immutability - How to initialize and "modify" a cyclic persistent data structure in Scala?

Question

I have searched and found some info on this topic but the answers are either confusing or not applicable.

I have something like this:

class Thing (val name:String, val refs:IndexedSeq[Ref])
class Ref (val name:String, val thing:Thing)

Now, I want to say, load in a file, parse it and populate this data structure from it. It being immutable and cyclic, how might one do so?

Also, let's say I do get this data structure populated, now I want to modify it, like change rootThing.refs(3).name, how might one do that?

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Thanks for the ideas posted here. At this point, I'm thinking that if one really wants persistent data structures for something like this, to think outside the box and consider what questions client code will need to ask. So instead of thinking of objects and fields, think of queries, indexes and such. To start with, I'm thinking in terms of: Is there a bidirectional multimap persistent data structure?

Answer 1

You can initialize a cyclic data structure of this form if you're prepared to modify it to introduce a degree of laziness,

scala> class Thing (val name:String, refs0: => IndexedSeq[Ref]) { lazy val refs = refs0 } ; class Ref (val name:String, thing0: => Thing) { lazy val thing = thing0 }
defined class Thing
defined class Ref

scala> val names = Vector("foo", "bar", "baz")                                                                                                                       
names: scala.collection.immutable.Vector[java.lang.String] = Vector(foo, bar, baz)

scala> val rootThing : Thing = new Thing("root", names.map { new Ref(_, rootThing) })
rootThing: Thing = Thing@1f7dab1

scala> rootThing.refs(1).name
res0: String = bar

However, you can't make it persistent: being cyclic, any change is visible via every element of the structure, so there are no opportunities for sharing between versions.