There are numerous ways to redesign the function signatures, but I would imagine the simplest would be (again, idk Haskell syntax):
getConfigurationDirectories: unit -> Maybe [FilePath]
nonEmpty: [a] -> Maybe [a]
head: [a] -> Maybe a
initializeCache: FilePath -> unit
Notice `nonEmpty` isn't really necessary because `head` could to the work. The above could be chained into a single, cohesive stack of calls where the result of each is piped through the appropriate `Maybe` method into the next call in a point-free style. I cannot imagine how this wouldn't be clearer. e.g:
maybeInitialized <- (getCofigurationDirectories >>= head >> initializeCache)
That's the whole thing. Crystal clear. The big takeaway of "Parse don't validate" should be about the predominant use of the `Maybe` monad as a construct to make "parsing" as ergonomic as possible! Each function that returns `Maybe` can be understood as a "parser" that, of course, can be elegantly combined to achieve your result.
My critique is exactly that unwrapping the `Maybe` immediately in order to throw an exception is kind of the worst of both worlds. I mentioned this in a sibling comment, but my sense is that the author is more concerned with have a concrete value (`configDirs`) available in the scope of `main` than best-representing the solution to the problem in code. It is a shame because I agree with the thesis.
