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    <p>Let's look at the classic add-n closure from <a href="https://stackoverflow.com/questions/36636/what-is-a-closure">What is a closure</a>.</p> <pre><code>(define (add a) (lambda (b) (+ a b))) (define add3 (add 3)) (add3 4) returns 7 </code></pre> <p>In the above lambda expression, <code>a</code> is the <a href="http://en.wikipedia.org/wiki/Free_variables_and_bound_variables" rel="nofollow noreferrer">free variable</a>, which is defined in the Wikipedia link to be:</p> <blockquote> <p>a variable referred to in a function that is not a local variable or an argument of that function. An upvalue is a free variable that has been bound (closed over) with a closure.</p> </blockquote> <p>Coming back to</p> <pre><code>def filesEnding(query: String) = filesMatching(_.endsWith(query)) </code></pre> <p>The implicit function <code>x =&gt; x.endsWith(query)</code> is the first-class function, which is assigned to first-class value <code>matcher</code>, and <code>_.endsWith()</code> is closed over <code>query</code>, similar to the way 3 closes up <code>a</code> in <code>(add 3)</code>. <code>(add3 4)</code> equivalent is done by <code>matcher(file.getName)</code>. </p> <p><strong>Edit</strong>: Tricky part is the feature in Scala called placeholder syntax anonymous functions. By using <code>_</code> in place of the sender or the parameter, Scala automatically creates an anonymous function, which we can consider it as lambda expression.</p> <p>For example,</p> <pre><code>_ + 1 creates x =&gt; x + 1 _ * _ creates (x1, x2) =&gt; x1 * x2 _.endsWith(query) creates x =&gt; x.endsWith(query) </code></pre> <p>Within the function <code>x =&gt; x.endsWith(query)</code>, <code>query</code> meets the two requirements of being a free variable:</p> <ol> <li><code>query</code> is not a local variable defined within the function (there is no local variable).</li> <li><code>query</code> is not an argument of the function (the only argument is <code>x</code>).</li> </ol>
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    1. POQuestion on Scala Closure (From "Programming in Scala")
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    1. POQuestion on Scala Closure (From "Programming in Scala")
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    1. COAm I understand correctly that because "matcher" method capture variable "query" therefore it use closure.
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    2. COYes, in this code "def filesEnding(query: String) = filesMatching(_.endsWith(query))" there's a lambda "_.endsWith(query)" which when desugared a bit looks like "{x => x.endsWith(query)}". In schemey notation would look like "(lambda (x) (endwith x query))". As you can see, in the lambda "query" is a free variable. It's not bound as an argument nor with a let in the lambda, so when the closure is formed query is captured from the containing environment, e.g. invocations of methods like "filesEnding".
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