Expects a String?

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Lyle Kopnicky

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Mar 16, 2015, 2:10:32 AM3/16/15
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Hi folks,

What the heck is going on here? Why does this expect a string?

scala> def double[A: Numeric](a: A) = a + a
<console>:7: error: type mismatch;
 found   : A
 required: String
       def double[A: Numeric](a: A) = a + a
                                          ^

Thanks,
Lyle

Robert Norris

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Mar 16, 2015, 2:14:59 AM3/16/15
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Yeah, you're getting the + from Predef.any2stringadd which is awful ... it basically lets you add a String to anything at all and get a String back. What you want is the + from Numeric.Ops which you can get by importing Numeric.Implicits._ thus:

scala> def double[A: Numeric](a: A) = a + a
<console>:7: error: type mismatch;
found : A
required: String
def double[A: Numeric](a: A) = a + a
^

scala> import Numeric.Implicits._
import Numeric.Implicits._

scala> def double[A: Numeric](a: A) = a + a
double: [A](a: A)(implicit evidence$1: Numeric[A])A

scala> double(1)
res0: Int = 2

scala> double(1.2)
res1: Double = 2.4
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Lyle Kopnicky

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Mar 16, 2015, 2:27:28 AM3/16/15
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Thanks, Rob!

Yeah, I don’t see the use in Predef.any2stringadd when you have string interpolation, and the format method on Strings.

I’m surprised that Numeric would be included by default, but not the implicits. But then again, I guess it’s safer not to include the implicits.

If only that safety rule were followed with any2stringadd.

So much simpler in Haskell:

Prelude> let double n = n + n
Prelude> double 1
2
Prelude> double 1.2
2.4
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