Scala Generic function assuming type

Question

When trying to define a generic method def f[T] (x:T) = x + 1 Scala gives below error

<console>:8: error: type mismatch; found : Int(1) required: String def f[T] (x:T) = x + 1 ^

The question is why Scala is assuming that it should be a String?

What if we want a function that can do +1 to Int, Char and String. I know I can do something like below, but it would work only on Int and Char.

def f[T <% Int](x:T) = x + 1

So what's the reason of this error and how to handle it in generic way.

kdgregory · Accepted Answer · 2014-02-05 12:50:00Z

The question is why Scala is assuming that it should be a String?

I can't guarantee this analysis, but it appears that Scala is applying the Predef.any2stringadd() implicit conversion to x, in an attempt to turn it into something that supports the + operator.

Here's a variant that will compile, and which demonstrates that implicit conversion:

scala> def f(x:Any) = { x + "1" } f: (x: Any)String scala> f("foo") res3: String = foo1 scala> f(123) res0: String = 1231 scala> f(classOf[String]) res2: String = class java.lang.String1

What if we want a function that can do +1 to Int, Char and String.

What does it mean to add 1 to any of these values?

If you simply want to invoke the + operator, then you need to use the match operator to select different behaviors depending on the actual type. This is because, while the name + is used for both, there's no common behavior between strings and numbers.

On the other hand, perhaps you want to deal with numbers that can be provided as either strings or numeric values (in which case, why Char?). To make that work, you need an implicit function that converts Any to a number.

scala> implicit def any2int(x:Any) : Int = { x.toString.toInt } warning: there were 1 feature warning(s); re-run with -feature for details any2int: (x: Any)Int scala> def f(x:Any) : Int = { x + 1 } f: (x: Any)Int scala> f(123) res0: Int = 124 scala> f("123") res1: Int = 124

I respectfully disagree that "there's no common behavior between strings and numbers"; the concept of a Monoid exists for a reason. :)

user64287 · Accepted Answer · 2015-04-23 19:13:40Z

The return type of the function will always be Int as I assume so it should be that the compiler choose Int but why String.

Collectives™ on Stack Overflow

Scala Generic function assuming type

2 Answers 2

1 Comment

Comments

Hot Network Questions