Code to solve the Advent of Code puzzles. This year, I’m using the puzzles to develop my skills in Haskell. I’m writing up a commentary on these puzzles and my solutions on my blog.

Learn you a Haskell, Introduction to Haskell 98, and Hackage are good resources.

The Cabal user guide and How I Start: Haskell are good sources of using the tools.

Toolchain

Install Ghcup following the instructions, making sure to load the updated environment with

source /home/neil/.ghcup/env

and then set the default GHC to use with ghcup set ghc 9.0.1 .

Install Haskell Language Server for Sublime Text

Creating the repository and project

Create the repository as normal: create the project in Gitolite, clone it, and insert the .gitignore and README.md files.

There’s one package per day, with the code for each package in sub-directories of the root directory.

Create the basic cabal project.

cabal init

Modify the advent-of-code21.cabal file as needed, such as updating the Cabal version and writing the common stanzas.

Creating subsequent days

Each day lives in a separate directory, with code in the src directory.

Compile with

cabal build

or

cabal build advent01

Run with

cabal run advent01

If you want to pass in additional RTS parameters, do it like this:

cabal run advent01 -- +RTS -K0 -RTS

Run interactively with

cabal repl advent01

or

stack ghci advent01:exe:advent01

if the first form is ambiguous.

Profiling

To profile, use

cabal run advent01 --enable-profiling -- +RTS -N -p -s -hT

Or, you can simplify the RTS options by adding them to a new stanza in the cabal file:

executable advent01prof
  import: common-extensions, build-directives
  main-is: advent01/Main.hs
  build-depends: text, containers, linear, array, pqueue, mtl, lens
  ghc-options:         -O2 
                       -Wall 
                       -threaded 
                       -eventlog
                       -rtsopts "-with-rtsopts=-N -p -s -hT"

Only include the -eventlog directive if you want to use Threadscope to investigate parallel behaviour.

then running

cabal run advent01prof --enable-profiling

Generate the profile graph with

hp2ps -M advent01.hp

Packages

Packages I used a lot:

• Containers (and some better documentation); Unordered containers is a mostly-equivalent alternative.
• Attoparsec (and Megaparsec, and ReadP once).

There are somewhat decent tutorials on Megaparsec and Attoparsec.

Packages I didn’t use much, but need to remember:

• Arithmoi for number theory
• Pointed List for zipper lists (sometimes circular)
• Vector for array-like things
• Linear for coordinate-vector like things
• Grid for 2-d grids
• Graph-wrapper for graphs
• Lens (and a summary of operators). I didn’t use these much this year, but did a lot last year.
• RWS (Reader-Writer-State monad stack); again, used a lot last year but not this year
• Monad loops, and a description
• Replace-Megaparsec, for using Mpc for all sorts of things traditionally done with regex substitutions.

Readme

Build this readme file wth

pandoc -s README.md > README.html

(Using the Modest style.)