Destructuring the conn object

Phoenix Forum Questions / Help
1

my headers

[
  {"accept", "*/*"},
  {"accept-encoding", "gzip, deflate, br"},
  {"authorization",
   "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjo2LCJzYWx0IjozMzQ4Mjd9.RtWI9w8uuxBue9Byi0tlZm6EUlN83HC1Ed9UzhPHpN4"},
  {"cache-control", "no-cache"},
  {"connection", "keep-alive"},
  {"content-length", "136"},
  {"content-type", "application/json"},
  {"experiment_data",
   "8Qd7CiAgICAiZXhwZXJpbWVudHMiOiBbFQAAAgACHwAEAgD1ByJuYW1lIjogIlZlcnNpb25UZXN0IiwtAAACADAiaWQhABxCFwCLcmVzdWx0IjpSAAACAPoFInhnLWRlbGl2ZXJ5LXgxIjogMjBVAAACAAomAF8yIjogNSYADmkzIjogMzCgABV9DgAWfWQACpQAD+YAATxPbmXSAA/pAJeACiAgICBdCn0="},
  {"host", "localhost:4000"},
  {"postman-token", "af95e65d-1261-4471-8191-b3f29b634254"},
  {"user-agent", "PostmanRuntime/7.26.5"}
]

def evaluate(%{req_headers: [%{"experiment_data"=> experiment_data}]} = conn, _) do
    IO.inspect("===============")
    IO.inspect(experiment_data)
    IO.inspect("===============")

    conn
    |> render_json_response(%{})
  end

I want the evaluate function to be called only when the experiment_data is present in the header and process it else go to other fallback function.
I tried all possible goggling stuff but couldn’t find the solution.

Thanks.

2

The req_headers is a list of 2-elem tuples and your matcher on evaluate is trying to match a map inside the list.

My suggestion is that on the function you are calling evaluate you use the Plug.Conn.get_req_header/2 function, that returns a list of values for that header and pattern match against that:

def call(%{req_headers: headers} = conn, _) do
  conn |> Plug.Conn.get_req_header("experiment_data") |> List.first() |> evaluate(conn)
end

def evaluate(nil, conn), do: conn

def evaluate(value, conn) do
  # do something with value
end

Bear in mind it’s possible to pattern match against list’s values:

ExampleA:

def evaluate(%{req_headers: [{"experiment_data", experiment_data} | _]}) do
  ...
end

Example B:

def evaluate(%{req_headers: [{"experiment_data", experiment_data}]}) do
  ...
end

But example A would only work if the {"experiment_data", "value"} tuple was the first element of the list and example B would only work if the list only had one element and that tuple was that element.

Hope I clarified your question :slight_smile:

5 Likes
3

Thanks for the explained answer.

But what required is there are legacy code where there is evaluate function that is being called based on pattern matching and I want this function to be called only when experiment_data is present in the header.

Thanks