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PODisconnect between :dynamic metadata and actually-dynamic Vars in Clojure
 

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  1. PODisconnect between :dynamic metadata and actually-dynamic Vars in Clojure
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    <p>I'm learning Clojure and am playing around with dynamic scope. I've discovered a few things:</p> <ul> <li>Adding <code>{:dynamic true}</code> metadata to an existing Var doesn't actually make the Var dynamic. I can kind of see why you'd want this to be the case, but it can be misleading.</li> <li>You can see if a Var is actually dynamic with <code>(.isDynamic #'whatever)</code></li> <li>You can (but probably shouldn't?) change whether a Var is dynamic with <code>(.setDynamic #'whatever)</code></li> </ul> <p>In short, it looks like the dynamic nature of a Var is stored in the guts of the clojure.lang.Var class itself, and setting <code>^:dynamic</code> metadata <em>at def time only</em> will affect that internal state. Changing metadata afterward doesn't appear to affect it. OK, fine.</p> <p>I discovered this when I was looking at the dynamic variables in namespace clojure.core. I checked to see what Vars in that namespace have <code>:dynamic</code> metadata set:</p> <pre><code>user=&gt; (def metadyn (map key (filter #(-&gt; % key resolve meta :dynamic) (ns-publics 'clojure.core)))) #'user/metadyn user=&gt; metadyn (*3 *2 *print-level* *data-readers* *e *print-length* *1 *verbose-defrecords* *clojure-version* *default-data-reader-fn* pr) </code></pre> <p>Rather fewer than I expected, missing things like <code>*out*</code> and <code>*in*</code>.</p> <p>I checked to see what Vars actually <em>are</em> dynamic:</p> <pre><code>user=&gt; (def realdyn (map key (filter #(-&gt; % key resolve .isDynamic) (ns-publics 'clojure.core)))) #'user/realdyn user=&gt; realdyn (*compile-path* *3 *2 *ns* *print-level* *allow-unresolved-vars* *data-readers* *unchecked-math* *math-context* *read-eval* *compile-files* *command-line-args* *warn-on-reflection* *e *flush-on-newline* *out* *print-length* *1 *file* *verbose-defrecords* *clojure-version* *use-context-classloader* *err* *default-data-reader-fn* *agent* pr *print-dup* *print-readably* *fn-loader* *print-meta* *in* *source-path* *compiler-options* *assert*) </code></pre> <p>Lots more.</p> <p>So, the following Vars are dynamic but don't claim to be in their metadata:</p> <pre><code>user=&gt; (clojure.set/difference (set realdyn) (set metadyn)) #{*compile-path* *ns* *allow-unresolved-vars* *unchecked-math* *math-context* *read-eval* *compile-files* *command-line-args* *warn-on-reflection* *flush-on-newline* *out* *file* *use-context-classloader* *err* *agent* *print-dup* *print-readably* *fn-loader* *print-meta* *in* *source-path* *compiler-options* *assert*} </code></pre> <p>My question is simply: is there any significance to this, something I'm missing? Or, is it just a case of sloppy bookkeeping in the Clojure implementation?</p> <p>No practical use for this; just trying to Understand.</p>
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    1. USAron
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    1. COThey are created in the runtime before def is available.
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    2. COThanks, good enough for me. If you put it in a reply, I'll mark it as the answer.
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