Why does ensuring work only on else?

Question

In scala, when I use the ensuring predef, it works only on the else part of an if-else expression:

def evenIt(x:Int) = {
    if(x % 2 == 0)
            x+1 //return odd on purpose!
    else{
      x + 1
    } ensuring( _ % 2 == 0)
}

//Test it:
evenIt(3)
> 4
evenIt(4)
> 5  //<--- ensuring does not catch this!

But I thought that "if-else" was an expression in scala. So it should just return a value - which in turn should be passed to ensuring?

Or am I confusing something here? Thanks.

EDIT: In the book Programming in Scala the author uses it as follows:

private def widen(x: Int) : Element =
   if(w <= width)
      this
   else {
      val left = elem(' ', (w - width) / 2, height)
      var right = elem(' ', w - width - left.width, height)
      left beside this beside right
   } ensuring ( w <= _.width

Does he apply it only to else part here?

Answer 1

Yes, if-else is an expression, but the way you bracketed it, you only apply ensuring to x+1, not to the if-expression. If you put the ensuring after the closing brace surrounding the if, it will do what you want:

def evenIt(x:Int) = {
    if(x % 2 == 0)
        x + 1 //return odd on purpose!
    else
        x + 1
} ensuring( _ % 2 == 0)