I'm trying to make a custom function to replace a given element in a list.

Code:
    let i = elemIndex toReplace lst in

        case i of
            Just i ->
                let z = splitAt i lst
                    x = fst z
                    y = (snd z)
                    in
                        init x
                        x ++ newNmr
                        x ++ y

            Nothing -> [5]

Error:

1)
* Couldn't match expected type `[a] -> [a]' with actual type `[a]'
    * The function `init' is applied to two arguments,
      but its type `[a] -> [a]' has only one
      In the first argument of `(++)', namely `init x x'
      In the expression: init x x ++ newNmr x ++ y
    * Relevant bindings include
        y :: [a] (bound at C:\users\niel\desktop\test2.hs:41:21)
        x :: [a] (bound at C:\users\niel\desktop\test2.hs:40:21)
        z :: ([a], [a]) (bound at C:\users\niel\desktop\test2.hs:39:21)
        newNmr :: [a] (bound at C:\users\niel\desktop\test2.hs:34:30)
        lst :: [a] (bound at C:\users\niel\desktop\test2.hs:34:26)
        toReplace :: a (bound at C:\users\niel\desktop\test2.hs:34:16)
        (Some bindings suppressed; use -fmax-relevant-binds=N or -fno-max-relevant-binds)
2)
* Couldn't match expected type `[a] -> [a]' with actual type `[a]'
    * The function `newNmr' is applied to one argument,
      but its type `[a]' has none
      In the first argument of `(++)', namely `newNmr x'
      In the second argument of `(++)', namely `newNmr x ++ y'
    * Relevant bindings include
        y :: [a] (bound at C:\users\niel\desktop\test2.hs:41:21)
        x :: [a] (bound at C:\users\niel\desktop\test2.hs:40:21)
        z :: ([a], [a]) (bound at C:\users\niel\desktop\test2.hs:39:21)
        newNmr :: [a] (bound at C:\users\niel\desktop\test2.hs:34:30)
        lst :: [a] (bound at C:\users\niel\desktop\test2.hs:34:26)
        toReplace :: a (bound at C:\users\niel\desktop\test2.hs:34:16)
        (Some bindings suppressed; use -fmax-relevant-binds=N or -fno-max-relevant-binds)

I've tried a lot, but I always got an error.

What am I doing wrong?

Thanks!