prefixmap | A prefix-enhanced map in Go

First of all it is important to understand that there is a difference between the type of a variable and the type of the value stored in that variable. In your example the actual type of the result ++ is still PrefixMap but the type of the variable (i.e. what the compilers can prove) is just mutable.Map and this is what you see in REPL. You can easily verify this by printing res2.getClass to get the actual type.

I think this happens because Map has actually two different ++ methods:

• one comes from TraversabelLike and it is a smart one with CanBuildFrom and all the other fancy stuff

• another comes from scala.collection.MapLike overridden at scala.collection.mutable.MapLike and it is much less generic

It is not clear what you mean by "inherited Map of PrefixMap". Map is a trait which if you are coming from the Java world is similar to interface. It means that Map on its own doesn't hold any value, it just specifies contract and provides some default implementation of various convenience methods via "core" methods (the ones you implement in your PrefixMap).

As to how this whole data structure works, you should imagine this PrefixMap implementation as a "tree". Logically each edge has a single char (in the prefix sequence) and each node potentially a value that corresponds to a string that is created by accumulation all chars on the way from the root to the current node.

So if you have a Map with "ab" -> 12 key-value, the tree will look something like this:

And if you add "ac" -> 123 to the tree, it will become

Finally if you add "a" -> 1 to the tree, it will become:

Important observation here is that if you take the "a" node as a root, what you'll be left with is a valid prefix tree with all strings shortened by that "a" prefix.

Physically the layout is a bit different:

1. There is the root node which is PrefixMap[T] which is Map[String,T] from the outside, and also a node for an empty string key.
2. Internal nodes which are value + suffixes i.e. optional value and merged list of children nodes with their corresponding characters on the edge into a Map[Char, PrefixMap[T]]

As you may see update implementation is effectively find something with withPrefix call and then assigning value to it. So what the withPrefix method does? Although it is implemented recursively, it might be easier to think about it in an iterative way. From this point of view, it iterates over the characters of the given String one by one and navigates through the tree creating missing nodes see