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copied!<p>Rob's answer is of course correct; I'm posting this one in response to the OP's comment on it -- perhaps it might be helpful in implementing the required functionality with <code>deftype</code>.</p> <p>I have once written an implementation of a "default map" for Clojure, which acts just like a regular map except it returns a fixed default value when asked about a key not present inside it. The code is in <a href="http://gist.github.com/468332" rel="nofollow noreferrer">this Gist</a>.</p> <p>I'm not sure if it will suit your use case directly, although you can use it to do things like</p> <pre><code>user> (:earth (assoc (DefaultMap. 0 {}) :earth 8000000000)) 8000000000 user> (:mars (assoc (DefaultMap. 0 {}) :earth 8000000000)) 0 </code></pre> <p>More importantly, it should give you an idea of what's involved in writing this sort of thing with <code>deftype</code>.</p> <p>Then again, it's based on <code>clojure.core/emit-defrecord</code>, so you might look at that part of Clojure's sources instead... It's doing a lot of things which you won't have to (because it's a function for preparing macro expansions -- there's lots of syntax-quoting and the like inside it which you have to strip away from it to use the code directly), but it is certainly the highest quality source of information possible. <a href="http://github.com/clojure/clojure/blob/1.2.0/src/clj/clojure/core_deftype.clj#L134" rel="nofollow noreferrer">Here</a>'s a direct link to that point in the source for the 1.2.0 release of Clojure.</p> <p><em>Update:</em></p> <p>One more thing I realised might be important. If you rely on a special map-like type for implementing this sort of thing, the client might <code>merge</code> it into a regular map and lose the "defaulting" functionality (and indeed any other special functionality) in the process. As long as the "map-likeness" illusion maintained by your type is complete enough for it to be used as a regular map, passed to Clojure's standard function etc., I think there might not be a way around that.</p> <p>So, at some level the client will probably have to know that there's some "magic" involved; if they get correct answers to queries like <code>(:mars {...})</code> (with no <code>:mars</code> in the <code>{...}</code>), they'll have to remember not to <code>merge</code> this into a regular map (<code>merge</code>-ing the other way around would work fine).</p>

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