Re: Parse a field in JSON given a path to the field

[2QdxY4RzWzUUiLuE]

On 2022-01-26 at 22:05:49 +0000,
Tim Landscheidt <tim@tim-landscheidt.de> wrote:

> Husain Alshehhi <husain@alshehhi.io> wrote:
> 
> > Does emacs provide a function that can return a path from a JSON object? I 
> > find something like this useful if I want a field from a nested, large 
> > JSON. Here is an example of a JSON string
> 
> > ,----
> > | {
> > | "field" : {
> > |   "field1" : {
> > |     "field2" : {
> > |       "field3" : "value"
> > |     }
> > |   }
> > | }
> > `----
> 
> > I do something like:
> 
> > ,----
> > | ;; assume that the value is in json-string
> > | (let ((json (json-parse json-string))
> > |       (field (gethash "field" json))
> > |       (field1 (gethash "field1" field))
> > |       (field2 (gethash "field2" field1))
> > |       (field3 (gethash "field3" field2)))
> > |   ;; process field3
> > |   )
> > `----
> 
> > I wonder if there is something like
> 
> > ,----
> > | (let ((field3 (json-parse-path "field/field1/field2/field3" json-string)))
> > |   ;; process field3
> > |   )
> > `----
> 
> If the path is fixed, you could also use let-alist:
> 
> | ELISP> (let ((json (json-parse-string "{\n  \"field\": {\n    \"field1\": 
> {\n      \"field2\": {\n        \"field3\": \"value\"\n      }\n    }\n  
> }\n}\n"
> |                                       :object-type 'alist)))
> |          (let-alist json
> |            (message "field/field1/field2/field3 = %S" 
> .field.field1.field2.field3)))
> | "field/field1/field2/field3 = \"value\""
> | ELISP>

I wrote this in Common Lisp; it should work (but I haven't tested it) in
Elisp:

    (defun json-drill (json keys)
      (let ((value json))
        (dolist (key keys value)
          (setf value (gethash key value)))))

The keys argument is a list of keys rather than a single string with the
keys separated by slashes.  Your example would look like this:

    (let ((json-data (json-parse json-string)))
      (let ((field3 (json-drill json '("field" "field1" "field2" "field3"))))
        ;; process field3
        ))

Obviously, whatever you pass to json-drill as keys doesn't have to be a
constant.