Elixir: Capturing the rest of a map using pattern matching

Question

I want to simultaneously match on a specific key in a map, and capture the rest of that map. I was hoping something like this would work:

iex(10)> %{"nodeType" => type | rest} = %{"nodeType" => "conditional", "foo" => "bar"}

** (CompileError) iex:10: cannot invoke remote function IEx.Helpers.|/2 inside match

The goal is to write a set of functions which take a map, pattern match on one of the map's fields, and perform some transformations on the rest of the map.

def handle_condition(%{"nodeType" => "condition" | rest}) do
  # do something with rest
done
def handle_expression(%{"nodeType" => "expression" | rest}) do
  # do something with rest
done

But it looks like I'll need the callee to pass the nodeType separately unless I'm missing something.

Answer 1

You can easily capture the whole map - maybe this will be enough?

def handle_condition(all = %{"nodeType" => "condition"}) do
  # do something with all
end

Or:

def handle_condition(all = %{"nodeType" => "condition"}) do
  all = Map.delete(all, "nodeType")
  # do something with all
end

Answer 2

Another nice way of achieving this is by using Map.pop/2:

def handle(%{} = map), do: handle(Map.pop(map, "nodeType"))

def handle({"condition", rest}) do
  # ... handle conditions here
end

def handle({"expression", rest}) do
  # ... handle expressions here
end