;; Destructuring Clojure's Maps ;; I can never ever remember how this works, so here is a note to self: ((fn [{a :a}] a) {:a 1}) ; 1 ;; And by let-lambda isomorphism (let [{a :a} {:a 1}] a) ; 1 ;; Why on earth is the syntax the wrong way round? Why can't {:a a} match {:a 1}? ;; Similarly ((fn [{a :a b :b}] [a b]) {:a 1 :b 2}) ; [1 2] (let [{a :a b :b} {:a 1 :b 2}] [a b]) ; ; [1 2] ;; And with the common pattern where the variables are like the keys: ((fn [{:keys [a b]}] [a b]) {:a 1 :b 2}) ; [1 2] (let [{:keys [a b]} {:a 1 :b 2}] [ a b ]) ; [1 2] ;; We can destructure recursively (although we may not be wise to if we keep forgetting how it works!) ((fn [{a :a {c :c d :d} :b}] [a c d]) {:a 1 :b {:c 2 :d 3}}) ; [1 2 3] (let [{a :a {c :c d :d} :b} {:a 1 :b {:c 2 :d 3}}] [a c d]) ; [1 2 3] ;; And we can remember the keys entire on which we have recursed, so: (let [{a :a {c :c d :d :as b} :b} {:a 1 :b {:c 2 :d 3}}] [a b c d]) ;-> [1 {:c 2, :d 3} 2 3] ;; Finally a 'real' example, a ring request map containing parameters and a session, both of ;; which have substructure (def ring-request {:params {:action "a" :key "k" :spurious "sp"} :session {:data "d" :state "s"} :irrelevant "irr"}) ;; So the parameters we're interested in look like {:params {:action :key} :session {:data :state}} ;; And we can extract all the pieces, naming each part, like so: (defn process-request [{{action :action key :key :as params } :params {data :data state :state :as session} :session :as request}] (println action) (println key) (println data) (println state) (println params) (println session) (println request)) (process-request ring-request) ;; a ;; k ;; d ;; s ;; {:key k, :action a, :spurious sp} ;; {:state s, :data d} ;; {:irrelevant irr, :params {:key k, :action a, :spurious sp}, :session {:state s, :data d}}

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## Sunday, February 15, 2015

### Destructuring Clojure Maps

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