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Unit formatting (volume, area, length, ...) functions for the Common Locale Data Repository (CLDR)
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Cldr for Units

Build Status Hex pm License

Getting Started

ex_cldr_units is an add-on library for ex_cldr that provides localisation and formatting for units such as weights, lengths, areas, volumes and so on. It also provides unit conversion and simple arithmetic for compatible units.


From ex_cldr version 2.0, a backend module must be defined into which the public API and the CLDR data is compiled. See the ex_cldr documentation for further information on configuration.

In the following examples we assume the presence of a module called MyApp.Cldr defined as:

defmodule MyApp.Cldr do
  use Cldr, locales: ["en", "fr"], default_locale: "en"

Supporting the String.Chars protocol

The String.Chars protocol underpins Kernel.to_string/1 and is also used in string interpolation such as #{my_unit}. In order for this to be supported by Cldr.Unit, a default backend module must be configured in config.exs. For example:

config :ex_cldr_units,
  default_backend: MyApp.Cldr

Public API

The primary api is defined by three functions:

  • MyApp.Cldr.Unit.to_string/2 for formatting units

  • to create a new Unit.t struct that encapsulated a unit and a value that can be used for arithmetic, comparison and conversion

  • MyApp.Cldr.Unit.convert/2 to convert one compatible unit to another

  • MyApp.Cldr.Unit.localize/3 to localize a unit by converting it to units customary for a given territory

  • MyApp.Cldr.Unit.add/2, MyApp.Cldr.Unit.sub/2, MyApp.Cldr.Unit.mult/2, MyApp.Cldr.Unit.div/2 provide basic arithmetic operations on compatible Unit.t structs.

Unit formatting and localization

MyApp.Cldr.Unit.to_string/2 provides localized unit formatting. It supports two arguments:

  • number is any number (integer, float or Decimal) or a Unit.t struct returned by

  • options which are:

    • :unit is any unit returned by Cldr.Unit.units/0. This option is required unless a Unit.t is passed as the first argument.

    • :locale is any configured locale. See Cldr.known_locales(). The default is locale: Cldr.get_current_locale()

    • :style is one of those returned by Cldr.Unit.available_styles. The current styles are :long, :short and :narrow. The default is style: :long

    • :per allows compound units to be formatted. For example, assume we want to format a string which represents "kilograms per second". There is no such unit defined in CLDR (or perhaps anywhere!). If however we define the unit unit =, 20) we can then execute Cldr.Unit.to_string(unit, per: :second). Each locale defines a specific way to format such a compount unit. Usually it will return something like 20 kilograms/second

    • Any other options are passed to Cldr.Number.to_string/2 which is used to format the number

iex> MyApp.Cldr.Unit.to_string 123, unit: :gallon
{:ok, "123 gallons"}

iex> MyApp.Cldr.Unit.to_string 1234, unit: :gallon, format: :long
{:ok, "1 thousand gallons"}

iex> MyApp.Cldr.Unit.to_string 1234, unit: :gallon, format: :short
{:ok, "1K gallons"}

iex> MyApp.Cldr.Unit.to_string 1234, unit: :megahertz
{:ok, "1,234 megahertz"}

iex> MyApp.Cldr.Unit.to_string 1234, unit: :foot, locale: "fr"
{:ok, "1 234 pieds"}

iex> MyApp.Cldr.Unit.to_string, 42), locale: "fr"
{:ok, "42 ampères"}

iex> Cldr.Unit.to_string 1234, MyApp.Cldr, unit: :foot, style: :narrow, per: :second
{:ok, "1,234′/s"}

iex> Cldr.Unit.to_string 1234, MyApp.Cldr, unit: :foot, per: :second
{:ok, "1,234 feet per second"}

Unit decomposition

Sometimes its a requirement to decompose a unit into one or more subunits. For example, if someone is 6.3 feet heigh we would normally say "6 feet, 4 inches". This can be achieved with Cldr.Unit.decompose/2. Using our example:

 iex> height =, 6.3)
 #Unit<:foot, 6.3>
 iex(2)> Cldr.Unit.decompose height, [:foot, :inch]
 [#Unit<:foot, 6.0>, #Unit<:inch, 4.0>]

A localised string representing this decomposition can also be produced. Cldr.Unit.to_string/3 will process a unit list, using the function Cldr.List.to_string/2 to perform the list combination. Again using the example:

 iex> c = Cldr.Unit.decompose height, [:foot, :inch]
 [#Unit<:foot, 6.0>, #Unit<:inch, 4.0>]
 iex> Cldr.Unit.to_string c, MyApp.Cldr
 "6 feet and 4 inches"
 iex> Cldr.Unit.to_string c, MyApp.Cldr, list_options: [format: :unit_short]
 "6 feet, 4 inches"
 # And of course full localisation is supported
 iex> Cldr.Unit.to_string c, MyApp.Cldr, locale: "fr"
 "6 pieds et 4 pouces"

Converting Units

Unit.t structs can be converted to other compatible units. For example, feet can be converted to meters since they are both the length unit type.

