Write what the program does.
The types keep up.

use std::effects::log::{ Console, println }
use std::effects::http::{ HttpClient, MockHttpClient, mock_get, request }

# weather says <Http> in its type. It never says which client answers,
# so the caller below picks that, not weather.
let weather = { url } -> request { url }.send {}.text_or { def = "offline" }

let main = {} -> perform Console in do
  let url = "https://wttr.in/Amsterdam?format=3"

  # The real client. This one goes over the network.
  perform HttpClient in println { msg = weather { url } }

  # Same function, same url, no network at all.
  let rules = [mock_get { body = "Amsterdam: Always raining", url = "*" }]
  perform MockHttpClient { rules } in println { msg = weather { url } }