When I found out about monads it was after I'd already implemented the same abstraction. It actually comes up quite a lot (async calls, error handling, audit logging, database access). http://m50d.github.io/2013/01/16/generic-contexts.html . Just like when I was told about "the visitor pattern", my reaction was mostly "that? I've been doing that for years. So that's what it's called".
Still, it's worth knowing the name for a concept so that you can talk to people about it, and it's certainly worth knowing the library tools that exist and can save you time. Pretty often I'll write two or three lines of code, look at it, and then realise "oh, that's just traverseM" or some such. Even if it doesn't save me any time writing, replacing three lines with one library call is a huge bonus to maintainability.
Still, it's worth knowing the name for a concept so that you can talk to people about it, and it's certainly worth knowing the library tools that exist and can save you time. Pretty often I'll write two or three lines of code, look at it, and then realise "oh, that's just traverseM" or some such. Even if it doesn't save me any time writing, replacing three lines with one library call is a huge bonus to maintainability.