Question

So I’m trying to teach myself Haskell. I am currently on the 11th chapter of Learn You a Haskell for Great Good and am doing the 99 Haskell Problems as well as the Project Euler Problems.

Things are going alright, but I find myself constantly doing something whenever I need to keep track of “variables”. I just create another function that accepts those “variables” as parameters and recursively feed it different values depending on the situation. To illustrate with an example, here’s my solution to Problem 7 of Project Euler, Find the 10001st prime:

answer :: Integer
answer = nthPrime 10001

nthPrime :: Integer -> Integer
nthPrime n
  | n < 1 = -1
  | otherwise = nthPrime' n 1 2 []


nthPrime' :: Integer -> Integer -> Integer -> [Integer] -> Integer
nthPrime' n currentIndex possiblePrime previousPrimes
  | isFactorOfAnyInThisList possiblePrime previousPrimes = nthPrime' n currentIndex theNextPossiblePrime previousPrimes
  | otherwise = 
    if currentIndex == n 
      then possiblePrime 
      else nthPrime' n currentIndexPlusOne theNextPossiblePrime previousPrimesPlusCurrentPrime
        where currentIndexPlusOne = currentIndex + 1
              theNextPossiblePrime = nextPossiblePrime possiblePrime
              previousPrimesPlusCurrentPrime = possiblePrime : previousPrimes

I think you get the idea. Let’s also just ignore the fact that this solution can be made to be more efficient, I’m aware of this.

So my question is kind of a two-part question. First, am I going about Haskell all wrong? Am I stuck in the imperative programming mindset and not embracing Haskell as I should? And if so, as I feel I am, how do avoid this? Is there a book or source you can point me to that might help me think more Haskell-like?

Your help is much appreciated,

-Asaf

Answer 1

It’s ok to have an imperative mindset at first. With time you will get more used to things and start seeing the places where you can have more functional programs. Practice makes perfect.

As for working with mutable variables you can kind of keep them for now if you follow the rule of thumb of converting variables into function parameters and iteration into tail recursion.