On Sun, Mar 22, 2026, 6:15 PM
Just to verify I understood it correctly:
It's declaring function finalizeConfigValue, with two parameters (disregarding currying), one is a function a -> Maybe b, second is a list of SourceIndex/a tuples; result is a Maybe B.
correct! What kind of type is a? A raw parse tree as delivered from the yaml parser, i.e. a hierarchical blob of Maps, Lists, and Strings for
terminals? (I decided to configure the parser for "everything is a string" because (a) no real need for numbers in my use case and (b) possibly YAML has some weird definition of what's number and what's a string.)
in that case, it's probably either a string or a list of strings. the assumption is that you've already dispatched on config keys: the output is the final canonical value of *one* key; the input list represents potentially multiple files, repeat definitions within a file, maybe a hard default for fallback. the "sorting" would be via custom logic for your SourceIndex type, indicating which alternative to prefer if there are multiple candidates. depending on specifics, this may not be versatile enough; just read that line as "disambiguate between multiple definitions." What does listToMaybe do?
head of list with an option wrapper, so it's safe on empty lists: listToMaybe [] = Nothing listToMaybe (x:_) = Just x Not sure what the mapMaybe serves. I guess it's dealing with error cases or some such, but I can't infer what kind of cases that would be and
what the intended effect it (my lack of Haskell knowledge shows again).
it takes a function with optional return (here, the final value parser) and maps it to a list, discarding failures and unwrapping the rest. mapMaybe :: (a -> Maybe b) -> [a] -> [b] so the whole process is: - sort by source preference - parse, discarding failures - return the first successful parse if one exists (list laziness skips the rest) I believe the main block to me understanding such code is that I don't
know how many parameters a function has - or, in the currying perspective, what order a function is.
yes, the type annotation is often critical for this. it includes all those things, which the function head might skip. it's common to read *only* types on a first pass through unfamiliar code! I guess I'm going to learn a whole lot here.
Which is exactly the point, actually :-)
cheers to that! 😄