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    <p>The <code>&lt;:&lt;</code> type is defined in <a href="https://lampsvn.epfl.ch/trac/scala/browser/scala/trunk/src/library/scala/Predef.scala" rel="noreferrer">Predef.scala</a> along with the related types <code>=:=</code> and <code>&lt;%&lt;</code> as follows:</p> <pre><code>// used, for example, in the encoding of generalized constraints // we need a new type constructor `&lt;:&lt;` and evidence `conforms`, as // reusing `Function2` and `identity` leads to ambiguities (any2stringadd is inferred) // to constrain any abstract type T that's in scope in a method's argument list (not just the method's own type parameters) // simply add an implicit argument of type `T &lt;:&lt; U`, where U is the required upper bound (for lower-bounds, use: `U &lt;: T`) // in part contributed by Jason Zaugg sealed abstract class &lt;:&lt;[-From, +To] extends (From =&gt; To) implicit def conforms[A]: A &lt;:&lt; A = new (A &lt;:&lt; A) {def apply(x: A) = x} // not in the &lt;:&lt; companion object because it is also intended to subsume identity (which is no longer implicit) </code></pre> <p>This uses the Scala feature that a generic type <code>op[T1, T2]</code> can be written <code>T1 op T2</code>. This can be used, as noted by aioobe, to provide an evidence parameter for methods that only apply to some instances of a generic type (the example given is the <code>toMap</code> method that can only be used on a <code>Traversable</code> of <code>Tuple2</code>). As noted in the comment, this generalizes a normal generic type constraint to allow it to refer to any in-scope abstract type/type parameter. Using this (<code>implicit ev : T1 &lt;:&lt; T2</code>) has the advantage over simply using an evidence parameter like (<code>implicit ev: T1 =&gt; T2</code>) in that the latter can lead to unintended in-scope implicit values being used for the conversion.</p> <p>I'm sure I'd seen some discussion on this on one of the Scala mailing lists, but can't find it at the moment.</p>
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    1. COWhat's the difference between <:< and =:= ?
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    2. CO@Eastsun `<:<` admits subclassing, while `=:=` doesn't.
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      1. USDaniel C. Sobral
    3. COAn implicit parameter `ev: T1 <:< T2` asserts that `T1` is a subtype of `T2`. An implicit parameter `ev: T1 =:= T2` asserts that they are the same type. See http://article.gmane.org/gmane.comp.lang.scala.user/18879 for an example of the latter.
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