Indentation Syntax

x2c can be written without braces or semicolons. Indentation groups the statements of a block, a line that opens a block ends with :, and a line break ends a statement. Name the file .xp, or put #pragma indent at the top of a .x file. The two spellings mean the same program, so an indented file can include, import, and call brace-form code and the reverse.

Here is a small program in brace form:

typedef enum Shape { CIRCLE, SQUARE } Shape;

double area(Shape shape, double size) {
  switch (shape) {
    case CIRCLE: return 3.14159 * size * size;
    default: return size * size;
  }
}

int main(void) {
  double total = 0;
  for (int i = 1; i <= 3; i++) {
    if (i == 2) continue;
    total += area(i == 1 ? CIRCLE : SQUARE, i);
  }
  printf("%.2f\n", total);
  return 0;
}

12.14

The same program in the indentation syntax:

#pragma indent

typedef enum Shape:
  CIRCLE,
  SQUARE
Shape

double area(Shape shape, double size):
  switch shape:
    case CIRCLE: return 3.14159 * size * size
    default: return size * size

int main(void):
  double total = 0
  for (int i = 1; i <= 3; i++):
    if i == 2: continue
    total += area(i == 1 ? CIRCLE : SQUARE, i)
  printf("%.2f\n", total)
  return 0

12.14

Conditions need no parentheses, except that a for header keeps its three clauses in them, and a short body can follow its colon on the same line. foreach separates its declaration from the collection with in. The enum keeps its commas, and its name after the dedent completes the typedef.

Long lines

A line break inside parentheses, brackets, or braces never ends a statement. Outside them, a line indented deeper than the one before it continues that line. A line that starts with . continues a method chain:

#pragma indent

int main(void):
  List words = %("indentation" "groups" "blocks")
  int letters = 0
  foreach String word in words:
    letters += word.len()
  String report = %"${words.len()} words, " +
    %"$letters letters"
  List lengths = words
    .map(%!(Var word) => word.str().len())
  printf("%s: %s\n", report, lengths.str())
  return 0

3 words, 23 letters: ( 11 6 6 )

Scopes and decorators

do: opens a bare scope when its block is not followed by a while line, which is how an indented macro keeps its temporaries to itself. A decorator line starts with @, so the layout knows it wraps the next statement rather than ending with ;:

#pragma indent
$(import "system-macros.xmacro")
#include <time.h>

int main(void):
  int total = 0
  do:
    int step = 2
    total += step
  @$time("sum")
  for (int i = 0; i < 1000; i++):
    total += i
  printf("%d\n", total)
  return 0

Converting a file

tools/indent-convert FILE... rewrites brace-form files in place, adding #pragma indent so each keeps its name; an executable script stays runnable by name. Every block header, including one whose body had no braces, ends with a colon. The converter proves each conversion by comparing the compiler’s AST for the result with the original’s, and it leaves a file unchanged when they differ, as they do when indentation misrepresents which statement an else or a body belongs to. --check reports without writing.

A .xpmacro file writes macros the same way. The language reference lists every layout rule. examples/tours/indentation.xp is a complete program.