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    <p><strong>Using the <em>implicitly</em> method</strong></p> <p>The most common and general approach is to use the <a href="https://stackoverflow.com/questions/3855595/scala-function-implicitly">implicitly method</a>, defined in Predef:</p> <pre><code>def add[T: Numeric](x: T, y: T) = implicitly[Numeric[T]].plus(x,y) </code></pre> <p>Obviously, this is somewhat verbose and requires repeating the name of the type class. </p> <p><strong>Referencing the evidence parameter</strong> (<em>don't!</em>)</p> <p>Another alternative is to use the name of the implicit evidence parameter automatically generated by the compiler:</p> <pre><code>def add[T: Numeric](x: T, y: T) = evidence$1.plus(x,y) </code></pre> <p>It's surprising that this technique is even legal, and it should not be relied upon in practice since the name of the evidence parameter could change. </p> <p><strong>Context of a Higher Kind</strong> (<em>introducing the <code>context</code> method</em>)</p> <p>Instead, one can use a beefed-up version of the <code>implicitly</code> method. Note that the implicitly method is defined as</p> <pre><code>def implicitly[T](implicit e: T): T = e </code></pre> <p>This method simply relies on the compiler to insert an implicit object of the correct type from the surrounding scope into the method call, and then returns it. We can do a bit better:</p> <pre><code>def context[C[_], T](implicit e: C[T]) = e </code></pre> <p>This allows us to define our <code>add</code> method as</p> <pre><code>def add[T: Numeric](x: T, y: T) = context.plus(x,y) </code></pre> <p>The <code>context</code> method type parameters <code>Numeric</code> and <code>T</code> are inferred from the scope! Unfortunately, there are circumstances in which this <code>context</code> method will not work. When a type parameter has multiple context bounds or there are multiple parameters with different context bounds, for example. We can resolve the latter problem with a slightly more complex version:</p> <pre><code>class Context[T] { def apply[C[_]]()(implicit e: C[T]) = e } def context[T] = new Context[T] </code></pre> <p>This version requires us to specify the type parameter every time, but handles multiple type parameters.</p> <pre><code>def add[T: Numeric](x: T, y: T) = context[T]().plus(x,y) </code></pre>
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    1. POHow do I get an instance of the type class associated with a context bound?
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    1. USCommunity
    1. USAaron Novstrup
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    1. POHow do I get an instance of the type class associated with a context bound?
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    1. COClever hack with the `context` method!
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      1. USDaniel Spiewak
    2. COBoy, if I thought anyone was relying on the name of the evidence parameter that way I'd change it once a week or so... also, the technique is not legal in my jurisdiction, but maybe the laws are different where you live.
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    3. COVery clever indeed.
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      1. USpedrofurla
 

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