Protocols - clarify access to parameters

[raymcdermott]

There are various ways to define and access properties defined with protocols and data types which are non-obvious and not explained or clarified in the Protocols or Data Types guides

I propose updating the Protocol guide with this examples:

Names and values when using defrecord directly

(defprotocol DeviceRegistration
  "Protocol to register various IOT devices"
  (register [device])
  (ping [device]))

(defrecord RaspberryPi [device-id location registry-id]
  DeviceRegistration
  (register [_]
    (->RaspberryPi device-id location 789))

  (ping [_]
    (= 789 registry-id)))

register and ping ignore the method parameters. Instead they uses the properties directly from the record.

Sample Usage

(def rasp-pi (map->RaspberryPi {:device-id 123 :location {:x 123.4 :y 432.1}}))
;=> #'practice1.core/rasp-pi

(def registered-pi (register rasp-pi))
;=> #'practice1.core/registered-pi

(println (type registered-pi))
;practice1.core.RaspberryPi

(def pinged? (ping registered-pi))
;=> #'practice1.core/pinged?

(println pinged?)
; true

Note: Diligent readers will have observed that we can only ever register or ping one RaspberryPi using this code. It's good enough for the example but maybe not production quality.

Names and values when using extend-protocol

(defrecord Arduino [device-id location registry-id])

(extend-protocol DeviceRegistration
  Arduino
  (register [device]
    (assoc device :registry-id 890))

  (ping [device]
    (= 890 (:registry-id device))))

register and ping do not have access to the record directly. They access record properties from the method parameter, which is the Arduino record.

register creates a new Arduino record via assoc. In this case it is simpler than creating a new Arduino record using either ->Arduino or map->Arduino functions supplied by defrecord.

Sample Usage

(def arduino (map->Arduino {:device-id 345 :location {:x 432.1 :y 987.6}}))
;=> #'practice1.core/arduino

(def registered-arduino (register arduino))
;=> #'practice1.core/registered-arduino

(println (type registered-arduino))
;practice1.core.Arduino

(def pinged? (ping registered-arduino))
;=> #'practice1.core/pinged?

(println pinged?)
;true

Code in the wild uses this

Most code I see reads more like, ahem, this:

(extend-protocol DeviceRegistration
  Arduino
  (register [this]
    (assoc this :registry-id 890))

  (ping [this]
    (= 890 (:registry-id this))))

In some languages there is a keyword this or self and it has a special meaning. It kind of does in these cases too - even though the symbol itself is not special in Clojure. Rather than use conventionally privileged words, I prefer the more descriptive option.

Obligatory Clojure documentation rant

Following on from the this mini-rant, I am not a fan of x y z and foo and bar as example parameters or method calls.

Documentation comes to life when it is illustrated with simple and easily understood examples rather than abstract characters and meaningless phrases.

I know 'real things' come and go but these guides are not written to see the heat death of the universe. Are they?

[puredanger]

Apologies on the tragically long period of time it has taken for me to respond to this. When you say "Protocols or Data Types guides", do you mean the Protocols or Data Types reference pages? Or are you referring to stuff proposed in PRs? In any case, these look like good examples and it would super awesome to see them in PR form.