reitit/doc/advanced/format_negotiation.md

Format Negotiation

reitit uses muuntaja as a library to handle format negotiation, encoding and decoding.

In order to have access to the options below, you need the following namespaces:

(require '[muuntaja.core :as m])

Then, a muuntaja instance must be installed in the router to your coercion benefit from the customizations.

(ring/ring-handler
   (ring/router
    ;; your route definitions
    ;; ....
    :data {:coercion reitit.coercion.spec/coercion
           :muuntaja m/instance}})

Changing default parameters

The current JSON formatter used by reitit already have the option to parse keys as keyword which is a sane default in Clojure. However, if you would like to parse all the double as bigdecimal you'd need to change an option of the JSON formatter

(def new-muuntaja-instance
  (m/create
   (assoc-in
    m/default-options
    [:formats "application/json" :decoder-opts :bigdecimals]
    true)))

Now you should change the m/instance installed in the router with the new-muuntaja-instance.

You can find more options for JSON and [EDN].

Adding custom encoder

The example below is from muuntaja explaining how to add a custom encoder to parse a java.util.Date instance.

(def muuntaja-instance
  (m/create
    (assoc-in
      m/default-options
      [:formats "application/json" :encoder-opts]
      {:date-format "yyyy-MM-dd"})))

(->> {:value (java.util.Date.)}
     (m/encode m "application/json")
     slurp)
; => "{\"value\":\"2019-10-15\"}"

Adding all together

If you inspect m/default-options it's only a map, therefore you can compose your new muuntaja instance with as many options as you need it.

(def new-muuntaja
  (m/create
   (-> m/default-options
       (assoc-in [:formats "application/json" :decoder-opts :bigdecimals] true)
       (assoc-in [:formats "application/json" :encoder-opts :data-format] "yyyy-MM-dd"))))