Let's code: Learning Haskell #1

Posted on
Learning Let's code

Introduction

Last week has been very immutable for me. I was looking into redux, Elm, React - just the ideas and trying to implement something similar for Angular.

For few years I have had a keen interest into functional programming and considered myself a good in the field. You know - using higher order functions, understand why null is evil - so code produced by me was “fairly” functional. Avoiding state, writing functions as pure as possible. During the last year I upgraded my level to good novice by learning concepts of partials functions, functors. What is the next step? Tackle the real beast - learn the haskell. Baby steps.

Exercises

Let’s start with Project Euler and go problem by problem googling haskell syntax and semantics on our way.

Problem #1

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below 1000.

Let’s go step by step.

  • take first 1000 of natural numbers [1..999]
  • filter the list to only have multiples of 3 or 5
disibleByThreeOrFive n =
    if n `mod` 3 == 0 || n `mod` 5 == 0 then
        True
    else
        False
filter disibleByThreeOrFive [1..999]

which returns a list of all numbers we need to sum

  • fold and sum them haskell foldl (\res prev -> res + prev) 0 (filter disibleByThreeOrFive [1..999]) which returns 233168. It is accepted by euler website - but the code we wrote could be improved.

First let’s get replace foldl with sum

sum (filter disibleByThreeOrFive [1..999])

Okay - those brackets are still there - good for us there is $ operator which is exactly for that - getting rid of them. Anything that comes after dollar sign will take precedence.

sum $ filter disibleByThreeOrFive [1..999]

Making it a one liner by extracting to lambda function

sum $
      filter
       (\x -> x `mod` 3 == 0
       || x `mod` 5 == 0)
       [1..999]

Summary

Remembered a little bit of haskell syntax, tackled the new editor plugin change and looking forward to doing more exercises.

Feedback

If you have any suggestions - I am eagerly waiting for feedback. https://benetis.me/posts/contact-me/