Question

Possible Duplicate:
Where does the name “xs” for pattern matching come from?

I’m learning Haskell. Here is a function that calculates the sum

sum' :: (Num a) => [a] -> a  
sum' [] = 0  
sum' (x:xs) = x + sum' xs  

I can’t figure out that xs means. x – is the head, xs – is the tail. But is it written is xs and not as just x or s?

Answer 1

x and xs are just variable names; you could use fred and wilma instead, for example. But it’s very common to call the head of a list x, and the tail xs (pronounced exes), because it gives you a hint that x is a single element, while xs may contain multiple elements.

The example you’ve given uses pattern matching, which is a nifty feature. The line

 sum' (x:xs) = x + sum' xs  

basically says “take the input parameter to sum’, and split it after the first element. Call the first element x and the rest of the list xs“. It would be essentially equivalent to:

 sum' list = x + sum' xs
     where x = head list
           xs = tail list