Enum.sum(for item <- message, do: byte_size(item)) + 4::signed-32>>

msg_startup =
  [length, version, message]
  |> List.flatten()
  |> IO.iodata_to_binary()

{:ok, socket} =
  :gen_tcp.connect(~c"localhost", 5432, [:binary, active: false], 5000)

:ok = :gen_tcp.send(socket, msg_startup)
{:ok, data} = :gen_tcp.recv(socket, 0, 2000)

:binpp.pprint(data)