Competitive programming in Haskell challenge: Letter Optimization

Now that I’ve wrapped up my series of posts on Infinite 2D Array (challenge, part 1, part 2, part 3), I’d like to tackle another topic related to competitive programming in Haskell. I won’t tell you what the topic is quite yet—for now, I challenge you to use Haskell to solve Letter Optimiztion! Feel free to post thoughts, questions, solutions, etc. in the comments (so don’t look at the comments if you don’t want spoilers!). In an upcoming post I will discuss my solution.