`tsc-current-field` doesn't work against starting node about elisp-tree-sitter

[ubolonton]

I think it's the same issue as tree-sitter/tree-sitter#567.
Basically the cursor can't walk out of the starting node, so it can't know which field it's on within the parent node.
We definitely should document this better.

[mathrick]

I did some more reading, and it turns out I was confused about the meaning of fields. I was under the impression that fields were the properties of nodes, but in reality, they're properties of the relationship between a parent and its children. So the API really doesn't have a concept of "a node's field", only "given child's field".

[ubolonton]

Yeah, field is basically the nickname by which the parent calls the child at home.

Semi-formally, it's the (name of the) edge between the parent node and the child node.

[mathrick]

This is apparently an issue for all traversal utils, for example:

;; This will also always return nil
(defun node-field (node)
  (block nil
    (tsc-traverse-do ([field] node)
     (return field))))

[mathrick]

Hmm, is it a shortcoming in the underlying treesitter API, or is there some functionality that's present in the base API but not exposed by tree-sitter.el? Because as things are right now, there's no straightforward way to get a node's field without gymnastics involving cursors and weird edge cases (see also #274). Something like tsc-node-field sounds like it should exist.

[mathrick]

Also, I don't know whether that is a thing that even makes sense to want, but this limitation means we can never have a field on the root node of a tree.

[sogaiu]

Yes, I found this confusing as well, but IIUC, this is close to my impression of how things work (my impression is vague (^^; ):

in reality, they're properties of the relationship between a parent and its children. So the API really doesn't have a concept of "a node's field", only "given child's field".