Modify code to use a generic underline json value

[MeirShpilraien]

Today the code assumes the underline json structure is serde_json::Value. We have a situation where this is not the case and we need to work with a custom structure. We still want to use this library with our structure. I was wondering if you will accept a PR that changes the code to work with any value that implement a defined trait, this way it can be use with serde_json::Value but with other structures as well. If you agree we can use this issue to define the details and I will be happy to contribute the PR.

[freestrings]

I have a question. What do you mean by custom struct?

[MeirShpilraien]

I mean that the json structure (in memory) is not a serde_json::Value. Instead, we use our own structure (specifically in our use-case it's even C implemented). Imagine for example that you want some extra metadata per node in the json structure, you can not use serde_json::Value but it is easy to create your own struct and just implement Serialize and Deserialize to be able to use it with serde_json for serialization and deserialization. But now I also want to be able to perform a jsonpath query on this struct and this is why I open this issue :).

I imagine we can define a trait such that each object that wants to be jsonpath "compatible" will need to implement this trait (for sure the implementation for serde_json::Value will come built-in). The trait can define functions like IterValues for array iteration or IterItems for object iteration, it will define a function to get the type of the json object and for sure get primitive values (strings, number, boolean, ...).

For existing users, the API will stay exactly the same, but it will open a door for more users to be able to use this library.

WDYT?

[freestrings]

I didn't understand exactly. Can I understand as below?

let v = TestStruct { ... };
let result = jsonpath::select(&v, "$.name").unwrap();

[MeirShpilraien]

Yes, exactly.

[freestrings]

That's a good idea. Thanks in advance for the PR. 👍

[mattmart3]

Maybe this trait could be useful to add (and retrieve) other metadata to the json node structure, such as the reference to the parent node or to the key string of the node. For instance I submitted a PR (#84) that implements the parent operator by the mean of a HashMap to relate each node to its parent. Of course by adding the parent reference to a more generic Node struct the implementation would be much more clean and efficient. Similarly, to implement the key operator (issue #69), one could add the key string reference to each node.
What do you think?