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copied!<p><em>Duck typing</em> means that an operation does <strong>not formally specify</strong> the requirements that its operands have to meet, but just <strong>tries it out</strong> with what is given.</p> <p>Unlike what others have said, this does not necessarily relate to dynamic languages or inheritance issues.</p> <p><strong>Example task:</strong> Call some method <code>Quack</code> on an object.</p> <p><em>Without</em> using duck-typing, a function <code>f</code> doing this task has to specify in advance that its argument has to support some method <code>Quack</code>. A common way is the use of interfaces</p> <pre><code>interface IQuack { void Quack(); } void f(IQuack x) { x.Quack(); } </code></pre> <p>Calling <code>f(42)</code> fails, but <code>f(donald)</code> works as long as <code>donald</code> is an instance of a <code>IQuack</code>-subtype.</p> <p>Another approach is <a href="http://en.wikipedia.org/wiki/Structural_type_system" rel="noreferrer"><em>structural typing</em></a> - but again, the method <code>Quack()</code> is formally specified any anything that cannot prove it <code>quack</code>s in advance will cause a compiler failure.</p> <pre><code>def f(x : { def Quack() : Unit }) = x.Quack() </code></pre> <p>We could even write</p> <pre><code>f :: Quackable a => a -> IO () f = quack </code></pre> <p>in Haskell, where the <code>Quackable</code> typeclass ensures the existence of our method.</p> <p><hr> So how does <strong>duck typing</strong> change this? </p> <p>Well, as I said, a duck typing system does not specify requirements but <em>just tries if anything works</em>.</p> <p>Thus, a dynamic type system as Python's always uses duck typing:</p> <pre><code>def f(x): x.Quack() </code></pre> <p>If <code>f</code> gets an <code>x</code> supporting a <code>Quack()</code>, everything is fine, if not, it will crash at runtime.</p> <p>But duck typing doesn't imply dynamic typing at all - in fact, there is a very popular but completely static duck typing approach that doesn't give any requirements too:</p> <pre><code>template <typename T> void f(T x) { x.Quack(); } </code></pre> <p>The function doesn't tell in any way that it wants some <code>x</code> that can <code>Quack</code>, so instead it just tries <em>at compile time</em> and if everything works, it's fine.</p>

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