defonce simply means "do not bind the var if it already has a root
value"... I think it only applies when reloading your namespace...to be
honest, I'm not sure why Chris has coded it like that (creating an atom
is not really expensive), but i find it hard to believe that you can' t
have 2 Noir apps on the same project...basically defonce it is the same
as def unless there is already a root value for that var...
can anyone else add anything?
Jim
On 12/09/12 15:30, the80srobot wrote:
Hello,
So I've been working on a project at work, that required me to code a
simple web interface. I considered going with Noir, and while reading
the code, I noticed a pattern that seems to repeat throughout most of
the code that Chris Granger has published in Clojure. This is what I'm
referring to:
; these are at the top level in (ns noir.core)
(defonce noir-routes (atom {}))
(defonce route-funcs (atom {}))
(defonce pre-routes (atom (sorted-map)))
(defonce post-routes (atom []))
(defonce compojure-routes (atom []))
Now, I am new to Clojure, but I am not new to (functional) programming
and I'd like to think that I know a singleton when I see one. Is that
really what these are? If I'm right then defining your 'globals' (for
lack of a better word) like this would mean, among other things, that
you really can't have two independent Noir apps defined/running in the
same project - is that a correct assessment?
Can someone more experienced shed some light on why it's done this
way? My experience in functional programming has taught me to always
limit my scope - I would think that either using thread-local bindings
(and then rebinding them to child threads) or relying on lexical scope
would be preferable to polluting the global state. Is this a Clojure
best practice?
Thanks. I'm looking to use Clojure a lot at work, and I'm trying to
really understand the language before I throw it our production problems.
~Adam
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