Optional Braces

As an experimental feature, Scala 3 enforces some rules on indentation and allows some occurrences of braces {...} to be optional.

Indentation Rules

The compiler enforces two rules for well-indented programs, flagging violations as warnings.

  1. In a brace-delimited region, no statement is allowed to start to the left of the first statement after the opening brace that starts a new line.

    This rule is helpful for finding missing closing braces. It prevents errors like:

    if (x < 0) {
      println(1)
      println(2)
    
    println("done")  // error: indented too far to the left
    
  2. If significant indentation is turned off (i.e. under Scala-2 mode or under -noindent) and we are at the start of an indented sub-part of an expression, and the indented part ends in a newline, the next statement must start at an indentation width less than the sub-part. This prevents errors where an opening brace was forgotten, as in

    if (x < 0)
      println(1)
      println(2)   // error: missing `{`
    

These rules still leave a lot of leeway how programs should be indented. For instance, they do not impose any restrictions on indentation within expressions, nor do they require that all statements of an indentation block line up exactly.

The rules are generally helpful in pinpointing the root cause of errors related to missing opening or closing braces. These errors are often quite hard to diagnose, in particular in large programs.

Optional Braces

The compiler will insert <indent> or <outdent> tokens at certain line breaks. Grammatically, pairs of <indent> and <outdent> tokens have the same effect as pairs of braces { and }.

The algorithm makes use of a stack IW of previously encountered indentation widths. The stack initially holds a single element with a zero indentation width. The current indentation width is the indentation width of the top of the stack.

There are two rules:

  1. An <indent> is inserted at a line break, if

    • An indentation region can start at the current position in the source, and
    • the first token on the next line has an indentation width strictly greater than the current indentation width

    An indentation region can start

    • after the condition of an if-else, or
    • after the leading parameter(s) of a given extension method clause, or
    • after one of the following tokens:
    =  =>  <-  if  then  else  while  do  try  catch  finally  for  yield  match  return  with
    

    If an <indent> is inserted, the indentation width of the token on the next line is pushed onto IW, which makes it the new current indentation width.

  2. An <outdent> is inserted at a line break, if

    • the first token on the next line has an indentation width strictly less than the current indentation width, and
    • the first token on the next line is not a leading infix operator.

    If an <outdent> is inserted, the top element if popped from IW. If the indentation width of the token on the next line is still less than the new current indentation width, step (2) repeats. Therefore, several <outdent> tokens may be inserted in a row.

    An <outdent> is also inserted if the next statement following a statement sequence starting with an <indent> closes an indentation region, i.e. is one of then, else, do, catch, finally, yield, } or case.

It is an error if the indentation width of the token following an <outdent> does not match the indentation of some previous line in the enclosing indentation region. For instance, the following would be rejected.

if x < 0
    -x
  else   // error: `else` does not align correctly
     x

Indentation tokens are only inserted in regions where newline statement separators are also inferred: at the toplevel, inside braces {...}, but not inside parentheses (...), patterns or types.

New Role of With

To make bracews optional for constructs like class bodies, the syntax of the language is changed so that a class body or similar construct may optionally be prefixed with with. Since with can start an indentation region, this means that all of the following syntaxes are allowed and are equivalent:

trait A {
  def f: Any
}
class C(x: Int) extends A {
  def f = x
}
type T = A {
  def f: Int
}

trait A with {
  def f: Int
}
class C(x: Int) extends A with {
  def f = x
}
type T = A with {
  def f: Int
}

trait A with
  def f: Int

class C(x: Int) extends A with
  def f = x

type T = A with
  def f: Int

The syntax changes allowing this are as follows:

TemplateBody ::=  [‘with’] ‘{’ [SelfType] TemplateStat {semi TemplateStat} ‘}’
EnumBody     ::=  [‘with’] ‘{’ [SelfType] EnumStat {semi EnumStat} ‘}’
Packaging    ::=  ‘package’ QualId [‘with’] ‘{’ TopStatSeq ‘}’
RefinedType  ::=  AnnotType {[‘with’] Refinement}

It is assumed here that braces following a with can be transparently replaced by an indentation region.

With the new indentation rules, the previously allowed syntax

class A extends B with
                C

becomes illegal since C above would be terated as a nested statement inside A. More generally, a with that separates parent constructors cannot be at the end of a line. One has to write

class A extends B
           with C

instead (or replace the "with" by a ","). When compiling in Scala-2 mode, a migration warning is issued for the illegal syntax and a (manual) rewrite is suggested.

Spaces vs Tabs

Indentation prefixes can consist of spaces and/or tabs. Indentation widths are the indentation prefixes themselves, ordered by the string prefix relation. So, so for instance "2 tabs, followed by 4 spaces" is strictly less than "2 tabs, followed by 5 spaces", but "2 tabs, followed by 4 spaces" is incomparable to "6 tabs" or to "4 spaces, followed by 2 tabs". It is an error if the indentation width of some line is incomparable with the indentation width of the region that's current at that point. To avoid such errors, it is a good idea not to mix spaces and tabs in the same source file.

Indentation and Braces

Indentation can be mixed freely with braces. For interpreting indentation inside braces, the following rules apply.

