Thanks for the explanation All. I have a much better grasp of what's going on
now.
Just one more question: It is defined behavior, or should I submit a patch for
Clojure 1.3?
Micah
On Oct 20, 2011, at 7:55 PM, Chouser wrote:
> On Thu, Oct 20, 2011 at 4:31 PM, Chris Perkins <chrisperkin...@gmail.com>
> wrote:
>> Note: I forgot to preface that with "I think..." :) Upon experimenting
>> briefly, it turns out I was wrong about how Clojure works (that seems to
>> happen a lot with me). A declare/def defines a var even when it's not
>> executed!
>> user> (defn xxx [] (declare yyy))
>> #'user/xxx
>> user> yyy
>> #<Unbound Unbound: #'user/yyy>
>> Well, I learned something today.
>
> But it only interns the Var, it doesn't fully set it up. Particularly
> relevant to the OP's example is that the metadata from the name symbol
> is not transferred to the Var (and the changes to the Var based on
> :dynamic are not applied) until runtime for the 'def', even though the
> Var exists at compile time.
>
> Here's a macro that expands at compile time to the *compile* time
> metadata of the var named in its argument:
>
> (defmacro compile-time-meta [x] (meta (resolve x)))
>
> Now observe how it behaves differently than a runtime call to 'meta':
>
> (vector
> (declare ^:dynamic *myvar*)
> (meta #'*myvar*)
> (compile-time-meta #'*myvar*)))
>
> The above returns:
>
> [#'user/*myvar*
> {:ns #<Namespace user>, :name *myvar*, :dynamic true, :declared true, ...}
> {:ns #<Namespace user>, :name #<Unbound Unbound: #'user/*myvar*>}]
>
> First is the Var itself.
> Next is the metadata of the Var at runtime, after the entire form has
> been compiled and therefore the metadata from the name has been
> applied to the Var, including the :dynamic flag.
> Finally we see that when our macro was expanded the Var existed but
> had minimal metadata. This was after the declare was compiled but
> before any part of the 'vector' form was run. There is no :dynamic
> flag, and anything that depends on that flag at compile time to work
> correctly (such as a function that refers the for Var) will fail to
> work correctly.
>
> --Chouser
>
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