To preface, I am on Windows 7 (64-bit), running Java version 6 (update 33) using clooj as my IDE. I have not tried to reproduce my problem in any other system. I am experienced with Clojure, but not at all with Java.
The entirety of the problem I am trying to solve is lengthy to describe, but it boils down to this: let's say I would like to a make a macro that takes one argument, an associative map, and returns a vector of the elements of the map with their order conserved.
=>(defmacro vectorize-a-map
[associative-map]
(vec associative-map))
=>#'ns/vectorize-a-map
=>(vectorize-a-map {:a 1 :b 2 :c 3 :d 4 :e 5 :f 6 :g 7 :h 8}
=>[[:a 1] [:b 2] [:c 3] [:d 4] [:e 5] [:f 6] [:g 7] [:h 8]]
That works, but add another element to the map and the order messes up...
=>(vectorize-a-map {:a 1 :b 2 :c 3 :d 4 :e 5 :f 6 :g 7 :h 8 :i 9}
=>[[:a 1] [:c 3] [:b 2] [:f 6] [:g 7] [:d 4] [:e 5] [:i 9] [:h 8]]
I believe I have discovered why this is happening. It seems like anything with 8 or fewer elements is instantiated as a PersistentArrayMap, which is exactly what I want, because from what I can tell, this class retains order. However, anything with 9 or more elements is instantiated as a PersistentHashMap, which does not retain order.
=>(type {:a 1 :b 2 :c 3 :d 4 :e 5 :f 6 :g 7 :h 8}
=>clojure.lang.PersistentArrayMap
=>(type {:a 1 :b 2 :c 3 :d 4 :e 5 :f 6 :g 7 :h 8 :i 9}
=>clojure.lang.PersistentHashMap
I would like my macro to be able to take associative maps of any size, so this is a problem. I have tried type hinting, destructuring binding, for list comprehension, and unquote splicing, all without success. To draw it out, none of the following will work:
(defmacro vectorize-a-map
[^clojure.lang.PersistentArrayMap associative-map]
(vec associative-map))
(defmacro vectorize-a-map
[[& associative-map]]
(vec associative-map))
(defmacro vectorize-a-map
[associative-map]
(vec
(for [x associative-map]
x)))
(defmacro vectorize-a-map
[associative-map]
`(vector ~@associative-map))
With this toy problem I present, I realize I could simply write my macro like so, and avoid the problem altogether:
=>(defmacro vectorize-kvs
[& elements]
(vec (map vec (partition 2 elements))))
=>#'ns/vectorize-kvs
=>(vectorize-kvs :a 1 :b 2 :c 3 :d 4 :e 5 :f 6 :g 7 :h 8 :i 9)
=>[[:a 1] [:b 2] [:c 3] [:d 4] [:e 5] [:f 6] [:g 7] [:h 8] [:i 9]]
However, for the actual problem I am trying to solve (which I have not gotten into), it is important (although not 100% necessary) that the macro be able to take associative maps. It seems like I'm looking for how to cast the argument into a PersistentArrayMap before anything has a chance to happen to it. There may be some other avenue to a solution that I am simply not considering or aware of.
I've researched the best I've known how and haven't found anything helpful yet. Does anybody have any thoughts/advice?
you can make your map with array-map
user> (map vec (array-map 1 2 3 4 5 6))
([1 2] [3 4] [5 6])
or with a larger map
user> (map vec (apply array-map (range 50)))
([0 1] [2 3] [4 5] [6 7] [8 9] [10 11] [12 13] [14 15] [16 17] [18 19] [20 21] [22 23] [24 25] [26 27] [28 29] [30 31] [32 33] [34 35] [36 37] [38 39] [40 41] [42 43] [44 45] [46 47] [48 49])
as a bonus you can avoid using a macro, which is useful because macros are not first-class and don't compose well*
Note that an array map will only maintain sort order when un-'modified'. Subsequent assoc-ing will eventually cause it to 'become' a hash-map.
If you find yourself depending on the order of keys in your maps you may want to consider if a sorted-map
will get you what you need. It will scale better than an array-map
. In the above example the output is the same:
(map vec (apply sorted-map (range 5000)))
[0 1] [2 3] ... [4998 4999]
*this is my opinion
a time comparason of sorted-map
vs. array-map
user> (time (dorun (map vec (apply sorted-map (range 500000)))))
"Elapsed time: 391.520491 msecs"
nil
user> (time (dorun (map vec (apply array-map (range 500000)))))
"Elapsed time: 674517.821669 msecs"
The problem as stated is not soluble. Maps are defined to have no order; any order you see in array-map
is coincidental. If you require that your macro receive a map, you have already lost the information you desire.
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