Wiki.haskell.org

Parsing the Command Line

lists a couple of libraries. and are the most popular options. Going with as it’s slightly more popular.

{-#  OPTIONS_GHC -Wall  #-}

module AoC2021Args (Args(..), aocArgParser) where

import Options.Applicative
import Data.Semigroup ((<>))

data Args = Args
    { day :: Int }

aocArgParser :: Parser Args
aocArgParser = Args
    <$> option auto
        ( long "day"
       <> help "The day to run code for. Use '0' to run all of the solutions."
       <> showDefault
       <> value 0
       <> metavar "DAY") --  The metavariable is displayed in the help text.

A type a is a Semigroup if it provides an associative function (<>) that lets you combine any two values of type a into one, and the following holds:

Day 8 - Advent of Code 2021. Eric Wastl. adventofcode.com . Accessed Mar 7, 2022.

Part I Description

You barely reach the safety of the cave when the whale smashes into the cave mouth, collapsing it. Sensors indicate another exit to this cave at a much greater depth, so you have no choice but to press on.

As your submarine slowly makes its way through the cave system, you notice that the four-digit seven-segment displays in your submarine are malfunctioning; they must have been damaged during the escape. You’ll be in a lot of trouble without them, so you’d better figure out what’s wrong.

Day 6 - Advent of Code 2021. Eric Wastl. adventofcode.com . Accessed Mar 2, 2022.

Part I Description

The sea floor is getting steeper. Maybe the sleigh keys got carried this way?

A massive school of glowing lanternfish swims past. They must spawn quickly to reach such large numbers - maybe exponentially quickly? You should model their growth to be sure.

Day 3 - Advent of Code 2021. Eric Wastl. adventofcode.com . Accessed Feb 23, 2022.

Problem Description

Part One

The submarine has been making some odd creaking noises, so you ask it to produce a diagnostic report just in case.

The diagnostic report (your puzzle input) consists of a list of binary numbers which, when decoded properly, can tell you many useful things about the conditions of the submarine. The first parameter to check is the power consumption.

This page contains remarks on Haskell that I encountered when working with source files that span multiple AoC 2021 problems.

and have Haskell solutions. It’ll be nice to compare how they solved the problems. I don’t want to end up perfecting the wrong approach!

Setting Up Haskell Env for AoC

To manage dependencies, Cabal and Stack are pretty popular. Stack incorporates the Cabal build system.

Problem Statement

Let \(d(n)\) be defined as the sum of proper divisors of \(n\) (numbers less than \(n\) which divide evenly into \(n\)).

If \(d(a) = b\) and \(d(b) = a\), where \(a \neq b\), then \(a\) and \(b\) are an amicable pair and each of \(a\) and \(b\) are called amicable numbers.

For example, the proper divisors of \(220\) are \(1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110\); therefore \(d(220) = 284\). The proper divisors of \(284\) are \(1, 2, 4, 71, 142\); so \(d(284) = 220\).