Destructuring the conn object

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

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