Hmm... since ISeq extends IPersistentCollection and overrides the latter's return type, cons must return an ISeq. So do I understand it correctly, that one *can't* have a deftype which returns itself as Seqable *and* has vector-style conj behavior (i.e. append at tail position)?
K. On 3 September 2014 18:13, Karsten Schmidt <i...@toxi.co.uk> wrote: > Hi all, > > I've defined a custom vector type and implemented various clj/cljs > protocols, but now ended up hitting my head against some weird behaviour > with conj (or rather cons, internally). The type > defines a 2-element vector-like construct and my cons > implementation would simply return a standard > clojure.lang.PersistentVector with the given arg added like this: > > (deftype Foo [a b] > clojure.lang.IPersistentCollection > clojure.lang.Indexed > clojure.lang.Sequential > clojure.lang.ISeq > clojure.lang.Seqable > clojure.lang.Reversible > ;; ... elided seq fn impls... > (cons [_ c] [a b c])) > > However, attempting a conj results in an exception I don't understand: > > (conj (Foo. 1 2) 3) > java.lang.ClassCastException: clojure.lang.PersistentVector cannot be > cast to clojure.lang.ISeq > at user.Foo.cons (foo.clj:13) > user.Foo.cons (foo.clj:-1) > clojure.lang.RT.conj (RT.java:562) > clojure.core$conj.invoke (core.clj:83) > ... > > As far as I can tell, clojure.core/conj simply calls RT.conj(coll, x), > which just calls > coll.cons(x): > > https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/RT.java#L604 > > static public IPersistentCollection conj(IPersistentCollection coll, Object > x){ > if(coll == null) > return new PersistentList(x); > return coll.cons(x); > } > > So where in that call path is there an attempt or requirement to cast > to an ISeq? > > I've been comparing notes (as far as this possible) with the default > PersistenVector implementation, but not sure where I'm going wrong > here... -- Karsten Schmidt http://postspectacular.com | http://toxiclibs.org | http://toxi.co.uk -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
