Example for consuming a list of objects

[justjoheinz]

I can succesfully consume a class person from the according json format, but how do I invoke json-to-clos if my json endpoint returns a list of person. Being fairly new to Common Lisp I do not grokk yet how to provide the according class information.

[gschjetne]

There's no API for it in json-mop. You'll have to parse your JSON response with Yason into a list of hash tables, iterate over it and calling json-to-clos with your person class on each element.

Maybe there should be an API for it. I feel bad for not maintaining this library all these years, I really need to do something about that.

[mooseyboots]

i had the same issue, and managed to implement it with a loop.

then i was reading up a little more on initializing, and discovered the initialize-instance method, which can be called with :after, and specialized on a given class. i tried to use it to iterate over a list of hash-tables in the slot of an initialized object, and call json-to-clos on each one, as suggested above.

but it doesn't work, even though the initialize-instance method is being called, and it does work when called using make-instance instead of json-to-clos.

is there some way that my initialize-instance perhaps needs to come after a similar method from this library? i see that json-mop:json-serializable-class has such a method (:around), and the yason library also does. i'm new to CLOS and so don't immediately understand the order of all the many initialize-instance methods.

or could there be something else in json-to-clos that prevents this from working?

seems like initialize-instance is a good way to go about handling a list of objects in a slot.

[kilianmh]

Consuming list / vectors can be implemented by adding these two additional json-to-clos methods

(defmethod json-to-clos ((input list) class &rest initargs)
  (declare (ignore initargs))
  (mapcar (lambda (element)
            (json-to-clos element class))
          input))

(defmethod json-to-clos ((input vector) class &rest initargs)
  (declare (ignore initargs))
  (coerce (json-to-clos (coerce input 'list) class)
          'vector))

Then we can parse an list object with json-to-clos the same way as one object.

What do you think @gschjetne ?

[gschjetne]

My CL is a bit rusty, but could we specialize on sequence instead, and kill two birds with one stone?

[kilianmh]

Sure we can also do it with a method specialized on sequence, such as:

(defmethod json-to-clos ((input sequence) class &rest initargs)
  (declare (ignore initargs))
  (etypecase input
    (list
     (mapcar (lambda (element)
               (json-to-clos element class))
             input))
    (vector
     (map 'vector
          (lambda (element)
            (json-to-clos element class))
          input))))