  1. The assumed indentation width of a multiline region enclosed in braces is the indentation width of the first token that starts a new line after the opening brace.

  2. On encountering a closing brace }, as many <outdent> tokens as necessary are inserted to close all open indentation regions inside the pair of braces.

Special Treatment of Case Clauses

The indentation rules for match expressions and catch clauses are refined as follows:

The rules allow to write match expressions where cases are not indented themselves, as in the example below:

x match
case 1 => print("I")
case 2 => print("II")
case 3 => print("III")
case 4 => print("IV")
case 5 => print("V")

println(".")

The End Marker

Indentation-based syntax has many advantages over other conventions. But one possible problem is that it makes it hard to discern when a large indentation region ends, since there is no specific token that delineates the end. Braces are not much better since a brace by itself also contains no information about what region is closed.

To solve this problem, Scala 3 offers an optional end marker. Example

def largeMethod(...) =
    ...
    if ... then ...
    else
        ... // a large block
    end if
    ... // more code
end largeMethod

An end marker consists of the identifier end which follows an <outdent> token, and is in turn followed on the same line by exactly one other token, which is either an identifier or one of the reserved words

if  while  for  match  try  new

If end is followed by a reserved word, the compiler checks that the marker closes an indentation region belonging to a construct that starts with the reserved word. If it is followed by an identifier id, the compiler checks that the marker closes a definition that defines id or a package clause that refers to id.

end itself is a soft keyword. It is only treated as an end marker if it occurs at the start of a line and is followed by an identifier or one of the reserved words above.

It is recommended that end markers are used for code where the extent of an indentation region is not immediately apparent "at a glance". Typically this is the case if an indentation region spans 20 lines or more.

Example

Here is a (somewhat meta-circular) example of code using indentation. It provides a concrete representation of indentation widths as defined above together with efficient operations for constructing and comparing indentation widths.

enum IndentWidth with
  case Run(ch: Char, n: Int)
  case Conc(l: IndentWidth, r: Run)

  def <= (that: IndentWidth): Boolean = this match
    case Run(ch1, n1) =>
      that match
        case Run(ch2, n2) => n1 <= n2 && (ch1 == ch2 || n1 == 0)
        case Conc(l, r)   => this <= l
    case Conc(l1, r1) =>
      that match
        case Conc(l2, r2) => l1 == l2 && r1 <= r2
        case _            => false

  def < (that: IndentWidth): Boolean =
    this <= that && !(that <= this)

  override def toString: String = this match
    case Run(ch, n) =>
      val kind = ch match
        case ' '  => "space"
        case '\t' => "tab"
        case _    => s"'$ch'-character"
      val suffix = if n == 1 then "" else "s"
      s"$n $kind$suffix"
    case Conc(l, r) =>
      s"$l, $r"

object IndentWidth with
  private inline val MaxCached = 40

  private val spaces = IArray.tabulate(MaxCached + 1)(new Run(' ', _))
  private val tabs = IArray.tabulate(MaxCached + 1)(new Run('\t', _))

  def Run(ch: Char, n: Int): Run =
    if n <= MaxCached && ch == ' ' then
      spaces(n)
    else if n <= MaxCached && ch == '\t' then
      tabs(n)
    else
      new Run(ch, n)
  end Run

  val Zero = Run(' ', 0)
end IndentWidth

Settings and Rewrites

Significant indentation is enabled by default. It can be turned off by giving any of the options -noindent, old-syntax and language:Scala2. If indentation is turned off, it is nevertheless checked that indentation conforms to the logical program structure as defined by braces. If that is not the case, the compiler issues a warning.

The Dotty compiler can rewrite source code to indented code and back. When invoked with options -rewrite -indent it will rewrite braces to indented regions where possible. When invoked with with options -rewrite -noindent it will rewrite in the reverse direction, inserting braces for indentation regions. The -indent option only works on new-style syntax. So to go from old-style syntax to new-style indented code one has to invoke the compiler twice, first with options -rewrite -new-syntax, then again with options -rewrite-indent. To go in the opposite direction, from indented code to old-style syntax, it's -rewrite -noindent, followed by -rewrite -old-syntax.

Variant: Indentation Marker :

Generally, the possible indentation regions coincide with those regions where braces {...} are also legal, no matter whether the braces enclose an expression or a set of definitions. There is one exception, though: Arguments to function can be enclosed in braces but they cannot be simply indented instead. Making indentation always significant for function arguments would be too restrictive and fragile.

To allow such arguments to be written without braces, a variant of the indentation scheme is implemented under option -Yindent-colons. This variant is more contentious and less stable than the rest of the significant indentation scheme. In this variant, a colon : at the end of a line is also one of the possible tokens that opens an indentation region. Examples:

times(10):
  println("ah")
  println("ha")

or

xs.map:
  x =>
      val y = x - 1
      y * y

Colons at the end of lines are their own token, distinct from normal :. The Scala grammar is changed in this variant so that colons at end of lines are accepted at all points where an opening brace enclosing a function argument is legal. Special provisions are taken so that method result types can still use a colon on the end of a line, followed by the actual type on the next.