Question

I know the “xs” can be used for expressing the rest elements in a list
but I totally have no idea what the “ls” mean in Haskell?

Answer 1

ls is not a predefined thing. It is whatever you bind it to, just like xs.

For instance, I think you’ve seen examples like this:

sum [] = 0
sum (x:xs) = x + sum xs

The variable xs, that you just defined here, gets bound (will have the value of) the rest of the list because of the pattern (x:xs). But this could equally well have been written as:

sum [] = 0
sum (l:ls) = l + sum ls

We prefer not to call a variable l though, because it is easily confused with the digit 1 (or even the pipe symbol | on really messed up fonts).

We could even write:

sum [] = 0
sum (head:tail) = head + sum tail

where we reuse the names of the built-in prelude functions head and tail, but this is bound to lead to confusion.