Dfyjkt

I have been trying to write a code which takes all the integers in a tree and return a sum of them. I'm trying to do this with type a, which is from a data time:

data Tree a = Nil | Value a (Tree a) (Tree a)
deriving Show

and we want to use:
tree = Value 2 (Value 2 (Value 2 Nil Nil) Nil) (Value 2 Nil Nil)

and my code is as follow:

countTree :: (a -> a -> a) -> a -> Tree a -> a
countTree p k (Nil) = h
countTree p k (Value x y z) = x (+) (countTree p k y) (+) (countTree p k z) 

and I want to run my code as countTree (+) 0 tree and the results should return 8.
The problem is that when I run my code it tells me that x has four arguments but it's type a has zero which I honestly don't understand why. I've modifying sections of my code, but no success once so ever, I could really use some assistance.

x (+) (countTree p k y) (+) (countTree p k z) 

is attempting to treat x as a function, and pass to it as arguments all of

(+) (countTree p k y) (+) (countTree p k z)

If you want to have "x + recur left + recur right", you'd want something like:

x + (countTree p k y) + (countTree p k z)

I'm pretty sure however you actually want to use p, not + hard coded. Using prefix notation, you'd have to rearrange it a bit to something like :

(p (p x (countTree p k y)) (countTree p k z))

Or, you could use backticks to inline the calls to p as @bipll suggested:

x `p` (countTree p k y) `p` (countTree p k z)

A side note, but I'm also pretty sure you want h to be k.