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    <p>These are called <strong>generalized type constraints</strong>. They allow you, from within a type-parameterized class or trait, to <em>further constrain</em> one of its type parameters. Here's an example:</p> <pre><code>case class Foo[A](a:A) { // 'A' can be substituted with any type // getStringLength can only be used if this is a Foo[String] def getStringLength(implicit evidence: A =:= String) = a.length } </code></pre> <p>The implicit argument <code>evidence</code> is supplied by the compiler, iff <code>A</code> is <code>String</code>. You can think of it as a <em>proof</em> that <code>A</code> is <code>String</code>--the argument itself isn't important, only knowing that it exists. <em>[edit: well, technically it actually is important because it represents an implicit conversion from <code>A</code> to <code>String</code>, which is what allows you to call <code>a.length</code> and not have the compiler yell at you]</em></p> <p>Now I can use it like so:</p> <pre><code>scala&gt; Foo("blah").getStringLength res6: Int = 4 </code></pre> <p>But if I tried use it with a <code>Foo</code> containing something other than a <code>String</code>:</p> <pre><code>scala&gt; Foo(123).getStringLength &lt;console&gt;:9: error: could not find implicit value for parameter evidence: =:=[Int,String] </code></pre> <p>You can read that error as "could not find evidence that Int == String"... that's as it should be! <code>getStringLength</code> is imposing <em>further restrictions</em> on the type of <code>A</code> than what <code>Foo</code> in general requires; namely, you can only invoke <code>getStringLength</code> on a <code>Foo[String]</code>. This constraint is enforced at compile-time, which is cool!</p> <p><code>&lt;:&lt;</code> and <code>&lt;%&lt;</code> work similarly, but with slight variations:</p> <ul> <li><code>A =:= B</code> means A must be exactly B</li> <li><code>A &lt;:&lt; B</code> means A must be a subtype of B (analogous to the <em>simple</em> type constraint <code>&lt;:</code>)</li> <li><code>A &lt;%&lt; B</code> means A must be <em>viewable</em> as B, possibly via implicit conversion (analogous to the simple type constraint <code>&lt;%</code>)</li> </ul> <p><a href="http://gist.github.com/229163" rel="noreferrer">This snippet</a> by @retronym is a good explanation of how this sort of thing used to be accomplished and how generalized type constraints make it easier now.</p> <p><strong>ADDENDUM</strong></p> <p>To answer your follow-up question, admittedly the example I gave is pretty contrived and not obviously useful. But imagine using it to define something like a <code>List.sumInts</code> method, which adds up a list of integers. You don't want to allow this method to be invoked on any old <code>List</code>, just a <code>List[Int]</code>. However the <code>List</code> type constructor can't be so constrainted; you still want to be able to have lists of strings, foos, bars, and whatnots. So by placing a generalized type constraint on <code>sumInts</code>, you can ensure that <em>just that method</em> has an additional constraint that it can only be used on a <code>List[Int]</code>. Essentially you're writing special-case code for certain kinds of lists.</p>
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    1. POWhat do <:<, <%<, and =:= mean in Scala 2.8, and where are they documented?
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    1. USTom Crockett
    1. USTom Crockett
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    1. POWhat do <:<, <%<, and =:= mean in Scala 2.8, and where are they documented?
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    1. COWell, ok, but there's also methods by the same names on `Manifest`, which you didn't mention.
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      1. USDaniel C. Sobral
    2. COThe methods on `Manifest` are `<:<` and `>:>` only... since OP mentioned exactly the 3 varieties of generalized type constraints, I'm assuming that's what he was interested in.
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    3. COYou do have a point there.
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