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    <p>REPL = read eval print loop. Step through the read-eval process.</p> <p>READ: Clojure sees the string <code>"(`a)"</code>, parses it and ends up with a data structure. At read time, reader macros are expanded and not much else happens. In this case, the reader expands the backquote and ends up with this:</p> <pre><code>user&gt; (read-string "(`a)") ((quote user/a)) </code></pre> <p>EVAL: Clojure tries to evaluate this object. Evaluation rules vary depending on what kind of object you're looking at. </p> <ul> <li>Some objects evaluate as themselves (numbers, strings, keywords etc.).</li> <li>A Symbol is evaluated by resolving it in some namespace to obtain some value (usually).</li> <li>A List is evaluated by macro-expanding the list until there are no macros left, then recursively evaluating the first item in the list to obtain some <strong>resulting value</strong>, then using the <strong>value</strong> of the first item in the list to decide what to do. If the first value is a special form, special stuff happens. Otherwise the first value is treated as a function and called with the values of the rest of the list (obtained by recursively evaluating all of the list's items) as parameters.</li> <li>etc.</li> </ul> <p>Refer to <code>clojure.lang.Compiler/analyzeSeq</code> in the Clojure source to see the evaluation rules for lists, or <code>clojure.lang.Compiler/analyzeSymbol</code> for symbols. There are lots of other evaluation rules there.</p> <h2>Example</h2> <p>Suppose you do this:</p> <pre><code>user&gt; (user/a) </code></pre> <p>The REPL ends up doing this internally:</p> <pre><code>user&gt; (eval '(user/a)) </code></pre> <p>Clojure sees that you're evaluating a list, so it evaluates all items in the list. The first (and only) item:</p> <pre><code>user&gt; (eval 'user/a) #&lt;user$a__1811 user$a__1811@82c23d&gt; </code></pre> <p><code>a</code> is not a special form and this list doesn't need to be macroexpanded, so the symbol <code>a</code> is looked up in the namespace <code>user</code> and the <strong>resulting value</strong> here is an <code>fn</code>. So this <code>fn</code> is called.</p> <h2>Your code</h2> <p>But instead you have this:</p> <pre><code>user&gt; (eval '((quote user/a))) </code></pre> <p>Clojure evaluates the first item in the list, which is itself a list. </p> <pre><code>user&gt; (eval '(quote user/a)) user/a </code></pre> <p>It evaluated the first item in this sub-list, <code>quote</code>, which is a special form, so special rules apply and it returns its argument (the Symbol <code>a</code>) un-evaluated.</p> <p>The symbol <code>a</code> is the <strong>value</strong> in this case as the <code>fn</code> was the value up above. So Clojure treats the Symbol itself as a function and calls it. In Clojure, anything that implements the <code>Ifn</code> interface is callable like an <code>fn</code>. It so happens that <code>clojure.lang.Symbol</code> implements <code>Ifn</code>. A Symbol called as a function expects one parameter, a collection, and it looks itself up in that collection. It's meant to be used like this:</p> <pre><code>user&gt; ('a {'a :foo}) :foo </code></pre> <p>This is what it tries to do here. But you aren't passing any parameters, so you get the error "Wrong number of args passed to: Symbol" (it expects a collection).</p> <p>For your code to work you'd need two levels of <code>eval</code>. This works, hopefully you can see why:</p> <pre><code>user&gt; (eval '((eval (quote user/a)))) Hello, world user&gt; ((eval (first l))) Hello, world </code></pre> <p>Note that in real code, using <code>eval</code> directly is usually a really bad idea. Macros are a better idea by far. I'm only using it here for demonstration. </p> <p>Look in <code>Compiler.java</code> in the Clojure source to see how this all plays out. It's not too hard to follow.</p>
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