New Control Syntax

Scala 3 has a new "quiet" syntax for control expressions that does not rely on enclosing the condition in parentheses, and also allows to drop parentheses or braces around the generators of a for-expression. Examples:

if x < 0 then
  "negative"
else if x == 0 then
  "zero"
else
  "positive"

if x < 0 then -x else x

while x >= 0 do x = f(x)

for x <- xs if x > 0
yield x * x

for
  x <- xs
  y <- ys
do
  println(x + y)

try body
catch case ex: IOException => handle

The rules in detail are:

• The condition of an if-expression can be written without enclosing parentheses if it is followed by a then.
• The condition of a while-loop can be written without enclosing parentheses if it is followed by a do.
• The enumerators of a for-expression can be written without enclosing parentheses or braces if they are followed by a yield or do.
• A do in a for-expression expresses a for-loop.
• A catch can be followed by a single case on the same line. If there are multiple cases, these have to appear within braces (just like in Scala 2) or an indented block.

Rewrites

The Scala 3 compiler can rewrite source code from old syntax to new syntax and back. When invoked with options -rewrite -new-syntax it will rewrite from old to new syntax, dropping parentheses and braces in conditions and enumerators. When invoked with options -rewrite -old-syntax it will rewrite in the reverse direction, inserting parentheses and braces as needed.