What's wrong with my lein/nrepl/emacs setup? - emacs

Question

Newbie Clojure programmer here. NREPL in Emacs isn't working for me.
Aquamacs 2.5 (Emacs 23.4.1)
nrepl.el 0.1.8
Leiningen 2.2.0 on Java 1.6.0_51 Java HotSpot(TM) 64-Bit Server VM
My project is just the lein-generated Hello World.
Running "lein repl" in a shell works, but nrepl.el doesn't work.
Rather than winding up in the project's namespace in the repl,
I just get the default toplevel in the "user" namespace.
Should see something like:
my$ lein repl
nREPL server started on port 57347
REPL-y 0.2.0
Clojure 1.5.1
Hello Project
my.core=>
But instead see:
; nREPL 0.1.8-preview
user> (in-ns my.core)
CompilerException java.lang.ClassNotFoundException: my.core, compiling:(NO_SOURCE_PATH:1:1)
user>
In a clean nrepl.el scenario, I see two java processes going and they look plausible. One has my project on the -classpath and is implementing the actual repl (server), while the other is the client side java (-D MyProjectPath -m leiningen.core.main repl :headless).
I get the same lossage whether I get there by m-x nrepl-jack-in or starting in the shell and then m-x nrepl. I get a repl that doesn't know my project.
I wonder if this stuff works for anyone, or if I have something installed or set up wrong? Lots of "0.xxx" versions of things going on here...

Answer 1

Looks like you are missing a quote before your namespace, try the following:
(in-ns 'my.core)
Note the ' before my.

Answer 2

NRepl always starts in the top-level user namespace. Use M-x nrepl-set-ns, or C-c M-n in a Clojure buffer, to set the namespace of NRepl to the one of the buffer.
In your specific case,
visit src/my/core.clj, press C-c M-j to start NRepl,
switch back to the src/my/core.clj buffer,
press C-u C-c C-z to set the NRepl namespace to my.core and switch back to the NRepl buffer.
C-u C-c C-z is a shortcut for C-c M-n followed by C-c C-z to switch to the NRepl buffer.