Question

Cutting right to the chase

“Convert” int to String

 implicit def int2string(i: Int): String = {
    "foo"
  }

Method that takes a String and prints

def printString(i: String) = print(i)

Calling printString with an Int argument

printString(_:Int)

Shouldn’t that display “foo”? However printString(i:String) never gets called.

printString(1) prints “foo”

Is there a problem here or I’m missing something?

Answer 1

That’s because what printString(_:Int) actually what it does is to turn that expression in a function that takes a Int and probably is never invoked… See:

scala> implicit def int2string(i: Int): String = "foo"
int2string: (i: Int)String

scala> def printString(i: String) = print(i)
printString: (i: String)Unit

No syntax error here mean it is working. As an illustration:

scala> printString(_:Int)       // Function from Int to Unit 
res0: Int => Unit = <function1>  

The compiler turns the outer expression into { x:Int => printString(x) }, and then applies the implicit conversion since the implicit is in scope, so the outcome is { x:Int => printString(int2string(x)) }.

A non-working one, since there is no conversion from Object to String:

scala> printString(_:Object)
<console>:10: error: type mismatch;
 found   : java.lang.Object
 required: String
              printString(_:Object)

Now to actually see the printing we need to invoke it:

scala> val foo = printString(_:Int)
foo: Int => Unit = <function1>

scala> foo(5)
foo