> print out a haiku while numbering the lines.

Little need for monads to do this:

    main = readFile "haiku.txt" >>= putStrLn . number
      where
        number = lines . zipWith addNumber [0..] . unlines
        addNumber n l = show n ++ " " ++ l

(Untested, but should work.)

> Little need for monads to do this:

What do you think >>= is? It is the piece unique to Monads.

So your “little need for monads” example shows the opposite.

If you’re willing to take input on stdin, you can rewrite it without >>=:

    main = interact number
      where
        number = lines . zipWith addNumber [0..] . unlines
        addNumber n l = show n ++ " " ++ l

My point is that the logic itself does not require the idea of monadic sequencing — it’s relevant only for the minimal amount of surrounding IO.

> Untested

But proven correct?

https://www-cs-faculty.stanford.edu/~knuth/faq.html

Sorry, nope. I didn’t bother proving correctness for such a small toy example. (Besides, I wouldn’t know how.)

Not concerned with whether the example as such is a poor use-case.

Just seeing the mechanics is the point.