Newbie: Creating a MapEntry

[samppi]

Is it possible to create a MapEntry from scratch? The reason why I'm
asking is because I want to mess with sequences of two-sized vectors,
and it would be really cool if I could use the key and val functions
on them rather than (get % 0) and (get % 1):

(map #(str "Key:" (key %) "Value:" (val %)) [(map-entry :a 3) (map-
entry :b 2) (map-entry :a 1) (map-entry :c 0)])

Just wondering. (And if not, is there any good reason why they should
only be available from sequences of maps? :))

[Stephen C. Gilardi]

On Nov 21, 2008, at 12:03 AM, samppi wrote:

Is it possible to create a MapEntry from scratch? 

user=> (def a (clojure.lang.MapEntry. 3 4))

#'user/a

user=> (key a)

3

user=> (val a)

4

user=>

You can also make it a little shorter by importing clojure.lang.MapEntry .

--Steve

[samppi]

Thank you very much.

[wlr]

On Nov 21, 12:03 am, samppi <rbysam...@gmail.com> wrote:
> I want to mess with sequences of two-sized vectors,

Maybe destructuring will do:
user> (dorun (map (fn [[k v]] (println "Key " k "Value " v)) [ [:a 3]
[:b 2] [:a 1] [:c 0]]))
=>
Key :a Value 3
Key :b Value 2
Key :a Value 1
Key :c Value 0
nil

[Rich Hickey]

I make no promises about the continued existence of MapEntry. Please
don't use it.

Rich

[J. McConnell]

On Fri, Nov 21, 2008 at 12:03 AM, samppi <rbys...@gmail.com> wrote:
>
> Is it possible to create a MapEntry from scratch? The reason why I'm
> asking is because I want to mess with sequences of two-sized vectors,
> and it would be really cool if I could use the key and val functions
> on them rather than (get % 0) and (get % 1):
>
> (map #(str "Key:" (key %) "Value:" (val %)) [(map-entry :a 3) (map-
> entry :b 2) (map-entry :a 1) (map-entry :c 0)])

Since Rich discouraged use of the MapEntry class, here are a couple of options.

First, one thing worth remembering is that vectors are functions of
their indexes, so you can do this and get rid of the "get"'s:

(map #(str "Key: " (% 0) "Value: " (% 1)) [[:a 3] [:b 2] [:a 1] [:c 0]])

Second, if it was the use of (key) and (val) that you were most
interested in, you could create your own (map-entry) function:

user=> (defn map-entry [k v] (first {k v}))
#'user/map-entry
user=> (map #(str "Key:" (key %) "Value:" (val %))
[(map-entry :a 3) (map-entry :b 2) (map-entry :a 1) (map-entry :c 0)])
("Key::aValue:3" "Key::bValue:2" "Key::aValue:1" "Key::cValue:0")

HTH,

- J.

[samppi]

Yes, thank you—(key) and (val) were what I was interested in, so I'll
use the latter function you gave. What I'm wondering though is, if
MapEntries aren't guaranteed for the future, what is being planned for
(key) and (val) too. Oh, well. :)

On Nov 21, 8:23 am, "J. McConnell" <jdo...@gmail.com> wrote:

[Rich Hickey]

On Nov 21, 11:40 am, samppi <rbysam...@gmail.com> wrote:
> Yes, thank you—(key) and (val) were what I was interested in, so I'll
> use the latter function you gave. What I'm wondering though is, if
> MapEntries aren't guaranteed for the future, what is being planned for
> (key) and (val) too. Oh, well. :)
>

There's a difference between supporting key and val and allowing you
to create MapEntries directly. key and val will stick around.

Rich

[Stephen C. Gilardi]

On Nov 21, 2008, at 10:23 AM, J. McConnell wrote:

user=> (defn map-entry [k v] (first {k v}))

Very nice!

--Steve