Solving a HackerRank problem using Haskell
20180922, last updated 20200905
I have been learning Haskell with this great book: Learn You a Haskell for Great Good!
When I learn something new, I split the learning process in two parts:
 Theoretical: typically by reading a book.
 Practical: typically by creating a project or solving HackerRank exercises.
In this blog post I want to show how Haskell can be used to solve HackerRank problems. I will present my solution written in Haskell for the "Diagonal Difference" problem. Please see the problem statement here: Diagonal Difference
I will show the solution in parts, explaining each part separately. By the end of the post I will show the solution as a whole. If you see something that could be done better I will greatly appreciate your feedback.
Solution general description
The solution will be shown following a topdown approach, starting with the main function, which is the entry point of the program, and then explaining each function until we reach the complete solution.
The implementation of the main function is as follows:
24 25 26 27 28 29 

The main function presents a high level view of how we are going to solve the problem:
 Read expected input from stdin: lines 25 and 26
 Give us a data structure to work with: line 26
 Perform logic to solve the problem: lines 27 and 28
 Print result to standard output: line 29
Now let's analyse what each function does to complete the solution.
getIntSquareMatrix
This is the implementation of getIntSquareMatrix
:
4 5 6 7 8 

The function getIntSquareMatrix
receives an Int
as parameter and returns a matrix of Int elements, inside an IO "wrapper" IO([[Int]])
. Allow me to explain this function line by line:
Line 6 reads a line from stdin N times, where N is defined by the parameter rows
, then it binds the result to lines
.
The type of lines
is [String]
. Why?
replicateM rows getLine
returns anIO([String])
<
binds the[String]
fromIO([String])
tolines
 Therefore
lines
is of type[String]
Line 7 applies some functions to lines
and defines intMatrix
of type [[Int]]
.
Finally line 8 wraps the intMatrix
into the IO
"wrapper" so the function returns a value of type IO([[Int]])
.
getDiagonals
The implementation of the function getDiagonals
is as follows:
10 11 12 13 14 15 16 

Given a matrix as parameter, this function returns a tuple with the anti diagonal and the main diagonal of the matrix. The anti diagonal goes from bottom left to top right. The main diagonal goes from top left to bottom right.
absDiagonalDifference
This is the implementation of the function absDiagonalDifference
:
18 19 20 21 22 

This function takes the two lists of Int elements [Int]
as parameters. Each list represents one diagonal of the matrix. Then it sums the contents of each list. Finally it returns the absolute value of the difference between the sums.
Complete solution
This is the complete solution of the problem. It contains the same functions we have explained in a single file.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 

Final messages
 With this post I wanted to show that Haskell could be used to solve HackerRank problems and that in fact it is a great language for doing it!
 Why I did not use the word "Monad" in this post? I want this entry to be simple and easy to understand for Haskell newcomers.
 Please DO NOT copy and paste the solution of the problem, I uploaded it here for educational purposes only.