Right perfect. And if you wanted to do a concat on a list that just took one item, you could do:

def concat([item], list) do
  [item | list]
end

concat([1], [2,3] #=> [1,2,3]

The trick then is to just apply this logic recursively. When doing recursive logic, it’s always a good idea to identify the “base case” which you can think of as “when do I stop recursing?”. In the case of this kind of concat function, you stop recursing when you have just 1 item in the list, because at that point you do a simple prepend.

When you have more than one item, you need to recursively prepend until you hit the base case:

def concat([], list), do: list

def concat([item], list) do
  [item | list]
end

def concat([item | rest], list) do
  [item | concat(rest, list)]
end

The first part of [item | concat(rest, list)] is [item | which you’re already familiar with, that’s setting up item to be prepended to whatever is on the right hand side of |. But this time, instead of prepending it to | list] we do | concat(rest, list)] because we want the item to come before the result of prepending all of the other items.