Hi! It is a special syntax in Elixir to pattern match a list.

Example:

• If arr is [] then this pattern does not match;
• If arr is [1] then leader is 1 and rest is [];
• If arr is [1, 2] then leader is 1 and rest is [2];
• If arr is [1, 2, 3] then leader is 1 and rest is [2, 3];
• And so on.

Usually, using functional programming jargon, leader is called head and rest is called tail.

thank you!

what does this argument mean/do -> [leader | rest] ? thanks🙏🏿

Yes.

https://hexdocs.pm/elixir/master/Kernel.html#length/1

does the function length exist in elixir?