Suppose we've got a simple representation of a user:

case class User(id: Long, name: String, email: String)

Now suppose we're writing a web service where we allow clients to post some JSON to a resource to create a new user. We get to pick the id, not the client, so we might accept something like this:

{
  "name": "Foo McBar",
  "email": "foo@mcbar.com"
}

If we're using a type class-based JSON library like Argonaut, we'll probably have written a codec instance for User (or we may be using a library like argonaut-shapeless that derives instances for our case classes automatically).

The problem is that our User codec won't work on JSON like the example above (since it's missing the id field).