allow user to set a preferred currency

[ThierryO]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

fmt_currency() uses the default currency = "USD". Allow the user to set a different default. Maybe something simple like currency = getOption("gt.currency", default = "USD"). When the user doesn't set gt.currency, the default currency is USD.

[rich-iannone]

I admit that the current implementation is not what I wanted so I'm quite glad you made the issue for this! I don't really prefer using options (I know there are a few, but I hope they are for non-essentials) because it tends to be bad for reproducibility. However, I do have an idea for this.

We have the locale argument in the gt() function. By default, it's set to NULL but you could use "fr-BE" and, from that, we can obtain the territory's current currency. Here's an example:

exibble |>
  dplyr::select(num, currency) |>
  gt(locale = "fr-BE") |>
  fmt_currency(
    currency = "EUR" # <- this line wouldn't be needed, `"EUR"` would be inferred from the `fr-BE` locale
  )

The CLDR (which gt makes heavy use of) has data on currencies per territory: https://github.com/unicode-org/cldr-json/blob/fb61c0afc2b49264b32d5d7bae6f660dd2ebee34/cldr-json/cldr-core/supplemental/currencyData.json. It's not a bad practice to obtain the currency code from the locale (several posts on different forums confirms that functionality written to do this is used often, e.g., https://stackoverflow.com/questions/8325002/how-to-get-the-default-currency-from-the-php-intl-icu-library).

This would involve making currency = NULL in fmt_currency() which is not bad at all. If locale is not specified in gt() or fmt_currency() then the default locale ("en") would be used, resulting in "USD" (same as before).

With these changes, you'd only need to specify the currency code to override the locale-provided currency. I know there are some strange cases where there are multiple currencies in current use, but that can be handled.

[rich-iannone]

In addition to this, the info_locales() table should be updated to indicate which territory belongs to a base locale (e.g., "pt" -> "BR", "fr" -> "FR", etc.). Furthermore, the info_currencies() table should have information on the locales associated with each currency code.