 # Test for unit compatibility
 iex> MyApp.Cldr.Unit.compatible? :foot, :meter
 iex> MyApp.Cldr.Unit.compatible? :foot, :liter

 # Convert a unit
 iex> MyApp.Cldr.Unit.convert, 3), :meter
 #Unit<:meter, 0.9144111192392099>

 # What units are compatible?
 iex> MyApp.Cldr.Unit.compatible_units :foot
 [:astronomical_unit, :centimeter, :decimeter, :fathom, :foot, :furlong, :inch,
  :kilometer, :light_year, :meter, :micrometer, :mile, :mile_scandinavian,
  :millimeter, :nanometer, :nautical_mile, :parsec, :picometer, :point, :yard]

Converting measurement systems

Different territories (countries) use different measurement systesm. CLDR recognises three measurement systems: :metric, :US and :UK. Users expect that data will presented to them in a familiar for. For example, a person from the US would expect to see the height of a person expressed in feet and inches. A person in Europe would expect to see the height of a person expressed in centimeters. Or in some cases in meters and centimeters.

CLDR provides data to facilitate this conversion and ex_cldr provides the Cldr.Unit.localize/3 function to convert a unit into the form expected for a given territory. Here is an example for the height of a person, expressed in meters but localized for a US audience:

 iex> height =, :meter)
 iex> us_height = Cldr.Unit.localize height, :person, territory: :US, style: :informal
 [#Unit<:foot, 5>, #Unit<:inch, 11>]
 iex> Cldr.Unit.to_string us_height
 "5 feet and 11 inches"

Note that conversion is dependent on context. The context above is :person reflecting that we are referring to the height of a person. For units of length category, the other contexts available are :rainfall, :snowfall, :vehicle, :visibility and :road. Using the above example with the context of :rainfall we see

 iex> height = Cldr.Unit.localize height, :rainfall, territory: :US
 [#Unit<:inch, 71>]
 iex> Cldr.Unit.to_string height
 "71 inches"

See Cldr.Unit.unit_preferences/0 to see what mappings are available, in particular what context usage is supported for conversion.

Unit arithmetic

Basic arithmetic is provided by Cldr.Unit.add/2, Cldr.Unit.sub/2, Cldr.Unit.mult/2, Cldr.Unit.div/2 as well as Cldr.Unit.round/3

 iex> MyApp.Cldr.Unit.Math.add!(:foot, 1),!(:foot, 1)
 #Unit<:foot, 2>

 iex> MyApp.Cldr.Unit.Math.add!(:foot, 1),!(:mile, 1)
 #Unit<:foot, 5280.945925937846>

 iex> MyApp.Cldr.Unit.Math.add!(:foot, 1),!(:gallon, 1)
 {:error, {Cldr.Unit.IncompatibleUnitError,
   "Operations can only be performed between units of the same type. Received #Unit<:foot, 1> and #Unit<:gallon, 1>"}}

 iex> MyApp.Cldr.Unit.round, 1031.61), 1
 #Unit<:yard, 1031.6>

 iex> MyApp.Cldr.Unit.round, 1031.61), 1, :up
 #Unit<:yard, 1031.7>

Available units

Available units are returned by MyApp.Cldr.Unit.units/0.

iex> MyApp.Cldr.Unit.units
[:acre, :acre_foot, :ampere, :arc_minute, :arc_second, :astronomical_unit, :bit,
 :bushel, :byte, :calorie, :carat, :celsius, :centiliter, :centimeter, :century,
 :cubic_centimeter, :cubic_foot, :cubic_inch, :cubic_kilometer, :cubic_meter,
 :cubic_mile, :cubic_yard, :cup, :cup_metric, :day, :deciliter, :decimeter,
 :degree, :fahrenheit, :fathom, :fluid_ounce, :foodcalorie, :foot, :furlong,
 :g_force, :gallon, :gallon_imperial, :generic, :gigabit, :gigabyte, :gigahertz,
 :gigawatt, :gram, :hectare, :hectoliter, :hectopascal, :hertz, :horsepower,
 :hour, :inch, ...]

Unit types

Units are grouped by unit type which defines the convertibility of different types. In general, units of the same time are convertible to each other. The function MyApp.Cldr.Unit.unit_categories/0 returns the unit types. MyApp.Cldr.Unit.unit_tree/0 returns the map of all unit types and their child units.

iex> MyApp.Cldr.Unit.unit_categories
[:acceleration, :angle, :area, :concentr, :consumption, :coordinate, :digital,
 :duration, :electric, :energy, :frequency, :length, :light, :mass, :power,
 :pressure, :speed, :temperature, :volume]

Further information

For help in iex:

iex> h
iex> h MyApp.Cldr.Unit.to_string
iex> h MyApp.Cldr.Unit.convert
iex> h MyApp.Cldr.Unit.units
iex> h MyApp.Cldr.Unit.unit_categories


Note that :ex_cldr_units requires Elixir 1.5 or later.

Add ex_cldr_units as a dependency to your mix project:

defp deps do
    {:ex_cldr_units, "~> 2.0"}

then retrieve ex_cldr_units from hex:

mix deps.get
mix deps.compile
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