Advent of Code 2025 (Haskell)

To run code for a day:

cabal run y2025d01 input/2025/day01/sample.txt

Day 1

• Haskell's scanl is handy for this sort of problem.
• I had a bunch of off-by-ones on part 2, for example if you landed exactly on zero, I'd double-count it.
• Looking at reddit posts, the cleanest solution is to replace the big turns with a bunch of L1s and R1s. Then you can reuse your code from part 1. replicate is the function for this.

Day 2

• Defining local helper functions with where is handy, and you don't need to write explicit types for them.
• You can't write 1 / 2 in Haskell, but you can write 1 div 2.
• The Haskell equivalent of str * n is concat (replicate n str).