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 listsum.hs

listsum :: [a] -> a

It should be listsum :: Num a => [a] -> a, because (+) restricts the type a to instances of the class Num (see Type constraints).

λ :t (+)            
(+) :: Num a => a -> a -> a

Also it is unorthodox to put a blank line between type signature and definition. Please remove such blank lines (line 3 in listsum.hs).

Re mykth.hs

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

The function should return k-th element of the list. What is the type of elements of that list? (Hint: look at the type of mykth's first argument.)

[1 of 1] Compiling Main             ( /tmp/YYY.hs, interpreted )

/tmp/YYY.hs:2:30: error:
    • Couldn't match expected type ‘Integer’ with actual type ‘a’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          mykth :: forall a. [a] -> Integer -> Integer
        at /tmp/YYY.hs:1:1-34
    • In the expression: x
      In an equation for ‘mykth’:
          mykth (x : xs) k
            | k == 1 = x
            | otherwise = mykth xs (k - 1)
    • Relevant bindings include
        xs :: [a] (bound at /tmp/YYY.hs:2:10)
        x :: a (bound at /tmp/YYY.hs:2:8)
        mykth :: [a] -> Integer -> Integer (bound at /tmp/YYY.hs:2:1)
  |
2 | mykth (x:xs) k | k == 1    = x
  |                              ^
Failed, 0 modules loaded.

Did you make an honest attempt to understand compiler error message? Sometimes that's all you have.

λ mykth [1..5] 0
*** Exception: /tmp/YYY.hs:(2,1)-(3,45): Non-exhaustive patterns in function mykth

λ mykth [1..5] 6
*** Exception: /tmp/YYY.hs:(2,1)-(3,45): Non-exhaustive patterns in function mykth

Please update the function to fail with meaningful error message if k is out of boundaries.

P.S.: [1, 2, 3, 4, 5] can be more easily expressed as [1..5].

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.