Hi Mark, Am 21.12.2008 um 16:51 schrieb Mark Volkmann:
Until this morning I was under the impression that ALL Clojure collections are sequences. Now I understand (from the screencast "Clojure Data Structures - Part 2") that vectors and maps are not and that you need to call "seq" on them to get a sequence representation. What are some situations where this is necessary? I'm confused because the following code works.
It is important to understand the difference between a collection and a seq. Collections are specific implementations of a container. Maybe a vector, maybe a map, ... A seq on the other hand is an abstract view on such a collection. (In particular a seq is not a collection.) Note that a list is still a collection, which just happens to implement the contract of a seq. So it can act as it's own seq.
(def my-map {:a 1 :b 2 :c 3})
(first my-map) -> [:b 2]
(rest my-map} -> ([:c 3] [:a 1])
Theoretically the calls would need a seq for the map. (first (seq my-map)) However, this is very hard on the user. He always has to take care, that he passes a seq. But since a seq is also it's own seq, ie. (seq a-seq) == a-seq, we can change the interface from "takes a seq" to "takes a sequable thing" and call seq on that thing in the function itself. Then we suddenly save a lot of seq calls on the user side. Also being more convenient for the user. Hope this helps. Sincerely Meikel
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