Oops, looks like the end of my message got cut off. Appended below.
Alex Osborne wrote:
> Dmitri wrote:
> > I notice that certain sequence operations such as concat and cons will
> > not retain the original type of sequence, for example if you combine
> > two vectors together a list will be returned:
> >
> > user=> (concat [1 2] [3 4])
> > (1 2 3 4)
> >
> > is this intentional behavior, and would it not be more consistent for
> > concat to retain the original type of the data structures, when both
> > data structures that were passed in are of the same type.
>
> It's because concat returns a lazy sequence, the concatenation only
> happens when you ask for relevant elements (which has the benefit that
> it doesn't need to do any copying, saving both time and memory). If you
> want to concatenate two vectors eagerly (so returning another vector)
> you could use 'into' instead:
>
> user=> (into [1 2] [3 4])
> [1 2 3 4]
>
> > Also, why
> > does cons behave differently from conj:
> >
> > user=> (conj [1 2] 3)
> > [1 2 3]
> >
> > user=> (cons 2 [1 2])
> > (2 1 2)
>
> Because cons always creates a list (which construct at the front), while
> conj "adds" it in the natural (ie fastest) way for that collection type,
> vectors "add" at the end.
>
> user> (conj '(1 2) 3)
> (3 1 2)
> user> (conj [1 2] 3)
> [1 2 3]
> user> (conj #{1 2} 3)
> #{1 2 3}
user> (cons 3 '(1 2))
(3 1 2)
user> (cons 3 [1 2])
(3 1 2)
user> (cons 3 #{1 2})
(3 1 2)
Usually you'll want to use conj, not cons.
Note that when we do (cons 3 [1 2]) cons is making a cons cell that
looks like this:
+---+---+
| 3 | @-----> (seq [1 2])
+---+---+
It's not converting the whole vector to a list, it just adds the element
you cons'd on and points at the existing list. You can see that here:
user> (type (cons 3 [1 2]))
clojure.lang.Cons
user> (type (rest (cons 3 [1 2])))
clojure.lang.APersistentVector$Seq
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