3 different sum functions in Haskell

sum.hs

module Sum where

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

b xs = b' 0 xs
    where
    b' q []     = q
    b' q (x:xs) = b' (q + x) xs

c xs = foldl (+) 0 xs

list = [1 .. 100]

main =
    do
    print $ a list
    print $ b list
    print $ c list