The problem is that defrecord explicitly defines the interface you are
trying to implement, so you are effectively attempting to declare the
interface twice. And since clojure.lang.seqable isnt a protocol, you
cant redefine it with extend. So, you will have to rewrite the
defrecord macro or similar that implements what defrecord does, except
it will actually look at your interfaces to see if you are attempting
to implement the same one, and then use the users interface if so.
Heres an example of a custom deftype i wrote which basically
reimplements defrecord + some more. It is an 'AtomHash', which means
it acts like a hash map (i.e. like a record),
but instead of accessing the record slots directly, it accesses an
atom, which itself contains the map. I never figured out how to make
(merge (atom-hash {:a 2}) {:a 4}) return an atom-hash, but oh well.
Actually, i think the new clojure 1.3 has internal protocols for
merge. Anyways, from this, you can build a macro which replaces user
defined sequences with any defaults. Or you could just use it as a
template.
(deftype AtomHash [val]
Object
(toString [this] (str "<AtomHash " @val ">"))
clojure.lang.IPersistentMap
clojure.lang.ILookup
(valAt [this key] (get @val key))
(valAt [this key notfound] (get @val key notfound))
clojure.lang.IPersistentCollection
(count [this] (.count @val))
(empty [this] {})
(cons [this e] (.cons @val e))
(equiv [this gs] (or (identical? this gs)
(when (identical? (class this) (class gs))
(= val (.val gs)))))
clojure.lang.Associative
(containsKey [this k] (or (and (get @val k) true) false))
(entryAt [this k] (get @val k))
clojure.lang.Seqable
(seq [this] (seq @val))
clojure.lang.IPersistentMap
(assoc [this k g] (assoc @val k g))
(assocEx [this k g] (assoc this k g))
(without [this k] (.without @val k))
clojure.lang.IDeref
(deref [this] @val))
;;REPLACE namespace with implementation namespace
(defmethod print-dup AtomHash [o w]
(.write w "#=(util/atom-hash ") (print-dup @o w) (.write w ")"))
(defmethod clojure.core/print-method AtomHash [o w]
(.write w (.toString o)))
(defn atom-hash
([] (atom-hash {}))
([a] {:pre [(map? a)]} (AtomHash. (atom a))))
;;examples
(let [{:keys [a b]} (atom-hash {:a 2})] (list a b))
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