firstDuplicate.hs

firstDuplicate :: [Int] -> Int
firstDuplicate xs = checkDuplicates xs []

checkDuplicates :: [Int] -> [Int] -> Int
checkDuplicates (x:[]) f = if elem x f then x else -1
checkDuplicates (x:xs) f = if elem x f then x else checkDuplicates xs (x:f)

test.hs

data Tree a = Tree {
  value :: a,
  left :: Tree a,
  right :: Tree a,
} | Null deriving Show

tree = {
  value: 1,

  left: {
    value: 2,
    left: Null,

    right: {
      value: 4,
      left: Null,
      right: Null,
    }
  },

  right: {
    value: 2,

    left: {
      value: 3,
      left: Null,
      right: Null,
    },

    right: {
      value: 3,
      left: Null,
      right: Null,
    }
  }
}

hasPathSum :: Tree a -> Int -> Bool
hasPathSum Null _ = False
hasPathSum tree sum
         | tree.value == sum = True
         | otherwise = (hasPathSum tree.left sum - tree.value) || (hasPathSum tree.right sum - tree.value)