Malli customer transformers

[jmglov]

Hi all!

I’m using Malli to decode and validate some data containing datetimes, and I’m a bit stuck trying to figure out how to write a customer transformer to deal with them. I’m using tick to deal with the datetimes themselves.

Here’s what I’ve got so far:

(require '[malli.core :as m]
         '[malli.transform :as mt]
         '[tick.core :as t])

;; Malli transformer for datetime strings; see
;; https://github.com/metosin/malli/issues/49#issuecomment-763957635
(def date-time-transformer
  (mt/transformer
   {:encoders {:date-time {:compile (fn [_ _] {:enter str})}}}
   {:decoders {:date-time {:compile (fn [_ _] {:enter t/date-time})}}}))

(def schema-registry
  {:registry {:date-time (m/-simple-schema {:pred t/date-time?
                                            :type :date-time})}})

(m/decode :date-time
          "2021-03-29T10:00:00"
          schema-registry
          date-time-transformer)
;; => #time/date-time "2021-03-29T10:00"

(m/encode :date-time
          #time/date-time "2021-03-29T10:00:00"
          schema-registry
          date-time-transformer)
;; => "2021-03-29T10:00"

All good! But when I try sticking my datetime in a vector:

(m/decode [:catn [:timestamp :date-time]]
          ["2021-03-29T10:00:00"]
          schema-registry
          date-time-transformer)

I get an exception:

1. Unhandled clojure.lang.ExceptionInfo
   :malli.core/invalid-schema {:schema :catn}
   {:type :malli.core/invalid-schema,
    :message :malli.core/invalid-schema,
    :data {:schema :catn}}

My guess is one of three things is going wrong:

1. My transformer needs to handle stuff that isn’t :date-time
2. My custom registry also needs to be merged with the default registry instead of replacing it
3. Something else

If someone can help nudge me in the right direction and/or point me to some documentation of how to write a custom transformer, I’d be really grateful!

[jmglov]

It turns out that my error was one of the things I explained to myself during the course of writing this, my first ClojureVerse post (though given the out I gave myself with hypothesis 3, I suppose there was no way for me to be wrong per se).

My custom registry replaced the default one, so it had no way to handle anything but :date-time. Merging it with the default schemas instead did the trick:

(def schema-registry
  {:registry (merge (m/default-schemas)
                    {:date-time (m/-simple-schema {:pred t/date-time?
                                                   :type :date-time})})})

(m/decode [:catn [:timestamp :date-time]]
          ["2021-03-29T10:00:00"]
          schema-registry
          date-time-transformer)
;; => [#time/date-time "2021-03-29T10:00"]

(m/decode [:catn [:timestamp :date-time] [:weight :int]]
          ["2021-03-29T10:00:00" "27"]
          schema-registry
          (mt/transformer date-time-transformer mt/string-transformer))
;; => [#time/date-time "2021-03-29T10:00" 27]

I would still love a pointer to any docs explaining how to write custom transformers, though! I can kinda muddle through, but I’d prefer to read something that helps me understand the best way to do things.