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    <p>Hope the following somewhat contrived example will clarify the whole matter a bit.</p> <p>Consider this code (which corresponds to your current one):</p> <pre><code>object Main { def main(args: Array[String]) { implicit val intIntable = new Intable[Int] { def toInt(obj: Int) = obj } println(squareAndDouble(10: Int)) } // Corresponds to your register[T] function def squareAndDouble[T](obj: T) = { val d = double[T](obj) d*d } // Corresponds to your extractEntity[T] function def double[T: Intable](obj: T) = { implicitly[Intable[T]].toInt(obj)*2 } } trait Intable[T] { def toInt(obj: T): Int } </code></pre> <p>This code does not compile with the following error:</p> <pre><code>test.scala:11: error: could not find implicit value for evidence parameter of type Intable[T] val d = double[T](obj) </code></pre> <p>Now let's rewrite it using implicit parameters (that's what the compiler do for us when it sees context bound):</p> <pre><code>object Main { def main(args: Array[String]) { implicit val intIntable = new Intable[Int] { def toInt(obj: Int) = obj } println(squareAndDouble(10: Int)) } def squareAndDouble[T](obj: T) = { val d = double[T](obj) d*d } def double[T](obj: T)(implicit intable: Intable[T]) = { intable.toInt(obj)*2 } } trait Intable[T] { def toInt(obj: T): Int } </code></pre> <p>This program does not compile as well:</p> <pre><code>test.scala:11: error: could not find implicit value for parameter intable: Intable[T] val d = double[T](obj) </code></pre> <p>And now it should be more obvious why it does not compile. It fails simply because there is no implicit value ('evidence') for <code>T</code> type parameter inside <code>squareAndDouble</code> function which is required by <code>square</code> function. Since <code>double</code> function need an evidence for its type parameter (which can be anything) the only way for this evidence to appear in scope inside <code>squareAndDouble</code> is from an implicit parameter again, but now in <code>squareAndDouble</code> function.</p> <p>Basically, that means that you have to 'pass through' the evidence for your generic parameter in order to use the typeclass.</p> <p>Let's fix it:</p> <pre><code>object Main { def main(args: Array[String]) { implicit val intIntable = new Intable[Int] { def toInt(obj: Int) = obj } println(squareAndDouble(10: Int)) } // Added context bound here def squareAndDouble[T: Intable](obj: T) = { val d = double[T](obj) d*d } def double[T: Intable](obj: T) = { implicitly[Intable[T]].toInt(obj)*2 } } trait Intable[T] { def toInt(obj: T): Int } </code></pre> <p>Now it compiles and runs successfully:</p> <pre><code>% scalac test.scala % scala Main 400 </code></pre> <p>Another (may be more simple) way to understand the problem is as follows. Our <code>squareAndDouble[T]</code> function accepts unbounded type parameter <code>T</code>, but then it tries to call another function, <code>double[T: Intable]</code>, whose type parameter is bounded. This is not allowed: if it was legal, then it would be possible to call <code>squareAndDouble[T]</code> with any type, even with that which does not have <code>Intable</code> typeclass instance, and this would consequently break <code>double</code> function. Hence we have to add <code>Intable</code> boundary to <code>squareAndDouble</code> function too in order for it to compile: <code>squareAndDouble[T: Intable]</code>, and then it works perfectly.<br> This is in fact very similar to upper/lower bounds which work with class inheritance instead of implicit values.</p>
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    1. USVladimir Matveev
    1. USVladimir Matveev
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    1. COthanks a lot, such a stupid mistake took away more then to hours =( Shame on me
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