Why implicit conversion doesn't work in Lists? - scala

Question

Could someone give a quick explanation why implicit conversion doesn't work in these cases? Thanks.
scala> implicit def strTwoInt (s: String):Int = s.toCharArray.map{_.asDigit}.sum
strTwoInt: (s: String)Int
scala> List[Int]("1","2","3") sum
res3: Int = 6
scala> List("1","2","3") sum
<console>:9: error: could not find implicit value for parameter num: Numeric[java.lang.String]
List("1","2","3") sum
scala> val a = List("1","2","3")
scala> a.foldLeft(0)((i:Int, j:Int) => i+j)
<console>:10: error: type mismatch;
found : (Int, Int) => Int
required: (Int, java.lang.String) => Int

Answer 1

Your implicit conversion converts a String in an Int. In your first example, it is triggered by the fact that you try to put Strings into a List of Ints.
In your second example, you have a List of String and you call the method sum, which takes an implicit Numeric[String]. Your conversion does not apply because you neither try to pass a String somewhere the compiler was expecting an Int, nor you tried to call a method which is defined in Int and not in String. In that case, you can either define a Numeric[String] which use explicitly your conversion, or use a method which takes a List[Int] as parameter (giving hint to the compiler):
scala> def sumIt( xs: List[Int] ) = xs sum
sumIt: (xs: List[Int])Int
scala> sumIt( List("1","2","3") )
res5: Int = 6
In the third example, the foldLeft second argument must be of type:
(Int,String) => Int
the one you actually passed is of type:
(Int,Int) => Int
however, you did not define any implicit conversion between these two types. But:
a.foldLeft(0)((i:Int, j:String) => i+j)
triggers your conversion and works.
Edit: Here's how to implement the Numeric[String]:
implicit object StringNumeric extends math.Numeric[String] {
val num = math.Numeric.IntIsIntegral
def plus(x: String, y: String) = num.plus(x,y).toString
def minus(x: String, y: String) = num.minus(x,y).toString
def times(x: String, y: String) = num.times(x,y).toString
def negate(x: String): String = num.negate(x).toString
def fromInt(x: Int) = x.toString
def toInt(x: String) = x
def toLong(x: String) = toInt(x)
def toFloat(x: String) = toInt(x)
def toDouble(x: String) = toInt(x)
def compare(x:String,y:String) = num.compare(x,y)
}
scala> List("1","2","3") sum
res1: java.lang.String = 6
It works, but the result is a String.

Answer 2

Here's a quick explanation: implicit conversions apply to the types the convert from and to directly (e.g., String and Int in your case), and not to any parametrized type T[String] or T[Int] — unless implicit conversions have been defined for T itself, and it is not the case for lists.
Your implicit conversion does not apply in your two cases (and even if you had an implicit conversion from List[String] to List[Int], it wouldn't apply). It would automatically applied only when you need a value of type Int and you're passing String instead. Here, in the first case, the method sum asks for a Numeric[String] implicit parameter — an implicit conversion from String to Int does not come into play here.
Similar problem for your next attempt: the foldLeft on your collection requires a function of type (Int, String) => Int. Imagine the mess we would get into if, based on an implicit conversion from String to Int, the compiler automatically provided an implicit conversion from (Int, Int) => Int to (Int, String) => Int…
For all these cases, the easy way to fix it is to explicitly call .map(stringToInt) on your collection beforehand.