I've been bitten by all three of these things in the past: 1) A MapEntry looks like a vector, but isn't. 2) Two element vectors can't be used as map entries. 3) Applying empty to a MapEntry returns nil.
On Saturday, July 19, 2014 4:04:13 PM UTC-4, Jozef Wagner wrote: > > While the c.l.MapEntry is a persistent vector, there is no such thing as > an empty MapEntry persistent vector. Returning empty HAMT persistent vector > instead is not a good solution, as 2 elements HAMT vector does not > implement IMapEntry interface. > > One approach on how to solve this is in Zach's clj-tuple [1] where every > 2-tuple automatically implements IMapEntry interface. > > [1] https://github.com/ztellman/clj-tuple > > Jozef > > On Saturday, July 19, 2014 9:49:21 PM UTC+2, puzzler wrote: >> >> As Mike points out, it does seem that MapEntry is considered a collection >> and is designed to emulate a vector so that you don't really have to worry >> about whether you have a MapEntry or a two-element vector (and as he points >> out, in ClojureScript there really is no distinction between a MapEntry and >> a vector). >> >> With that in mind, I would agree that MapEntry's lack of implementation >> for empty is most likely an oversight, and ideally should behave as if it >> were a vector. >> >> >> >> >> On Sat, Jul 19, 2014 at 11:46 AM, Mike Fikes <mike...@me.com> wrote: >> >>> MapEntry is a collection: >>> >>> (coll? (clojure.lang.MapEntry. "a" 1)) >>> >>> ;=> true >>> >>> >>> (ancestors (class (clojure.lang.MapEntry. "a" 1))) >>> >>> ;=> (a set that includes clojure.lang.IPersistentCollection) >>> >>> >>> The docstring for empty implies it would return an empty MapEntry. But >>> perhaps since MapEntry is a special collection type that must have a size >>> exactly equal to 2, empty must return nil. >>> >>> >>> Perhaps ClojureScript will one day introduce a MapEntry type and behave >>> the same way as Clojure (ClojureScript currently returns a >>> PersistentVector). >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Clojure" group. >>> To post to this group, send email to clo...@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+u...@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+u...@googlegroups.com. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- 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.
