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Closer to Clojure 02: Collection

Collection of Clojure has linear list, tree, map (hash table), set, and vector type. You cannot do destructive assignment or modification to the collection. Instead, Clojure creates a new copy for you, however, the contents of the collection are shared with the original one and the copied one.

user=> (let [my-vector [1 2 3 4]
    my-map {:fred "ethel"}
    my-list (list 4 3 2 1)]
  (list
    (conj my-vector 5)
    (assoc my-map :ricky "lucy")
    (conj my-list 5)
    my-vector
    my-map
    my-list))
([1 2 3 4 5] {:ricky "lucy", :fred "ethel"} (5 4 3 2 1) [1 2 3 4] {:fred "ethel"} (4 3 2 1))

Vector is as same as #(a b c ...) of Scheme but not std::vector of C++.

There is a map function.

user=> (map #(+ % 3) [2 4 7])
[5 7 10]

Any data can have metadata.

user=> (def v [1 2 3])
user=> (def attributed-v (with-meta v {:source :trusted}))
user=> (:source (meta attributed-v))
:trusted
user=> (= v attributed-v)
true

The collection has common interface as Java does. Let's try first and rest, which are parts of seq interface.

user=> (let [my-vector [1 2 3 4]
    my-map {:fred "ethel" :ricky "lucy"}
    my-list (list 4 3 2 1)]
  [(first my-vector)
    (rest my-vector)
    (keys my-map)
    (vals my-map)
    (first my-list)
    (rest my-list)])
[1 (2 3 4) (:ricky :fred) ("lucy" "ethel") 4 (3 2 1)]

The collection who has seq interface is called sequence.

References:
http://clojure.org/functional_programming
http://java.ociweb.com/mark/clojure/article.html
コレクションには線形リスト，木の他に，マップ（ハッシュテーブル），集合，ベクタがあります．コレクションに対する破壊的代入，変更はできません．代わりに新しいコレクションが作られますが，新しいコレクションと古いコレクションはデータを共有します．

user=> (let [my-vector [1 2 3 4]
    my-map {:fred "ethel"}
    my-list (list 4 3 2 1)]
  (list
    (conj my-vector 5)
    (assoc my-map :ricky "lucy")
    (conj my-list 5)
    my-vector
    my-map
    my-list))
([1 2 3 4 5] {:ricky "lucy", :fred "ethel"} (5 4 3 2 1) [1 2 3 4] {:fred "ethel"} (4 3 2 1))

ベクタはSchemeで言うベクタ #(a b c ...) のことであり，C++のstd::vectorとは違う概念です．

map関数があります．

user=> (map #(+ % 3) [2 4 7])
[5 7 10]

すべてのデータはメタデータを持つことができます．

user=> (def v [1 2 3])
user=> (def attributed-v (with-meta v {:source :trusted}))
user=> (:source (meta attributed-v))
:trusted
user=> (= v attributed-v)
true

コレクションはJavaのように共通のインタフェースを持ちます．seqインタフェースのfirstとrestを使ってみましょう．

user=> (let [my-vector [1 2 3 4]
    my-map {:fred "ethel" :ricky "lucy"}
    my-list (list 4 3 2 1)]
  [(first my-vector)
    (rest my-vector)
    (keys my-map)
    (vals my-map)
    (first my-list)
    (rest my-list)])
[1 (2 3 4) (:ricky :fred) ("lucy" "ethel") 4 (3 2 1)]

seqインタフェースを持つコレクションをシーケンスと呼びます．

参考
http://clojure.org/functional_programming
http://java.ociweb.com/mark/clojure/article.html
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