Just use first and second for both cases.
On Nov 11, 9:52 am, samppi <rbysam...@gmail.com> wrote:
> Clojure 1.1.0-alpha-SNAPSHOT
> user=> (conj (first {1 2}) 3)
> [1 2 3]
> user=> (conj {1 2} [2 5])
> {2 5, 1 2}
> user=> (key (first {1 2}))
> 1
> user=> (key [1 2])
> java.lang.ClassCastException: clojure.lang.PersistentVector cannot be
> cast to java.util.Map$Entry (NO_SOURCE_FILE:0)
>
> In all respects but one, two-sized vectors act like map entries, and
> map entries act like two-sized vectors. This single exception is the
> exception that the key and val forms throw when called on two-sized
> vectors.
>
> Almost one year ago, when I was completely new at Clojure, I asked
> about creating map entries (http://groups.google.com/group/clojure/
> browse_thread/thread/15572b6c7269096e/ab113323410dc39e). Rich Hickey
> made it clear, though, that while he wants key and val to be used on
> map entries, he does not want map entries to be created directly by
> the user, and that map entries could be removed at any time.
>
> I presumed in the past year that he wanted us to use vectors whenever
> we wanted to create map entries, since he designed Clojure so that map
> entries are equivalent to vector pairs and vice versa. But in one
> aspect, this does not work. Let me give an example—processing pairs:
>
> user=> (defn a [pairs]
> (map #(vector (key %) (inc (val %))) pairs))
> #'user/a
> user=> (defn b [pairs]
> (map #(vector (key %) (* (val %) 2)) pairs))
> #'user/b
> user=> (a {:a 3, :b 2})
> ([:a 4] [:b 3])
> user=> (b {:a 3, :b 2})
> ([:a 6] [:b 4])
> user=> ((comp a b) {:a 3, :b 2})
> java.lang.ClassCastException: clojure.lang.PersistentVector cannot be
> cast to java.util.Map$Entry
>
> If we are not to create map entries directly, but use vector pairs
> instead, then I would like vector pairs to act more like map entries:
> the key and val functions should work on vectors too. This would be
> elegant and complete the equivalence between map entries and vector
> pairs. If this should not be, then what would be the rationale?
> Otherwise, the user should be able to just create map entries instead,
> since Clojure 1.0 definitely has map entries now.
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