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    <p>A language that considers procedures to be "first-class" <strong><em>allows functions to be passed around just like any other value</em></strong>.</p> <p>Languages like Java 7 (and earlier) and C "kind of" have this capability: C allows function pointers to be passed around, but you can't dynamically define a function in those languages and suddenly pass that somewhere else. Java before version 8 can <em>simulate</em> this to a certain extent with anonymous classes, but it doesn't technically have first-class functions.</p> <p>On the other hand, C++, D, C#, Visual Basic .NET, Java 8+, and functional languages (like Scheme and Haskell) <em>do</em> allow you to pass around functions like variables. For example, the code below <em>returns</em> a function that adds <code>addend</code> to its input:</p> <p>Written in D:</p> <pre><code>int delegate(int) makeAdder(int addend) //Returns a function { return delegate int(int x) //Long way { return x + addend; //Notice that addend came from _outside_ the function }; return (int x) { return x + addend; }; //Short way return x =&gt; addend + x; //Super-short way, introduced in D 2.058 } </code></pre> <p>Written in C#:</p> <pre><code>Func&lt;int, int&gt; MakeAdder(int addend) //Returns a function { return delegate(int x) //The long way. Note: Return type is implicitly 'int' { return x + addend; }; return x =&gt; x + addend; //Short way: x "goes to" (x + addend); inferred types } </code></pre> <p>Written in C++:</p> <pre><code>#include &lt;functional&gt; std::function&lt;int(int)&gt; make_adder(int addend) { return [=](int x) { return addend + x; }; } </code></pre> <p>Written in Scala:</p> <pre><code>def makeAdder(addend: Int) = (x: Int) =&gt; addend + x </code></pre> <p>Written in Python:</p> <pre><code>def make_adder(addend): def f(x): return addend + x return f # or... return lambda x: addend + x </code></pre> <p>Written in Erlang:</p> <pre><code>make_adder(Addend) -&gt; fun(X) -&gt; Addend + X end. </code></pre> <p>Written in JavaScript:</p> <pre><code>function makeAdder(addend) { return function(x) { return addend + x; }; } </code></pre> <p>Written in JavaScript (ES2015 arrow function syntax):</p> <pre><code>const makeAdder = addend =&gt; x =&gt; addend + x; </code></pre> <p>Written in Scheme:</p> <pre><code>(define (makeAdder addend) (lambda (x) (+ x addend))) </code></pre> <p>Written in Haskell:</p> <pre><code>makeAdder :: Int -&gt; (Int -&gt; Int) makeAdder addend = \x -&gt; addend + x </code></pre> <p>Written in Visual Basic 2008:</p> <pre><code>Function MakeAdder(addend As Integer) As Func(Of Integer, Integer) Return Function(x) (x + addend) End Function </code></pre> <p>Written in Swift (both verbose and short-hand implementations):</p> <pre><code>func makeAdder(append: Int) -&gt; (x: Int) -&gt; Int { return { (x: Int) -&gt; Int in return x + append }; } func makeAdder(append: Int) -&gt; (Int) -&gt; Int { return {$0 + append}; } </code></pre> <p>(By the way, a "lambda" is just a function without a name. Lambdas are only supported in languages that support first-class functions.)</p>
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    1. POWhat is a first class citizen function?
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    1. COin C++ you can using template function objects: http://www.boost.org/doc/libs/1_46_0/libs/spirit/phoenix/doc/html/index.html
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    2. COThey're not functions, they're function objects. They behave similarly, but they're not lambdas like in C# or D, and making them is a *huge* pain compared to languages that have lambdas, since you have to declare an entirely new class, etc.
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    3. COthere isnt a difference between a stateless functor and function. do look at phoenix, you'll be surprised
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