Can Scala try-catch-finally expression be without braces? - scala

Question

I'm learning Scala and confused about the try-catch-finally syntax.
In Scala Syntax Specification, it says:
Expr1 ::= ‘try’ Expr [‘catch’ Expr] [‘finally’ Expr]
| ...
Can I write expression without { } blocks like this:
try
println("Hello")
catch
RuntimeException e => println("" + e)
finally
println("World")
Or the expression must be a block expression ?

Answer 1

Scala 3 (Dotty) is experimenting with optional braces (significant indentation) so the following works
scala> try
| 1 / 0
| catch
| case e => println(s"good catch $e")
| finally
| println("Celebration dance :)")
|
good catch java.lang.ArithmeticException: / by zero
Celebration dance :)
val res1: AnyVal = ()
where we note the handler
case e => println(s"good catch $e")
did not need braces as in Scala 2. In fact due to special treatment of case clauses after catch keyword the following would also work
scala> try
| 1 / 0
| catch
| case e => println(s"good catch $e")
| finally
| println("Celebration dance :)")
|
good catch java.lang.ArithmeticException: / by zero
Celebration dance :)
val res2: AnyVal = ()
where we note the handler did not have to be indented after catch
catch
case e => println(s"good catch $e")

Answer 2

Sure.
import scala.util.control.NonFatal
def fourtyseven: PartialFunction[Throwable, Int] = {
case NonFatal(_) => 47
}
def attempt(n: => Int): Int =
try n catch fourtyseven finally println("done")
println(attempt(42))
println(attempt(sys.error("welp")))
This compiles and runs as expected, although I had to define the catch expression separately (as it requires braces).
You can play around with this code here on Scastie.
A few of notes:
try/catch/finally is an expression that returns a value (this could be a bit unfamiliar if you're coming from Java) -- you can read more about it here on the docs
catch always takes a PartialFunction from a Throwable to some type, with the overall return type of the try/catch to be the closest common superclass of the two (e.g. say you have class Animal; class Dog extends Animal; class Cat extends Animal, if the try returns a Dog and the catch returns a Cat the overall expression will return an Animal)
I used the NonFatal extractor in the catch partial function, you can read more about it in this answer, while extractor objects in general are described here in the official docs