[scala] Why is this a diverging implicit?

[Aaron Harnly]

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>

*/

[Henry Ware-2]

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>
>
> */
>
>

[Aaron Harnly]

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>
>>
>> */
>>
>>