Question

Assuming a binary search tree, I would like to return an error in case we are trying to insert an element that is already there. Is there a way to make this work?

data BST2 a = EmptyBST2 | Node2 a (BST2 a) (BST2 a)  deriving Show

insert2 :: a -> Either b (BST2 a) -> Either b (BST2 a)
insert2 elem (Right EmptyBST2) = Right (Node2 elem EmptyBST2 EmptyBST2)
insert2 elem (Right (Node2 root left right))
  | (elem == root) = Left "Error: Element already exist."
  | (elem < root) = (Node2 root (insert2 elem left) right)
  | otherwise = (Node2 root left (insert2 elem right))

Note: I am new to Haskell.

Answer 1

This problem is simpler to solve using helper functions:

insert2 :: (Ord a) => a -> BST2 a -> Either String (BST2 a)
insert2 newVal tree
  | contains newVal tree = Left "Error:  element already in tree"
  | otherwise = Right $ insert newVal tree

Now you need contains and insert:

contains :: (Ord a) => a -> BST2 a -> Bool
contains .... implementation .... -- checks whether a BST2 contains an element

insert :: (Ord a) => a -> BST2 a -> BST2 a
insert .... implementation .... -- inserts an element if not already there,
                                --   otherwise returns original tree

Alternatively, instead of Either String (BST2 a), you could use one of Haskell’s many other approaches to errors.