What is the difference between curly bracket and parenthesis in a Scala for loop?

Question

This is my first day of studying Scala (using the "Beginning Scala" book). When I read the for loops in Scala, there are 2 examples:

val books = List("Beginning Scala", "Beginning Groovy", "Beginning Java", "Scala in easy steps", "Scala in 24 hours")

[1]

for (book<-books if book.contains("Scala")) println(book)

[2]

for { book <- books
  bookVal = book.toUpperCase()
}
println(bookVal)

The thing that confused me is:

In [1] for uses parentheses "()" to wrap the loop control block while in [2] it uses curly braces "{}". I wonder if this is just different syntax but the same purpose or if they actually mean something different?

Thanks

Answer 1

Curly braces are usually used if you have multiline expression or expression which contains few other experession. If you're able (or want) to write in single line using semicolons you may use parentheses. Every for loop you can write with curly braces but using of parentheses is reduced.

If some other case curly braces allow you to use easier syntax to write partial function or pattern matching.

If you write in REPL following code:

for (
    i <- List(1,2,3)
    y = i * i
) yield y

it wouldn't compile e.g.