Return value of a block

A common misunderstanding is that a code block (without parameters) is a function. That is not the case. A code block is a sequence of statements that are executed and result the last statement is returned. That sounds like a Function0, however, if the block is passed to a method/function only the last statement will be returned to the function/method. If that method/function expects a function as the parameter the last statement maybe returned as a function not a value, this means that the block itself is not a function.

1. scala> var count = 0                                                                                                                         
2. count: Int = 0
4. // the last statement is returned as a function so count
5. // is incremented only one during the creation of the function
6. scala> List(1,2,3,4).map{count += 1;_ + 1}
7. res9: List[Int] = List(2, 3, 4, 5)
9. scala> count
10. res10: Int = 1
12. // now the count increment is within the function
13. scala> List(1,2,3,4).map{i => count += 1;i + 1}
14. res11: List[Int] = List(2, 3, 4, 5)
16. scala> count
17. res12: Int = 5

The previous example demonstrates a Gotcha if I ever saw one. Map expects a function so the block essentially constructs a function. The last statement being the function. The first line count += 1 executed only once because it is part of creating the function not part of the resulting function. This is equivalent to:

1. scala> val x = {count += 1 ; i:Int => i +1}
2. x: (Int) => Int = < function1>
4. scala> List(1,2,3,4).map(x)
5. res15: List[Int] = List(2, 3, 4, 5)

Beginning a block with the parameter list signals that the entire block is a function.

Rule of thumb: Functions with placeholder parameters should be a single statement.