Functions in FTL

Functions provide additional functionality available to the localizers. They can be either used to format data according to the current language's rules or can provide additional data that the localizer may use (like, the platform, or time of the day) to fine tune the translation.

FTL implementations should ship with a number of built-in functions that can be used from within localization messages.

The list of available functions is extensible and environments may want to introduce additional functions, designed to aid localizers writing translations targeted for such environments.

Using Functions

FTL Functions can only be called inside of placeables. Use them to return a value to be interpolated in the message or as selectors in select expressions.

Example:

today-is = Today is { DATETIME($date) }

Function parameters

Functions may (but don't have to) accept positional and keyword arguments. Some keyword parameters are only available to developers and cannot be used inside of FTL translations. Instead they can be set on external arguments passed into translations by the developer.

See the reference below for more information about the arguments accepted for each built-in function.

Built-in Functions

Built-in functions are very generic and should be applicable to any translation environment.

NUMBER

Formats a number to a string in a given locale.

Example:

dpi-ratio = Your DPI ratio is { NUMBER($ratio, minimumFractionDigits: 2) }

Parameters:

currencyDisplay
useGrouping
minimumIntegerDigits
minimumFractionDigits
maximumFractionDigits
minimumSignificantDigits
maximumSignificantDigits

Developer parameters:

style
currency

See the Intl.NumberFormat for the description of the parameters.

DATETIME

Formats a date to a string in a given locale.

Example:

today-is = Today is { DATETIME($date, month: "long", year: "numeric", day: "numeric") }

Parameters:

hour12
weekday
era
year
month
day
hour
minute
second
timeZoneName

Developer parameters:

timeZone

See the Intl.DateTimeFormat for the description of the parameters.

Implicit use

In order to simplify most common scenarios, FTL will run some default functions while resolving placeables.

NUMBER

If the variable passed from the developer is a number and is used in a placeable, FTL will implicitly call a NUMBER function on it.

Example:

emails = Number of unread emails { $unreadEmails }

emails2 = Number of unread emails { NUMBER($undeadEmails) }

Numbers used as selectors in select expressions will match the number exactly or they will match the current language's CLDR plural category for the number.

The following examples are equivalent and will both work. The second example may be used to pass additional formatting options to the NUMBER formatter for the purpose of choosing the correct plural category:

liked-count = { $num ->
        [0]     No likes yet.
        [one]   One person liked your message
       *[other] { $num } people liked your message
    }

liked-count2 = { NUMBER($num) ->
        [0]     No likes yet.
        [one]   One person liked your message
       *[other] { $num } people liked your message
    }

DATETIME

If the variable passed from the developer is a date and is used in a placeable, FTL will implicitly call a DATE function on it.

Example:

log-time = Entry time: { $date }

log-time2 = Entry time: { DATETIME($date) }

Partial arguments

In most cases localizers don't need to call Functions explicitly, thanks to the implicit formatting. If the implicit formatting is not sufficient, the Function can be called explicitly with additional parameters. To ease the burden this might have on localizers, Fluent allows developers to set the default formatting parameters for the external arguments they pass.

In other words, developers can provide externals which are already wrapped in a partial Function call.

today = Today is { $day }
ctx.format('today', {
  day: Intl.MessageDateTimeArgument(new Date(), {
    weekday: 'long'
  })
})

If the localizer wishes to modify the parameters, for example because the string doesn't fit in the UI, they can pass the external argument to the same Function and overload the parameters set by the developer.

today = Today is { DATETIME($day, weekday: "short") }

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