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

User · Answer

This topic is actually a little more complicated as described in the answers so far. This blogpost by Rob Norris explains it in more detail and gives examples on when using return will actually break your code (or at least have non-obvious effects).

At this point let me just quote the essence of the post. The most important statement is right in the beginning. Print this as a poster and put it to your wall :-)

The return keyword is not “optional” or “inferred”; it changes the meaning of your program, and you should never use it.

It gives one example, where it actually breaks something, when you inline a function

// Inline add and addR
def sum(ns: Int*): Int = ns.foldLeft(0)((n, m) => n + m) // inlined add

scala> sum(33, 42, 99)
res2: Int = 174 // alright

def sumR(ns: Int*): Int = ns.foldLeft(0)((n, m) => return n + m) // inlined addR

scala> sumR(33, 42, 99)
res3: Int = 33 // um.

because

A return expression, when evaluated, abandons the current computation and returns to the caller of the method in which return appears.

This is only one of the examples given in the linked post and it's the easiest to understand. There're more and I highly encourage you, to go there, read and understand.

When you come from imperative languages like Java, this might seem odd at first, but once you get used to this style it will make sense. Let me close with another quote:

If you find yourself in a situation where you think you want to return early, you need to re-think the way you have defined your computation.

User · Answer

By default the last expression of a function will be returned  In your example there is another expression after the point  where you want your return value  If you want to return anything prior to your last expression  you still have to use return   You could modify your example like this  to return a Boolean from the first part  def balanceMain elem  List Char    Boolean       if  elem isEmpty              is a Boolean resulting function as well  so your can write it this way     count    0     else          keep the rest in this block  the last value will be returned as well     if  elem head                 balanceMain elem tail  open  count   1               some more statements                just don t forget your Boolean in the end     someBoolExpression

User · Answer

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

User · Answer

I don t program Scala  but I use another language with implicit returns  Ruby   You have code after your if  elem isEmpty  block -- the last line of code is what s returned  which is why you re not getting what you re expecting   EDIT  Here s a simpler way to write your function too  Just use the boolean value of isEmpty and count to return true or false automatically   def balanceMain elem  List Char    Boolean         elem isEmpty  amp  amp  count    0

User · Answer

Don t write if statements without a corresponding else  Once you add the else to your fragment you ll see that your true and false are in fact the last expressions of the function   def balanceMain elem  List Char    Boolean           if  elem isEmpty        if  count    0          true       else         false     else       if  elem head                 balanceMain elem tail  open  count   1        else

User · Answer

Use case match for early return purpose   It will force you to declare all return branches explicitly  preventing the careless mistake of forgetting to write return somewhere

[scala] Return in Scala

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