Hi all,

I have some design questions - not for Haskell but for the Java+Vavr
combo, but I am about to dip my toes into actual work in a functional
manner, as far as that is possible, and I thought I might as well go for
the community that's best subscribed to a purely functional style.

Bird's eye view:

Java everybody knows (I don't like it either, no worries), Vavr is a -
to my eyes - pretty nice functional library, https://docs.vavr.io/ for
details if you're actually interested.
I want to leverage Vavr to get a Java program to be as Haskell-ish as is
reasonable; that's not going to be much by a Haskeller's standards,
but... baby steps.

Application is a simple command-line thing: Read configuration from
command line and configuration file, emit diagnostics about any errors
in the config, then process.
I want the configuration processing to be as functional as reasonable,
given the constraints. However, I'm undecides about many things, no
doubt because I simply don't know the best design patterns, and it's
frustrating to see multiple options and not knowing which ones will
paint me into a corner and which ones will not.

Things I'm undecided about:

a) Data type variations
During the configuration phase, I need to carry information about where
some configuration item came from (its ("context", usually file, line
number, column number).
In the processing phase, configuration is considered final and
error-free, so context is not needed (that's a done design decision).
I could carry context information into the processing phase, but it's
going to be awkward: Say, we have the following types (forgive the most
un-Haskellish pseudo syntax but I don't dare to use Haskell style
because I'd almost certainly get that wrong and provoke misunderstandings)
   DirectoryConfig {
     ConfigData<Path> path
     ConfigData<String> title
     ...
   }
   data ConfigData<a> {
     String fileName
     int lineNumber
     int columnNumber
     a value
   }
but in the processing phase I don't want my config objects polluted with
context, so I want
   DirectoryConfig {
     Path path
     String title
     ...
   }
No idea how to deal with that. I'd use code generation in Java I guess,
but that's horribly inelegant and complicated to set up (no, I don't
particularly like Java, it's just what I'm currently using).
So... how would one do such a thing in a functional language?
Not necessarily Haskell, I guess some language extensions exist for that
kind of stuff, but I'm more-or-less tied to Java + functional libraries,
so I'll have to stick with the more basic approaches most likely.
Besides, even if I did Haskell, I'd want to avoid the advanced stuff
until I get confident in the basics.

TL;DR: I have a deeply nested configuration data structure where each
field has a "context", i.e. the place it came from; how to I make it so
that the context is available during configuration evaluation but is
unavailable in the later processing phase?

I hope this is understandable; it's really hard to do that when you
don't even know enough to ask the questions precisely enough.

Regards,
Jo
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