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    <p>It is not possible in scala to require an implicit conversion to exist for a type parameter of a trait. There is a good reason for this. Suppose we defined a trait like:</p> <pre><code>trait ATrait[T &lt;% Int] { def method(v: T) { println(v: Int) } } </code></pre> <p>And then made instances of it in two places:</p> <pre><code>package place1 { implicit def strToInt(s: String) = 5 val inst = new ATrait[String] } package place2 { implicit def strToInt(s: String) = 6 val inst = new ATrait[String] } </code></pre> <p>And then used these instances like:</p> <pre><code>val a = if (someTest) place1 else place2 a.method("Hello") </code></pre> <p>Should this print <code>5</code> or <code>6</code>? That is, which implicit conversion should it use? Implicits have to be found at compile time, but you don't know which implicit conversion was present for the creation of the object.</p> <p>In other words, implicits are provided by the scope in which they are used, not by the objects they are used on; the latter would be impossible.</p> <p>So, about your problem. Instead of using an implicit, you could use an ordinary member:</p> <pre><code>trait Summable[T] { def -= (v: T): Unit def -= (v: Int) { this -= (encode(v)) } def encode(i: Int): T } class Int4 (var x: Int, var y: Int, var z: Int, var w: Int) extends Summable[Int4] { def -= (v : Int4) : Unit = { x -= v.x; y -= v.y; z -= v.z; w -= v.w } def encode(i: Int) = Int4.int2Int4(i) } </code></pre> <p>Now the <code>decrement</code> method compiles correctly.</p> <p>Another way of saying this is, don't think of implicits as properties belonging to a type (ie, "can be implicitly converted from an Int" isn't a property of Int4). They are values, which can be identified using types.</p> <p>Hope this helps.</p>
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    1. POIn Scala, how do i tell an abstract base class that type parameter T supports implicit conversion from Int (or Float, or...)?
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    1. COI don't follow your objection to implicits on traits. Implicits on traits are not possible because traits do not support construtors. Classes can and often do have implicits, including ones such as requested here -- so why can classes have it but not traits (besides lacking constructors)?
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      1. USDaniel C. Sobral
    2. CO@Daniel exactly that. Because the class has a constructor the implicit becomes a parameter, and so becomes a member. With a trait all members are declared explicitly. Basically my point is that without a member you can't carry the implicit value around, so it can't appear in the signature as [T <% S]. And even a class with an implicit parameter, people using an instance of the class won't have it automatically in scope.
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