you can also use map cons, if you would like for your code to crash when you try to assign a nonexistent field, lets you catch errors in tests (how good is your test coverage? lol)

defmodule S do
  defstruct [x: 0]
  def do_something(val = %S{}) do
     %{val | y: "oops"}
  end
end

will crash on do_something. Whenever it’s a struct, I prefer using map cons for that bailout behaviour.

I prefer the previous answer, but another option would get a list of keys expected in the struct and take them from the map.

Building on the other example:

iex> Map.take(m, Map.keys(%URI{}))
%URI{
  authority: "elixirforum.com",
  fragment: nil,
  host: "elixirforum.com",
  path: nil,
  port: 80,
  query: nil,
  scheme: "http",
  userinfo: nil
}

Two downsides to this approach compared to the previous:

1. This solution won’t replace any missing keys.
2. This approach doesn’t let you create the struct dynamically (not without wrapping it in struct/0 at least, which at that point you might as well do the other).