On Nov 14, 5:42 pm, André Thieme <splendidl...@googlemail.com> wrote:
> But in real programs things are not so easy. We have refs in refs.
This is just a thought experiment. But what about actually having refs
in refs? I'm not sure if I am reinventing mutable object here, so
please shoot me down ;-)
Clojure 1.1.0-alpha-SNAPSHOT
user=> (deftype Person [name age friends]
[clojure.lang.IPersistentMap])
#'user/Person
user=> (def *persons* (ref {}))
#'user/*persons*
user=> (dosync (alter *persons* assoc :ethel (ref (Person "ethel" 24
[]))))
{:ethel #<r...@585976c2: #:Person{:name "ethel", :age 24, :friends []}
>}
user=> (dosync (alter *persons* assoc :fred (ref (Person "fred" 42
[]))))
{:fred #<r...@2c78bc3b: #:Person{:name "fred", :age 42, :friends []}
>, :ethel #<r...@585976c2: #:Person{:name "ethel", :age 24, :friends []}
>}
user=> @*persons*
{:fred #<r...@2c78bc3b: #:Person{:name "fred", :age 42, :friends []}
>, :ethel #<r...@585976c2: #:Person{:name "ethel", :age 24, :friends []}
>}
;;now...
user=> (dosync (alter (:fred @*persons*)
(fn [f] (assoc f :friends (conj (:friends f)
(:ethel @*persons*))))))
#:Person{:name "fred", :age 42, :friends [#<r...@585976c2: #:Person
{:name "ethel", :age 24, :friends []}>]}
user=> @*persons*
{:fred #<r...@2c78bc3b: #:Person{:name "fred", :age 42, :friends
[#<r...@585976c2: #:Person{:name "ethel", :age 24, :friends []}>]}
>, :ethel #<r...@585976c2: #:Person{:name "ethel", :age 24, :friends []}
>}
user=> (identical? (first (:friends @(:fred @*persons*))) (:ethel
@*persons*))
true
Note that Fred and Ethel are identities, and their corresponding refs
still refer to *immutable* values (the refs themselves).
Now to inc age:
user=> (dosync (alter (:ethel @*persons*) update-in [:age] inc))
#:Person{:name "ethel", :age 25, :friends []}
user=> @*persons*
{:fred #<r...@da0225b: #:Person{:name "fred", :age 42, :friends
[#<r...@7ae35bb7: #:Person{:name "ethel", :age 25, :friends []}>]}
>, :ethel #<r...@7ae35bb7: #:Person{:name "ethel", :age 25, :friends []}
>}
In a way, I like the modeling: the people are actually identities and
there are relations between identities. STM ensures consistent
snapshots.
So am I reinventing objects in a bad way, or is this just recognizing
the true identities?
:-)
/K
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