Why does binding affect the type of my map?

I was playing around in the REPL and I got some weird behavior:

Clojure 1.4.0
user=> (type {:a 1})
clojure.lang.PersistentArrayMap
user=> (def x {:a 1})
#'user/x
user=> (type x)
clojure.lang.PersistentHashMap

I thought that all small literal maps were instances of PersistentArrayMap, but apparently that's not the case if it's been bound with def. Why would using def cause Clojure to choose a different representation for my litte map? I know it's probably just some strange implementation detail, but I'm curious.

Why does binding affect the type of my map?

1 Answers