why not add another function to handle that?

def with_random_bytes(%Email{} = email) do
  Map.put(email, :bytes, random_bytes())
end

Then in your context or anywhere else you need it can always just call it like:

%Email{}
|> Email.with_random_bytes()
|> ...

There’s no difference, really.

def with_random_bytes(struct) do
  Map.put(struct, :bytes, random_bytes()
end

…

%MyStruct{email: email}
|> MyStruct.with_random_bytes()

However, I think new is the way to go. Don’t overcomplicate things. Document that new structs should be created using the initializer in order to set dynamic defaults.

Are Map.new, Range.new, and MapSet.new non-idiomatic?

100% agree, Session.new(email) or Session.new(opts) with documentation feels right IMO.

The thing is attrs is an Enumerable.t(), so it can be a map or a keyword. So that is why i do it after i create the struct.

But yea very valid if you want to have the two functions as you showed

We can even mix and match both solutions:

def new(attrs) do
  __MODULE__
  |> struct(attrs)
  |> maybe_with_random_bytes()
end

defp maybe_with_random_bytes(%{bytes: nil} = struct) do
  Map.put(struct, :bytes, random_bytes())
end

defp maybe_with_random_bytes(struct), do: struct