Return in Scala

Question

I am a newbie scala programmer and came across a weird behavior.

def balanceMain(elem: List[Char]): Boolean =   {     if (elem.isEmpty)       if (count == 0)         true;       else false;      if (elem.head == '(')       balanceMain(elem.tail, open, count + 1);.... 

Above basically I want to return true if elem.isEmpty and count == 0. Otherwise, I want to return false.

Now above I have read that there is no need to add a return statement in scala. So I have omitted return above. But it doesn't return the boolean. If I add a return statement as return true. it works perfectly. Why is it so?

Also, why is it considered a bad practice to have return statements in scala

Answer 1

It's not as simple as just omitting the return keyword. In Scala, if there is no return then the last expression is taken to be the return value. So, if the last expression is what you want to return, then you can omit the return keyword. But if what you want to return is not the last expression, then Scala will not know that you wanted to return it.

An example:

def f() = {   if (something)     "A"   else     "B" } 

Here the last expression of the function f is an if/else expression that evaluates to a String. Since there is no explicit return marked, Scala will infer that you wanted to return the result of this if/else expression: a String.

Now, if we add something after the if/else expression:

def f() = {   if (something)     "A"   else     "B"    if (somethingElse)     1   else     2 } 

Now the last expression is an if/else expression that evaluates to an Int. So the return type of f will be Int. If we really wanted it to return the String, then we're in trouble because Scala has no idea that that's what we intended. Thus, we have to fix it by either storing the String to a variable and returning it after the second if/else expression, or by changing the order so that the String part happens last.

Finally, we can avoid the return keyword even with a nested if-else expression like yours:

def f() = {   if(somethingFirst) {     if (something)      // Last expression of `if` returns a String      "A"     else      "B"   }   else {     if (somethingElse)       1     else       2      "C"                // Last expression of `else` returns a String   } 

}