Scala type miss match

Question

I am fairly new to scala and I am doing my assignment. This is the code that I am using:

case EqNumC(l,r)  => (interp(l),interp(r)) match{
      case (NumV(s),NumV(x)) => if(s == x) BoolV(true) else BoolV(false)
      case _ => throw InterpException("Value not found!")
    }

When I run this code, it works fine and I get my result correctly. However this is somehow wrong. and I decided to make it better, by doing so:

case EqNumC(l,r)  => (interp(l),interp(r)) match{
      case (NumV(s),NumV(x)) => if(s==x) BoolV(true)
      case (NumV(_),NumV(_)) => BoolV(false)
      case _ => throw InterpException("Value not found!")
    }

However When I run this I get this error:

Status: CompilationFailure
solution.scala:129: error: type mismatch;
 found   : Unit
 required: Value
      case (NumV(s),NumV(x)) => if(s==x) BoolV(true)
                                ^

I don't see the problem here, because it's almost the same as the other one. How can I get rid of this problem and what is the best way to get this done?

Answer 1

The if control structure needs to come before the => when pattern-matching. When it comes after, the compiler sees an if without an else and infers it to be Unit (no return type).

It should look like this:

case (NumV(s),NumV(x)) if(s == x) => BoolV(true)

The first version works because if/else returns a value, but a single if does not.