simple-bank-account.clj

(ns bank.account
  (:require
   [clojure.spec.alpha :as s]))

(s/def ::fname string?)
(s/def ::lname string?)
(s/def ::cents integer?)

;; It's currently valid to have a negative balance.
;; Could alternatively do something like (s/and ::cents (complement neg?))
(s/def ::balance ::cents)

;; maybe use an inst here?
(s/def ::date-of-birth string?)

;; The first name is optional, the others are required.
(s/def ::account
  (s/keys :req [::lname ::balance ::date-of-birth]
          :opt [::fname]))

(s/fdef deposit
  :args (s/cat :account ::account, :quantity ::cents)
  :ret ::account)

;; This is the implementation I would use regardless of whether I used spec or not.
(defn deposit [account quantity]
  (update account ::balance + quantity))

(s/fdef withdraw-valid?
  :args (s/cat :account ::account, :quantity ::cents)
  :ret boolean?)

(defn withdraw-valid? [{::keys [balance] :as account} quantity]
  (>= balance quantity))

(s/fdef withdraw
  :args (s/cat :account ::account, :quantity ::cents)
  :ret ::account)

(defn withdraw [account quantity]
  (update account ::balance - quantity))

(comment

  ;; Usage
  (def me #::{:lname "madstap"
              :balance 100
              :date-of-birth "01-01-17"})

  (withdraw me 100) ;=> #:bank.account{,,, :balance 0 ,,,}

  (deposit me 1000) ;=> ;=> #:bank.account{,,, :balance 1100 ,,,}

  (withdraw-valid? me 1000) ;=> false

  ;; Bonus: Generate random, valid bank accounts.
  ;; NB: Needs test.check as a dependency [org.clojure/test.check "0.10.0-alpha2"]
  (clojure.pprint/pprint (s/exercise ::account))

  ([#:bank.account{:lname "", :balance -1, :date-of-birth "", :fname ""}
    #:bank.account{:lname "", :balance -1, :date-of-birth "", :fname ""}]
   [#:bank.account{:lname "", :balance 0, :date-of-birth "6", :fname ""}
    #:bank.account{:lname "", :balance 0, :date-of-birth "6", :fname ""}]
   ,,,)
  )