When brute-force doesn't work for part 1, but you end up getting part 2 for free.

Graph search, that should have been much quicker to implement if I hadn't found a strange issue in a library I was using.

A simple virtual machine, with reverse engineering I avoided.

Trying a Search.Algorithms library, then having to resort to hand-building search.

Using Maybe to say if blocks can be moved, and the monad instance to simplify the plumbing.

A "part 2" that I didn't see coming!

Linear algebra FTW.

Reusing a classic data structure.

The return of Lanternfish.

I guessed wrong for part 2, so vastly over-complicated part 1.

A change of representation needed.

Actual proper vector addition!

Surprisingly simple application of fold and concatMap

The first real need for optimisation.

Arrays, refactoring, and point-free style.

Taming parsing, and folding the results.

Mostly writing down the problem as predicate definitions.

Back to Advent of Code, with a problem that fits Haskell well.