Escape

Escape is one category of instruction that allows control to dramatically leap from end of a program to another.

Jumps and Branches

The grandfather of all jumps is the goto command that allow programmers to specify the label of the next instruction that should be executed:

start: if (x > 10) goto end // a branch or conditional goto
x = x + 1
println("x = " + x)
goto start                  // a jump or unconditional goto
end: println("done")

Once the only way to alter sequence control, the goto is now much discredited for its ability to lead to spaghetti code. It is not included in Java or Scala.

Call/Return

The ability to call procedures and functions and have them return values to the caller enabled programmers to create reusable, custom instructions.

Format:

operator(operands) // calling a function
return result      // returning a value

Example:

result = sqrt(49)

Catch/Throw

Format:

try {
   // call risky functions in the try-block
} catch { // catch-block
   case thrown: Type1 => Branch1
   case thrown: Type2 => Branch2
   case thrown: Type3 => Branch3
   // etc.
   case thrown: Throwable => default
} finally {
   // finally-block, do this if an exception was thrown or not
}

Example:

try {
   var cmmd = readLine("-> ")
   cmmd match {
      case "deposit" =>
      case "withdraw" =>
      case _ => throw new BadCommandException
   }
} catch {
   case e:NegativeAmountException => println("amount must be positive")
   case e:InsufficientFundsException => println("Insufficient funds")
   case e: BadCommandException => println("Invalid command, type help")
   case e: ATMException => println("Command failed")
   case e: NumberFormatException => println("amount must be a number")
   case _: Throwable => {
      println("fatal error");
      more = false
   }
} finally {
   println("...")
   Console.flush()
}

Break and Continue

In Java (C, C++) there are two ways to escape from a loop:

break; // end loop and go on to the next command

continue; // go back to the top of the loop

For example, the loop:

for(int i = 0; i < 100; i++) {
   if (i % 3 == 0) continue; // skip if i divisible by 3
   if (i == 10) break; // terminate loop when i is 10
   System.out.println("i = " + i);
}
System.out.println("done");

Should print:

i = 0
i = 1
i = 2
i = 4
i = 5
i = 7
i = 8
done

Scala Break Blocks

Although break and continue aren't in the Scala, a more general form of break can be imported from the scala library:

import scala.util.control.Breaks._

object TestBreak {
   def main(args: Array[String]): Unit = {
     println("entering main")
     breakable {
       println("A")
       breakable {
         println("B")
         break
         println("C")
       }
       println("D")
       break
       println("E")
     }
     println("exiting main")
   }
}

Output:

entering main
A
B
D
exiting main