What is the formal difference in Scala between braces and parentheses, and when should they be used?

Question

What is the formal difference between passing arguments to functions in parentheses () and in braces {}?

The feeling I got from the Pro

Answer 1

There are a couple of different rules and inferences going on here: first of all, Scala infers the braces when a parameter is a function, e.g. in list.map(_ * 2) the braces are inferred, it's just a shorter form of list.map({_ * 2}). Secondly, Scala allows you to skip the parentheses on the last parameter list, if that parameter list has one parameter and it is a function, so list.foldLeft(0)(_ + _) can be written as list.foldLeft(0) { _ + _ } (or list.foldLeft(0)({_ + _}) if you want to be extra explicit).

However, if you add case you get, as others have mentioned, a partial function instead of a function, and Scala will not infer the braces for partial functions, so list.map(case x => x * 2) won't work, but both list.map({case x => 2 * 2}) and list.map { case x => x * 2 } will.