Question

There are a number of times I’ve run into a simple pattern when programming in Java or C++ for which a custom control structure could reduce the boilerplate within my code. It goes something like:

 if( Predicate ){
     Action

     return Value
 }

that is, a “return if”-type statement. I’ve tried making functions with signature like foo[A,B]( pred:((A,A)=>Boolean), value:Option[B] ) but then I wind up checking if I’ve returned Some or None. I’m tripped up by the return statement.

Is there an inherit way of making such control structures in functional languages or more specifically Scala?

Edit:

I was not as clear with my description and it’s confusing people who are trying to help me. The key reason my foo doesn’t work is that it can’t short-circuit the evaluation of the containing function. That is

def intersect( geometry:Geometry, reference:Geometry ):Geometry = {
    return_if( withinBounds( geometry, projection ), logToString( logger, "Geometry outside " + projection.toString ), EmptyGeometry() )
    return_if( topologicallyCorrect( geometry ), intersect( correct( geometry ), reference )
    //rest of the function
}

and still allow for tail recursion within the return_if.

Answer 1

Hmm, as far as I understand it you want the return in the control structure to exit the function it is embedded in.

So in your example it should exit the method intersect?

I am not sure if thats possible. Because a return inside the return_if will always exit return_if and I don’t think there is a way to tell scala, that the return should exit the function return_if is embedded in.

I hope I understood what you wanted to do 🙂