Hi Henry,
Yes, thanks -- I see that explicitly invoking a method that takes an
implicit Giver[T] works fine. But it's still unclear to me why the two-
step combination of
(1) implicitly available Giver[T]
(2) implicit method for extracting a T from an implicit Giver[T]
does not suffice for supplying an implicit T.
Any other notions?
best,
aaron
On Jul 17, 2008, at 9:59 PM, Henry Ware wrote:
> I don't ken the implicits system either, so I can't explain much.
>
> But, depending on what you are trying to do, this might help.
>
> trait Giver[T] { def give: T }
>
> implicit def IntGiver(i:Int)=new Giver[Int] { def give = 3 }
>
> def getGiver[T <% Giver[T]]:Giver[T] = null.asInstanceOf[T];
> def giveMeA[T <% Giver[T]]:T = getGiver[T].give;
> def main(a:Array[String])={
> Console.println("I have an int: " + giveMeA[Int])
> }
>
> -Henry Ware
>
> On Thu, Jul 17, 2008 at 9:07 AM, Aaron Harnly
> <
scala@...> wrote:
>> I'm puzzled as to why the following short example yields a
>> "diverging implicit
>> expansion" error. I placed the AnyVal bound on the type parameter T
>> specifically to prevent the compiler from worrying that it could
>> generate
>> Giver[Giver[Int]], Giver[Giver[Giver[Int]]], etc.
>>
>> Can someone who kens the implicits system better than I explain
>> what goes awry
>> here? much thanks, ~aaron
>>
>> trait Giver[T <: AnyVal] { def give: T }
>> implicit def fromGiver[T <: AnyVal](implicit giver: Giver[T]): T =
>> giver.give
>>
>> implicit object IntGiver extends Giver[Int] { def give = 3 }
>>
>> def anImplicit[T](implicit t: T) = t
>> Console.println("I have an int: " + anImplicit[Int])
>>
>> /**
>> The above yields this error:
>> [08:32 AM aaron@macbookpro: implicits] scala giver.scala
>>
>> (fragment of giver.scala):7: error: diverging implicit expansion
>> for type Int
>> starting with method fromGiver
>> Console.println("I have an int: " + anImplicit[Int])
>> ^
>> one error found
>> !!!
>> discarding <script preamble>
>>
>> */
>>
>>
