what is the consensus on using local data classes? yay or nay?

# getting-started

[vineethraj49]

what is the consensus on using local data classes? yay or nay?

[nanodeath]

local, as in...defined inside a method? hm...I don't think I've ever done that

[Matteo Mirk]

never done that… I can’t see the advantage at the moment

[vineethraj49]

well; it beats Map<Triple<String, String, String>, T> in my case so blob shrug

[Matteo Mirk]

of course, a custom type always beats primitive obsession and complex collections combination, just I can’t see why declaring one local, as “inside a method”…is that what you mean?

[vineethraj49]

hmm

[Matteo Mirk]

In general I think it would be cleaner to declare it as top-level in a file, you can even make it private if its scope is restricted to only classes/functions inside the file

[vineethraj49]

so I had this case where I had a collection<T> where T had 10+ attributes; but each T is uniquely identified by T.a, T.b, T.c

and a stupid bug I caused was due to me doing

val map = collection.associateBy { Triple(it.a, it.b, it.c) }

and then doing map.keys.filter { it.first == x } instead of `map.keys.filter { it.second == x }`;

simplest way to avoid that seemed to be; in the context of the function;

val map = collection.associateBy { K(it.a, it.b, it.c) }
and then map.keys.filter { it.b == x }

and given K was only needed in the context of said function, didn't need to be pulled out or exposed (even with internal)

what I'm not sure of; if this is a "recommended" pattern

[Matteo Mirk]

If you pull it up and mark it private it won’t be exposed anywhere. If I were to read code with local data types it would look less readable to me, but I guess it’a a matter of taste. There are no guidelines on this that I know of.

[mkrussel]

I would be fine with a local data class as long as the use for it seemed very specific to that function.

I've never done it before, and I never used local classes in Java either. I always wind up feeling it is better to pull it up