I have an application (several actually) which decode JSON data in a Map using Jackson. Data appears to either be in a Map or ArrayList (in the case of JSON arrays.) The data that comes in on these streams is unstructured, so this won't be changing.
I own some Clojure code which accesses nested properties in these objects. Ideally I'd like to extend the Associative abstraction to these Java types so that get-in
works on them. Something like the following:
(extend-protocol clojure.lang.Associative
java.util.Map
(containsKey [this k] (.containsKey this k))
(entryAt [this k] (when (.containsKey this k)
(clojure.lang.MapEntry/create k (.get this k))))
java.util.ArrayList
(containsKey [this k] (< (.size this) k))
(entryAt [this k] (when (.containsKey this k)
(clojure.lang.MapEntry/create k (.get this k)))))
There are two problems with this; The first being that Associative is not a protocol (if it were this appears it would work). The second being that the types are already defined so I cannot add Associative with deftype.
I'm pretty new to the JVM interop part of Clojure. Is there a method I'm not seeing? Or is there a protocol which wraps Associative and will work with get-in
that I've missed?
Thanks SO!
The answer is that half of the extension you want to do is already done, and the other half cannot be done. The get-in
function calls get
, which calls clojure.lang.RT/get
, which calls clojure.lang.RT/getFrom
, which calls java.util.Map/get
if the first argument is a Map
. So if you have any Java Map
, then get-in
works (I'm borrowing this example directly from the doto
docstring):
(let [m (doto (new java.util.HashMap) (.put "a" 1) (.put "b" 2))]
[(get-in m ["b"])
(get-in m ["a"])])
;;=> [2 1]
However, Clojure does not have a get
implementation for List
s that support RandomAccess
. You could write your own get
that does:
(ns sandbox.core
(:refer-clojure :exclude [get])
(:import (clojure.lang RT)
(java.util ArrayList List RandomAccess)))
(defn get
([m k]
(get m k nil))
([m k not-found]
(if (and (every? #(instance? % m) [List RandomAccess]) (integer? k))
(let [^List m m
k (int k)]
(if (and (<= 0 k) (< k (.size m)))
(.get m k)
not-found))
(RT/get map key not-found))))
Example:
(get (ArrayList. [:foo :bar :baz]) 2)
;;=> :bar
Then you could copy the implementation of get-in
so it would use your custom get
function.
I'm pretty sure this isn't what you want, though, because then every bit of code you write would have to use your get-in
rather than Clojure's get-in
, and any other code that already uses Clojure's get
would still not work with ArrayList
s. I don't think there's really a good solution to your problem, unfortunately.