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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Data Structure    

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Red Black Tree data structure implemented in Haskell.

The goal of this project is to provide an efficient generic structure that can insert and find elements in O(log(n)) time.


Implement BinaryTreeNode

To insert values to a RedBlackTree, their type must have an instance of BinaryTreeNode. Members of this typeclass must implement Ord, and consequently Eq, so that values can be compared and sorted within the tree. Since inserting duplicate values can corrupt the tree, this typeclass provides the following function:

  mergeNodes :: a -> a -> a

This function takes two BinaryTreeNode values and returns a new "merged" node. It is only called when a value is about to be inserted, but the tree already contains another value that is equal to it. This new "merged" node replaces the existing one without doing any further modifications to the tree.

If you want to ignore duplicate nodes, you can just return the first parameter, since that one is guaranteed to be the one that is already in the tree, whereas the second parameter is the node that we are are trying to insert. On the other hand, if you want to replace older nodes with newer ones then you can return the second parameter. If your use case is completely different then you can create a new node with the best parts from each node and return that. Implementing mergeNodes is all you need to do to get a working BinaryTreeNode instance.


To use this data structure you only need to know about these three functions:

emptyRedBlackTree :: RedBlackTree a

insert :: (BinaryTreeNode a) => RedBlackTree a -> a -> RedBlackTree a

find :: (BinaryTreeNode a) => RedBlackTree a -> a -> Maybe a

emptyRedBlackTree is pretty self explanatory, you can use this as a constructor.

insert takes an existing tree and a new value and returns a new tree that contains the new value.

find takes an existing tree and a target value and returns the value inside that tree that is equal to the target. Returns Nothing if no such value is found.


Suppose we wanted to store our app's users on a RedBlackTree. For simplicity, let's assume that we only know two things about the user: the user's email address, a string guaranteed to be unique for each user, and the user's name. The type User could look like this:

data User = User {
  userEmail :: String,
  userName :: String
} deriving Show

Since e-mail adresses are guaranteed to be unique, we don't care about duplicates so the User instances for Eq, Ord and BinaryTreeNode would look like this:

instance Eq User where
  (==) leftNode rightNode = userEmail leftNode == userEmail rightNode

instance Ord User where
  (<=) leftNode rightNode = userEmail leftNode <= userEmail rightNode

instance BinaryTreeNode User where
  mergeNodes leftNode _ = leftNode

The final step is to write a small program that inserts 8 users, and then looks up the name of the user whose address is "[email protected]".

import Data.List (foldl')
import Data.RedBlackTree
import Data.User -- This module defines our User type

main = do
-- create users to insert
let users = [
              User "[email protected]" "Gabriel",
              User "[email protected]" "Jimmy",
              User "[email protected]" "Paul",
              User "[email protected]" "George",
              User "[email protected]" "Frank",
              User "[email protected]" "David",
              User "[email protected]" "Bryan",
              User "[email protected]" "John"

-- insert one by one            
let treeWithOneUser = insert emptyRedBlackTree (User "[email protected]" "gabriel")
let treeWithTwoUsers = insert treeWithOneUser (User "[email protected]" "jimmy")

-- insert the whole list with foldl'
let userTree = foldl' insert emptyRedBlackTree users

-- find the username with address "[email protected]". The Eq instance of User only
-- checks for email address, so we can leave the username empty in the target value
let targetNode1 = User "[email protected]" ""
print $ fmap userName (find userTree targetNode1)
-- should print "Just frank"

-- find the username with address "[email protected]"
let targetNode2 = User "[email protected]" ""
print $ fmap userName (find userTree targetNode2)
-- should print Nothing


  • Currently the only operations supported are find and insert. There is no remove at the moment.
  • Uses a lot of memory. A RedBlackTree with 10 million integers needs over 1 GB of RAM. I'm not sure how to fix this. Suggestions are welcome!


To build this library, clone this repo and use Stack.

git clone https://github.com/GAumala/red-black-tree
stack setup
stack build

To run unit tests with RSpec:

stack test