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

Under Flutter there is one more constraint to satisfy, and it is not this package's: flutter_localizations pins intl to a single exact version, so that pin and the constraint here have to overlap. The intl range above spans both versions Flutter pins — 0.19.0 up to Flutter 3.31.x and 0.20.2 from 3.32.0 — so the two overlap on every Flutter from 3.27.0, which is also the first one format itself installs on.

Which intl your application resolves is visible in the output

This package is an adapter: it reports the symbols the resolved intl carries, and that data is not identical across the range. Of the 119 locales intl ships, four differ:

Locale	intl 0.19.0	0.20.2	0.20.3
en_ZA	decimal ., group ,	decimal ,, group NBSP	same as 0.20.2
de_CH, gsw, it_CH	group ’	group ’	group '

Only en_ZA changes the decimal separator, so it is the one where a number can be read wrongly rather than merely look different. Note that three of the four already move inside a ^0.20.2 constraint: pinning the range up would not have made the output version-independent, only narrower. Every other locale is identical across all three versions.

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.