listsum.hs

listsum :: [a] -> a

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

main = do
  let l = [1,2,3,4,5]
  print $ listsum l

mykth.hs

mykth :: [a] -> Integer -> Integer 

mykth (x:xs) k | k == 1    = x 
               | otherwise = mykth xs (k - 1) 

main = do
  let l = [1, 2, 3, 4, 5]
  let k = 3
  print $ mykth l k 

mylast.hs

mylast :: [a] -> a
mylast [] = error "Empty list"
mylast [x] = x
mylast (_:xa) = mylast xa

main = do
  let l = [1, 2, 3, 4]
  let cl = "rajanikant"
  print $ mylast l
  print $ mylast cl

secondlast.hs

secondlast :: [a] -> a

secondlast[] = error "Empty list."

secondlast(x:xs) = if length xs == 1 then x
                   else secondlast xs

main = do
  let l = [1, 2, 3, 4, 5]
  print $ secondlast (l)

Re secondlast.hs
Lines 2, 4: No blank line please.
Lines 3, 5: Put a space between function name and its argument.
Line 10: No brackets are needed.