Namespaces in Clojure

The only succinct documentation of namespaces I found on Clojure is here.  To quote Brian there:

> Hi,
> just started looking at Clojure, and it looks promising as a modern-
> day Lisp for practical use.

> I saw this presentation on Clojure: http://clojure.blip.tv/ and I
> wholeheartedly agree with the design principles. Unified access to all
> kinds of collections (list, array, struct, hash) is something I sorely
> missed in CL back in the day (late 90's), and I like the focus on
> practical programming in the modern world (multicore/multithread,
> leveraging existing libraries, a stable and mature VM plaform etc.).
> Looking forward to getting to know Clojure better.

> But I have a question: Why are there *four* ways to do an import, each
> with slightly different syntax and semantics?

:refer shouldn't be used by you at all.

:import brings in java classes

:use brings in the names from another clojure namespace without requiring namespace qualifications

:require brings in the names from another clojure namespace requiring namespace qualifications.

So if ns foo has a var bar, if you do (use 'foo), you can just refer to bar, but if you do (require 'foo), you have to say foo/bar, not just bar.

I should add: make sure file names do not include the "-" character even if the namespace you define with (ns ...) has a "-" character in it; replace it with a "_" instead in the file name (the namespace can continue to use the "-" though).

(edit: I will write more about namespaces, especially the syntax of the (ns ...) later.)