[codemonkey-zeta]

> you trade away your ability to make structural guarantees about which functions depend on which fields

You might make this trade off using map keys like strings or keywords, but not if you use namespace qualified keywords like ::my-namespace/id, in combination with something like spec.alpha or malli, in which case you can easily make those structural guarantees in a way that is more expressive than an ordinary type system.

[bccdee]

Spec & Malli look cool. But my concern is more with something like this (reusing my earlier example):

  let mut authn = UserLoginView.build(userDataRepository);
  let session = authn.login(user, pwd);
  // vs
  let session = userLogin(userDataMap);

In the first case, we know that `login` only has access to the fields in `UserLoginView`. In the second case, `userLogin` has access to every field in `userDataMap`. It's not simple to know how changes to other facets of the user entity will bleed across into logins. With `UserLoginView`, the separation is explicit, and the exchange between the general pool of user info and the specific view of it required for handling authorization is wrapped up in one factory method.

In the first case, it makes sense to unit test logins using every conceivable variation of `UserLoginView`s. In the second case, your surface area is much larger. `userDataMap` is full of details that are irrelevant to logins, so you only test the small relevant subset of user data variations. As the code ages and changes, it becomes harder and harder to assess at a glance whether your test data really represents all the test cases you need or not.

I worry that Clojure-style maps don't fix the problems pointed out by the article. In a codebase that passes around big dumb data objects representing important entities (incrementally processing them, updating fields, etc), the logic eventually gets tangled. Every function touches a wide assortment of fields, and your test data is full of details that are probably inconsequential but you can't tell without inspecting the functions. I don't see how Clojure solves this without its own UserLoginView-style abstraction.

[codemonkey-zeta]

To be clear, there's nothing wrong with your approach, and many people implement systems exactly the way you are describing in Clojure using Records (which are Java classes).

  (defrecord UserLoginView [email password])

  ;; DIFFERENCE: compile-time validation
  (defn login [^UserLoginView view]
    (authenticate (:email view) (:password view)))

  ;; Usage
  (let [user-data {:user/email "[e-mail]"
                   :user/password-hash "hash123"
                   :user/address "123 Main St"
                   :user/purchase-history []}

        ;; DIFFERENCE: construct the intermediary data structure - ignore extra stuff explicitly
        login-view (->UserLoginView
                     (:user/email user-data)
                     (:user/password-hash user-data))]
    (login login-view))

I prefer not to work this way though. The spec-driven alternative could be:

  (require '[clojure.spec.alpha :as s])
  (require '[clojure.spec.gen.alpha :as gen])

  (s/def :user.login/email string?)
  (s/def :user.login/password-hash string?)
  (s/def :user.login/credentials
     (s/keys :req [:user.login/email ;; spec's compose
                   :user.login/password-hash]))

  (defn login [credentials]
    ;; DIFFERENCE: runtime validation
    {:pre [(s/valid? :user.login/credentials credentials)]}
    (authenticate (:user.login/email credentials)
                  (:user.login/password-hash credentials)))

  (let [user-data {:user.login/email "[e-mail]"
                   :user.login/password-hash "hash123"
                   :user/address "123 Main St"  
                   :user/purchase-history []}]
    ;; DIFFERENCE: extra data ignored implicitly
    (login user-data))

  ;; Can also pass a minimal map
  (login {:user.login/email "[e-mail]"
          :user.login/password-hash "hash123"})

  ;; or you can generate the data (only possible because spec is a runtime construct)
  (let [user-data 
        (gen/generate (s/gen :user.login/credentials)) ; evaluates to #:user.login{:email "cWC1t3", :password-hash "Ok85cHMP5Bhrd4Lzx"}
        ]
    (login user-data))

The drawbacks of Records are the same for Objects - Records couple data structure to behavior (they're Java classes with methods), while spec separates validation from data, giving you:

- generative testing (with clojure.spec.gen.alpha/generate)

  You say "it makes sense to unit test logins using every conceivable variation of `UserLoginView`", well, with spec you can actually *do that*:

  (require '[clojure.test.check.properties :as prop])
  (require '[clojure.test.check.clojure-test :refer [defspec]])

  (defspec login-always-returns-session 100
    (prop/for-all [creds (s/gen :user.login/credentials)]
      (let [result (login creds)]
        (s/valid? :user.session/token result))))

  This is impossible with Records/Objects - you can't generate arbitrary Record instances without custom generators.

- function instrumentation (with clojure.spec.test.alpha/instrument)

- automatic failure case minimization (with clojure.spec.alpha/explain + explain-data)

- data normalization / coercion (with clojure.spec.alpha/conform)

- easier refactoring - You can change specs without changing data structures

- serialization is free - maps already serialize, whereas you have to implement it with Records.

Plus you get to leverage the million other functions that already work on maps, because they are the fundamental data structure in Clojure. You just don't have to create the intermediate record, let your data be data.