format_intl

format_intl adapts the number symbols and grouping rules in intl for use with format. It keeps format independent of intl while letting an application opt into locale-aware number formatting.

Installation

Add both packages to the application that creates the formatting engine:

dependencies:
  format: ^4.0.0
  format_intl: ^1.0.0

Usage

Inject an IntlNumberLocale when constructing Format. The resulting engine is independent of the package-level default API, so retain whichever method tear-offs your application needs:

import 'package:format/format.dart';
import 'package:format_intl/format_intl.dart';

void main() {
  final kazakh = Format(
    numberLocale: IntlNumberLocale('kk_KZ'),
  );
  final formatKk = kazakh.format;
  final sprintfKk = kazakh.sprintf;

  print(formatKk('{:n}', 1234567.5));
  print(sprintfKk('%.2f', 12.5));
}

Format provides four formatting method tear-offs:

• format accepts a brace-format template and up to ten positional values.
• formatWith accepts a brace-format template plus positional and named collections.
• sprintf accepts a printf-style template and up to ten positional values.
• vsprintf accepts a printf-style template and a list of values.

The n specifier applies the adapter's grouping rule automatically; it does not need a grouping flag. The printf dialect uses the locale's decimal separator, signs, exponent separator, and digits.

That exponent separator is worth knowing about before it surprises you: intl spells it E for 108 of its 119 locales, so %e and %E produce the same text under most of them and stop being distinguishable. Seven locales spell it as something that is not a letter at all — ×10^ in sv, أس in ar_EG — and there %E has nothing to uppercase either. Brace formatting is unaffected, because {:e} does not read the locale at all; only {:n} does. IntlNumberLocale supplies symbols and grouping only: it does not round values. Precision and rounding remain the responsibility of format. IntlNumberLocale does not select or apply number notation, including compact or scientific notation; notation remains the responsibility of format.

Decimal double values use Dart SDK conversion by default. Locale symbols are applied after that conversion. Select the compatible profile when Python brace or C++ printf rounding and exponent layout are required:

final compatibleKazakh = Format(
  numberLocale: IntlNumberLocale('kk_KZ'),
  doubleFormatMode: DoubleFormatMode.compatible,
);

DoubleSpecialValueSpelling.short selects nan/inf in Dart SDK mode; compatible mode always uses those short spellings. These profile settings belong to Format, not IntlNumberLocale.

Default-locale snapshot

Use IntlNumberLocale.fromDefault() when the intl current locale should be chosen at construction time:

final locale = IntlNumberLocale.fromDefault();
final engine = Format(numberLocale: locale);

This takes an immutable snapshot of the locale's number symbols and grouping rules. Changing Intl.defaultLocale later does not update an existing IntlNumberLocale or Format; construct a new adapter and engine to use the new default.

Localized digits are a Dart extension

IntlNumberLocale also maps ASCII digits to the locale's ZERO_DIGIT symbol. That applies to both brace and printf formatting. Localized digits and locale specific signs or exponent symbols are Dart extensions provided by format's NumberLocale interface; they go beyond the C locale behavior that printf compatibility targets.