clojure.data.json/write-str: specifying a key function for placing values into aggregate arrays

Question

Suppose I have a simple map, example-map:

(def example-map {"s" {"f" "g"} 
                  "m" {"r" "q"}})

I can use clojure.data.json/write-str to JSON-ify this map as such:

(clojure.data.json/write-str example-map) =>
"{\"s\":{\"f\":\"g\"},\"m\":{\"r\":\"q\"}}"

I'd like to conditionally place some of the values into lists according to the value of their keys.

write-str provides an optional :key-fn, which applies some function to key value pairs. For example, the desired function might specify that all values associated with entries that match "s" are placed in lists.

(clojure.data.json/write-str example-map :key-function desired-function) =>
"{\"s\":[{\"f\":\"g\"}],\"m\":{\"r\":\"q\"}}"

Does anyone know how to specify such a key function that checks for membership of a key in a set and places the values associated with members into an array rendered in the output JSON?

amalloy · Accepted Answer · 2017-04-25 19:08:05Z

Like your previous question, this is not a job for the JSON parser. You don't need to rely on write-time features of your JSON library to adjust the shape of your JSON maps. Instead, you have a fully functional Turing complete language at your disposal: Clojure! If the maps don't already look the way you want them to be output, then write a function that takes one Clojure map as input and produces a different one as output; then ask your JSON library to write the output map, without any special rules for fiddling with the output.

Now, as it happens this particular JSON library does provide an option named value-fn (not key-function as you claim) to let you modify a value in a map based on its key. So you could use that, in which case you simply need to write a function with a signature like:

(fn [k v]
  (...compute new value...))

There are many ways you could write such a function, but they are all entirely divorced from your JSON parser. If you need help writing it, mention some specific things you need help with, so you can get a clear explanation for the part of the process that is actually giving you trouble.

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