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Number format skeleton support

Introduction

Often people want to be able to do with number patterns what they can with dates - Specify that they want a currency, with min fractions=x and max fractions=y, etc. But that they want the pattern that is appropriate for the locale.

http://bugs.icu-project.org/trac/ticket/8610

Background

ICU number formatters support different styles, like percent, currency, scientific or decimal. Those produce locale appropriate results, but are not flexible.
One can create a formatter using a user specified pattern, with full control of significant digits, min/max number of fractional digits... Problem with this approach is that the result is usually not locale appropriate.

Solution

Introduce skeleton feature to number formatting, similar to what date formatters have today. The skeleton would allow user to specify:
  • min and max number of fraction digits, using .0# notation
  • min number of integer digits, using 0. notation
  • number of significant digits, using @ character
  • type of the formatter - currency or percent if corresponding symbols are used, otherwise decimal
  • if grouping separator is on or off - user can’t specify actual placement of the separator.
  • whether the currency symbol, ISO 4217 code, or name (eg “US Dollar”) should be used for currency formatting - the actual placement of the symbol/ISO/plural is locale specific.
The skeleton syntax would be similar to current pattern syntax with couple restrictions/changes in meaning:
  • Skeleton would be specified with leading !! mark
  • There is no support for a separate negative pattern using the ; character. The negative format is derived from the locale info.
  • If <comma> is present it means use grouping separator, if it’s not present then the grouping separator is off. The actual grouping (the character and position) is locale dependent.

Questions

  • cira: Should we allow 0-9 for rounding, @ for significant digits, \u2030 for per mile and E for scientific?
    • mark: @ is the most useful.
  • markus: Should there be a separate builder class, like DateTimePatternGenerator vs. DateFormat?
  • markus: Should we support date and number skeletons in MessageFormat?

Examples

!!0,0.000#%
  • since there is a %, pick up the percent pattern for the locale
  • since there is a grouping separator, set grouping separator = true
  • set the min/max fractions and min integers to match the skeleton - min 2 integer digits, min 3 fraction digits, max 4 fraction digits.

!!¤¤¤0,0.00
  • since ¤ occurs, pick up the currency pattern for the locale.
  • replace the ¤ by ¤¤¤ to match the request.
  • since there is a grouping separator, set grouping separator = true
  • set the min integers to match the skeleton. (The min/max fractions are set by the currency when formatting)

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