Implicit function in Scala not working as expected

Question

If I define the following function which expects a Long, and I define the following implicit function, the implicit function is used when I pass a Date to the first function and everything works as expected:

def millisToDays(in: Long): Int = (in / (1000L * 3600L * 24L)).toInt
implicit def dateToLong(d: Date) = d.getTime
println(millisToDays(new Date))

But for the following second example, I get a compiler error on the third line: "inferred type arguments [Int] do not conform to method mySum's type parameter bounds [t <: java.lang.Number]"

def mySum[T <: Number](as: T*): Double = as.foldLeft(0d)(_ + _.doubleValue)
implicit def intToInteger(n: Int): Integer = new Integer(n.toInt)
var r = mySum(2, 3)

What have I done wrong? Why isn't the intToInteger implicit function being used?

I am guessing that the problem is that the implicit function does not return a "T <: Number", but rather an Integer, so the compiler can't guess that the implicit function is actually useful.

Is there anyway which I can give the compiler a hint that it should use the implicit function?

Thanks! Ant

Dan Simon · Accepted Answer · 2012-01-03 21:21:16Z

5

The [T <: Number] type bounds means that T must be a subtype of Number. The implicit conversion from Int to Integer doesn't give you this, since even though the conversion is available it still doesn't mean that Int is a subtype of Number.

Luckily, there is something similar called view bounds, written [T <% Number], which specifies exactly what you want, that there is an implicit conversion available from T to Number

answered Jan 3, 2012 at 21:21

Dan Simon

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4 Comments

hellectronic Over a year ago

If you do this, then your custom implicit intToInteger can be deleted, because Scala does it for you.

Ant Kutschera Over a year ago

almost, but no cigar: I get a different error, when I use this function definition and no implicit: "def mySum[T <% Number](as: T*): Double = as.foldLeft(0d)(_ + _.doubleValue)". The new error is: ambiguous implicit values: both method int2Integer in object Predef of type (x: Int)java.lang.Integer and method int2bigInt in object BigInt of type (i: Int)scala.math.BigInt match expected type Int => java.lang.Number

Dan Simon Over a year ago

Are you importing scala.math.BigInt._? I was able to reproduce your error after doing that import. The error is occurring because BigInt contains its own implicit conversion that conforms to Int => Number, and the compiler doesn't know which one to use.

Ant Kutschera Over a year ago

yes, removing the BigInt wildcard import fixes it. I just found this link (scala-lang.org/node/136) which states <: means subtype - the book im reading says subtype OR same type... thanks for the answer - ill accept it :-)

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