filterM' f xs = map fst . filter snd . zip xs <$> mapM f xs
nexist n xs = take n <$> filterM (fmap not . doesFileExist) xs
notes = cycle ["c", "c#", "d", "d#", "e", "f", "f#", "g", "g#", "a", "a#", "b"]
majorSteps = [2, 2, 1, 2, 2, 2] :: [Int]
scale note = map (dropWhile (/= note) notes !!) . scanl (+) 0
scale note = map head . scanl (flip drop) (dropWhile (/= note) notes)
λ> scale "d" majorSteps
["d","e","f#","g","a","b","c#"]
Some thoughts and responses to what you've said:
1. I try to make sure anything I enter into my notes are intended to last, filtering/processing is a key step to that. This often means I won't enter notes unless they can contribute to my understanding of some lasting higher level concept. For example, I don't have any notes on tech library X, Y, Z. But I do have notes on concepts, like "Required accidental complexity should be avoided or separated" and if a library Z has a unique way of doing so, I may incorporate my understanding of the strategy to further my understanding of the concept.
2. My goal is less to simply collect links/articles, it's to process and assimilate. I write everything in my own words to clarify my understanding. I link it to other topics in my notes to help build stronger associations, which helps me ground and recall the knowledge. I'm also exploring whether I want to put some spaced repetition in with my notes to help keep pieces of information fresh.
3. I treat my notes as separate from my todo lists. The extra bit that these tools give you is the linking and clustering of information. I find the associations useful for encouraging identification/contextualization of where that knowledge sits in my current understanding and, periodically, discovering serendipitous connections.
4. I rarely revisit old books as well, but I also find that my recall of those books are terrible. Part of my note taking is to make sure that I have a better chance of understanding and taking away valuable information from those books. It's also worth mentioning that that I don't aim to have notes on the book, I aim to have new concept notes or updating existing concepts with new information.
So what domains do I find these apps useful? It's unclear to me how much I can attribute to the linking the app encourages. But, I do find myself recalling what I've noted much more frequently in my day to day as an software engineer. The identification and concretization of patterns/concepts has helped me be more articulate and have more clarity in my work, particularly when giving feedback to others.
All that said, everybody is different. This process might not work for you, or you may find you don't need it to get the same effects. I know plenty of successful people that don't. And if it doesn't work for you, I wouldn't worry too much about it.