To process lists, we can simply pattern match on the constructors of the list type:

listSum :: [Int] -> Int
listSum []          = 0
listSum (x:xs) = x + listSum xs

We can match more values by specifying a more elaborate pattern:

sumTwoPer :: [Int] -> Int
sumTwoPer [] = 0
sumTwoPer (x1:x2:xs) = x1 + x2 + sumTwoPer xs
sumTwoPer (x:xs) = x + sumTwoPer xs

Note that in the above example, we had to provide a more exhaustive pattern match to handle cases where an odd length list is given as an argument.

The Haskell Prelude defines many built-ins for handling lists, like map, filter, etc.. Where possible, you should use these instead of writing your own recursive functions.