Advent Of Code 2020 - in Haskell
A few months ago, I was introduced to the Advent Of Code contest series. The 2020 round has just begun, and looking at the problems from previous years, I felt it would be fun and challenging to program them in Haskell. And so I got to work and got 40 out of 50 stars at the end. This was incredibly fun, and surprisingly difficult at certain stages, and I want to share my thoughts with others.
I have put up my solutions on GitHub.
§ Elegant Solutions
It is really amazing how elegant Haskell solutions look, usually with very less effort.
-- Given list of ints `xs`, find the product of some triple (x, y, z) of elements of `xs` that sum to 2020.
solve :: [Int] -> Int
solve xs = head [x * y * z | x <- xs, y <- xs, z <- xs, x + y + z == 2020]
Though one might dismiss this as some syntactic sugar, it is much more than that. List comprehensions in Haskell are a specific case of Monad comprehensions, which are an important aspect of Haskell’s pure design.
§ Text Parsing
Lexing and parsing text is a big pain, and providing easy-to-use libraries for this is very difficult. I have previously used Flex and GNU Bison for implementing a compiler, and it is a nightmare to use them.
Haskell makes life simpler (in some cases) by providing parser combinators as part of the Text.Parsec package. This helps write very beautiful monadic parsing code, which quite closely resembles imperative code, while still being pure.
Here is a snippet from the solution for Day 13. This parses input of the form mxmxvkd kfcds sqjhc nhms (contains dairy, fish).
type Ingredient = String
type Allergen = String
type Dish = ([Ingredient], [Allergen])
type Input = [Dish]
mkDish :: String -> Dish
mkDish = either undefined id . parse parseDish ""
where
parseDish = (,) <$> fil parseIngredients <*> fil parseAllergens
parseIngredients = parseWord `sepBy1` space
parseAllergens =
string "(contains " *> (parseWord `sepBy1` string ", ") <* char ')'
parseWord = many $ choice [char c | c <- ['a' .. 'z']]
fil = fmap (filter (not . null))
§ More
There are a lot more interesting patterns one can observe in the solutions/implementation. It has been a while since I did the contest, so I may have forgotten some things. I will update this post later if I remember.
