Problem mocking a function when doing unit testing

I’m testing a function that call another function to fetch external data using API calls, here is the main module code :

defmodule MyApp.MyModule.ExternalData do
  require Logger
  
  def fetch_external_data(params) do
    with {:ok, data} <-
      http_get_resource(params["url"], params["headers"]) do
        data
    else
      :error ->
        Logger.info("ERROR executing API call to : #{source} with path : #{url} ")
        %{}
    end
  end

  def http_get_resource(url, headers) do
    case HTTPoison.get(url, headers) do
      {:ok, %HTTPoison.Response{status_code: 200, body: body}} ->
        {:ok, Poison.decode!(body)}

      {:ok, %HTTPoison.Response{status_code: 404}} ->
        :error

      {:error, %HTTPoison.Error{reason: _reason}} ->
        :error
    end
  end
end

And here is my test file :

defmodule MyApp.MyModule.ExternalData.Test do
  use ExUnit.Case

  # Replace `http_get_resource/2` with a mock function to simulate the HTTP request
  def mock_http_get_resource(url, headers) do
    case url do
      # Simulate a successful request with a 200 status code
      "http://valid.com/resource" -> {:ok, %{status_code: 200, body: %{key1: "value1", key2: "value2"}}}
      # Simulate a failed request with a 404 status code
      "http://invalid.com/resource" -> {:ok, %{status_code: 404, body: "Resource not found"}}
      # Simulate a request that times out
      "http://timeout.com/resource" -> {:error, :timeout}
    end
  end

  def http_get_resource(url, headers) do
    mock_http_get_resource(url, headers)
  end

  test "fetch_one_external_resource/1" do
    # Replace the actual `http_get_resource/2` function with the mock function
    alias MyApp.MyModule.ExternalData, as: M
    M.http_get_resource = &http_get_resource/2

    # Test a successful request
    params = %{
      "url": "aaa", 
      "headers": "bbb"
    }
    assert M.fetch_one_external_resource(params) == %{new_key1: "value1", new_key2: "value2"}

    # Test a failed request
    params = %{
      "url": "aaa", 
      "headers": "bbb"
    }
    assert M.fetch_one_external_resource(params) == :error

    # Test a request that times out
    params = %{
      "url": "aaa", 
      "headers": "bbb"
    }
    assert M.fetch_one_external_resource(params) == :error
  end
end

I’m getting this error :

== Compilation error in file test/fetch_data_test.exs ==
** (CompileError) test/fetch_data_test.exs:26: cannot invoke remote function MyApp.MyModule.ExternalData.http_get_resource/0 inside a match
    test/fetch_data_test:23: (module)
    (elixir 1.14.2) lib/kernel/parallel_compiler.ex:455: Kernel.ParallelCompiler.require_file/2
    (elixir 1.14.2) lib/kernel/parallel_compiler.ex:348: anonymous fn/5 in Kernel.ParallelCompiler.spawn_workers/7

Any ideas or fix please, I think the problem is in the line M.http_get_resource = &http_get_resource/2

thanks