CODEWITHSUNDEEP
clojure
Sam Gibson
Sam Gibson

Reputation: 145

Clojure - Why does into behave differently with a list than a vector?

Why does into behave differently when the collection being inserted into is different? For example:

user=> (into [] [1 2 3])
[1 2 3]

So far, so good. Exactly as I would expect. However:

user=> (into () [1 2 3])
(3 2 1)

Why does this reverse the arguments? I assume it's for performance, and the items are inserted sequentially with cons? This still seems to break the abstraction to me, in two differently typed ordered collections I would expect the result to be ordered consistently.

Upvotes: 2

Views: 156

Answers (2)

omiel
omiel

Reputation: 1593

As an addendum to @MarkFisher 's answer: into is analogous to reduce conj ; the difference of behaviour of into follows from the difference in behaviour of conj :

(conj '(:b :c) :a)             ;; '(:a :b :c)      ;; prepend to lists
(conj  [:a :b] :c)             ;; [:a :b :c]       ;; append to vectors

(conj #{:a :b}    :c)          ;; #{:a :b :c}      ;; add to sets
(conj #{:a :b :c} :c)          ;; #{:a :b :c}      ;; nothing if already there

(conj {:a 1 :b 2}      [:c 3]) ;; {:a 1 :b 2 :c 3} ;; add key-value
(conj {:a 1 :b 2 :c 3} [:c 4]) ;; {:a 1 :b 2 :c 4} ;; replace value if present

EDIT: I recently used conj 's polymorphism at my advantage. Writing a palindrome generator, the same function could be used to generate both head and tail of the palindrome, simply by feeding either a vector or a list.

Upvotes: 0

Mark Fisher
Mark Fisher

Reputation: 9886

into is conj and seq together.

conj adds to lists at the beginning, and vectors at the end for efficiency so that there is no traversal of either type in order to add to it.

This is explained nicely in Programming Clojure when talking about the Collection abstraction in Clojure:

These functions (conj/seq/count/empty/=) are all polymorphic with regard to the concrete type of collection being operated upon. Said another way, each operation provides semantics consistent with the constraints of each data structure implementation.

Upvotes: 7

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