Hi Alyssa,
Alyssa Kwan <alyssa.c.k...@gmail.com> writes:
> ns-unmap isn't typically used. But for durability, we can think of
> JVM shutdown/startup as unmapping everything and starting fresh.
> Therefore, expected behavior for ns-unmap should be the same as
> behavior for deserializing and loading an object after a new JVM
> startup.
I think the point that you're missing is that vars are just plain old
first class objects. This includes being managed by the garbage
collector so they'll continue to exist until all references to them are
released. You can pass them around, put them in vectors etc.
> Here's another issue I ran into:
>
> => (def a 1)
> #'user/a
>
> => (defn b [] a)
> #'user/b
When you compile this, a reference to the var #'user/a is embedded in
your b function. This reference is directly to the var object.
> So far so good...
>
> => (ns-unmap *ns* 'a)
> nil
>
> => a
> java.lang.Exception: Unable to resolve symbol: a in this context
>
> => (b)
> 2
The var has been unmapped from the namespace, but it still exists
because the function b has a reference to it. Vars don't need to live
in a namespace. For example, the "with-local-vars" macro creates a
"local" var which doesn't belong to a namespace.
>
> => (binding [a 3]
> (b))
> java.lang.Exception: Unable to resolve var: a in this context
> So what's the expected behavior here? I would think that after a is
> unmapped, b should no longer work. Instead, it's hanging onto a with
> the last value.
The var still exists, b holds a reference to it.
> And the binding form doesn't work anymore, so there's
> no way to dynamically bind over it. It's like a weird hybrid of
> lexical and dynamic binding...
The only reason this is erroring is that the symbol 'a can no longer be
mapped to your var.
Actually there is a way to dynamically bind over a var which has been
unmapped, all you need is a reference to it:
user> (def a 1)
#'user/a
user> (def my-a #'user/a)
#'user/my-a
user> (defn b [] a)
#'user/b
user> (ns-unmap *ns* 'a)
nil
user> (b)
1
user> (with-bindings* {my-a 30} #(b))
30
--
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
