Well, I just didn't understand the usefulness.
def input(%{field: {f, field}} = assigns) do
assigns
# Why `nil`?
|> assign(field: nil)
|> assign_new(:name, fn ->
name = Phoenix.HTML.Form.input_name(f, field)
if assigns.multiple, do: name <> "[]", else: name
end)
|> assign_new(:id, fn -> Phoenix.HTML.Form.input_id(f, field) end)
|> assign_new(:value, fn -> Phoenix.HTML.Form.input_value(f, field) end)
|> assign_new(:errors, fn -> translate_errors(f.errors || [], field) end)
|> input()
end