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DD 51: Fractional Digits
#########################
Summary
=======
This design document specifies how an amount's fractional digits should be rendered.
Note that UIs that cannot render amounts as specified (e.g. because the display does
not support super script digits) may ignore the rendering advice provided by the
protocol under this DD.
Motivation
==========
Since different currencies have different ways to show/render fractionals, the
end-user apps should follow these guidelines.
Requirements
============
There is already a specification for ScopedCurrencyInfo - this needs to change
We need three core characteristics for fractional digits for each currency:
a) the number of fractional digits [0..8] the user may enter in a TextInputField
b) the number of fractional digits [0..8] to be rendered as normal characters (same font and size as the integer digits).
All additional fractional digits will be rendered as SuperScriptDigits as known from gas filling stations.
The UI should never round or truncate any amount, but always render all existing digits (except trailing zeroes, see c).
c) the number of fractional digits [0..8] to be rendered as trailing zeroes (including SuperScript digits).
E.g. if this is 2 (and normal == 2), then render $5 as “$ 5.00”.
If this is 3 (and normal == 2), then render $5 as “$ 5.00⁰” with two normal trailing zeroes and one superscript trailing zero.
Usually, all these three numbers have the same value, which means that in case
of e.g. “2” (used for €,$,£) the user can enter cent/penny values (but not a
fraction of those), these cents/pennies are always shown (even if they are 0)
as two normal digits after the decimal separator, and fractions of a
cent/penny are rendered as SuperScriptDigits, but only if they are not
trailing zeroes.
Additionally, some cryptocurrencies have such huge units, that they are
commonly rendered in milli-units, such as mBTC (milliBTC, 1/1000 of a BTC),
Gwei (Giga-WEI), MWei (Million-WEI), Kwei (Kilo-WEI), or
Mether/Kether/Gether/Tether and more "logical" units such as Szabo and
Finney. See `https://coinguides.org/ethereum-unit-converter-gwei-ether/` if
you want a good laugh. Regardless of the self-inflicted insanity here, this
could also make sense for inflated currencies in some cases. So we probably
should also have the ability to ship such a conversion map.
iOS has a built-in currency formatter, which knows how to deal with
thousands-separators and where to apply them (e.g. India uses a mixture of
hundreds and thousands instead of putting the separator after each 3 digits
like western currencies). However, this formatter will round after two (or
three) fractional digits and thus cannot be used for the whole amount.
(please add information about Android and WebEx here)
Proposed Solution
=================
Protocol considerations
-----------------------
The exchange, bank and merchant backends would need to be configured (via
their configuration files) to return the following ScopedCurrencyInfo in their
``/config`` and/or ``/keys`` endpoints. The bank returns this so that the
bank SPA can render amounts correctly, the exchange informs the wallets about
the desired way to render the currency, and the merchant backend informs the
merchant SPA --- independently of any particular exchange being used --- how
the merchant SPA should render amounts. Hence, the information will need to be
provisioned by all three services.
.. code-block:: swift
public struct ScopedCurrencyInfo: Codable, Sendable {
// e.g. “.” for $ and ¥; “,” for €
let decimal_separator: String
// how many digits the user may enter after the decimalSeparator
let num_fractional_input_digits: Integer
// €,$,£: 2; some arabic currencies: 3, ¥: 0
let num_fractional_normal_digits: Int
// usually same as numFractionalNormalDigits, but e.g. might be 2 for ¥
let num_fractional_trailing_zero_digits: Int
// true for “$ 3.50”; false for “3,50 €”
let is_currency_name_leading: Bool
// map of powers of 10 to alternative currency names / symbols, must
// always have an entry under "0" that defines the base name,
// e.g. "0 => €" or "3 => k€". For BTC, would be "0 => BTC, -3 => mBTC".
// This way, we can also communicate the currency symbol to be used.
let alt_unit_names: Map<Int, String>
}
For very large (2400000) or very tiny amounts (0.000056) the software would
then first represent the number compactly without any fraction (so for our
examples above, 24 * 10^6 and 56 * 10^-6) and then search for the nearest fit
in the alt_unit_names table. The result might then be 24000 KGELD or 0.056
mGELD, assuming the map had entries for 3 and -3 respectively. Depending on
the table, the result could also be 24 MGELD (6 => MGELD), or 5.6 nGELD
(assuming -6 => nGeld). Fractional rendering rules would still be applied
to the alternative unit name, alas the "num_fractional_input_digits" would
always apply to the unit currency and may need to be adjusted if amounts
are input using an alternative unit name.
Configuration syntax
--------------------
Each currency should be specified in its own subsystem-independent
currency, with the section name prefixed with "currency-". In that
section. The map could be given directly in JSON. For example:
.. code-block:: ini
[currency-euro]
name = "Euro"
code = "EUR"
decimal_separator = ","
num_fractional_input_digits = 2
num_fractional_normal_digits = 2
num_fractional_trailing_zero_digits = 2
is_currency_name_leading = NO
alt_unit_names = {0:"€"}
[currency-japanese-yen]
name = "Japanese Yen"
code = "JPY"
decimal_separator = "."
num_fractional_input_digits = 2
num_fractional_normal_digits = 0
num_fractional_trailing_zero_digits = 2
is_currency_name_leading = YES
alt_unit_names = {0:"¥"}
[currency-bitcoin-mainnet]
name = "Bitcoin-Mainnet"
code = "BITCOINBTC"
decimal_separator = "."
num_fractional_input_digits = 8
num_fractional_normal_digits = 3
num_fractional_trailing_zero_digits = 0
is_currency_name_leading = NO
alt_unit_names = {0:"BTC",-3:"mBTC"}
[currency-ethereum]
name = "WAI-ETHER"
code = "EthereumWAI"
decimal_separator = "."
num_fractional_input_digits = 0
num_fractional_normal_digits = 0
num_fractional_trailing_zero_digits = 0
is_currency_name_leading = NO
alt_unit_names = {0:"WAI",3:"KWAI",6:"MWAI",9:"GWAI",12:"Szabo",15:"Finney",18:"Ether",21:"KEther",24:"MEther"}
Implementation considerations
-----------------------------
For iOS, we plan to format the integer part of the amount with the built-in
currency formatter, then add the fractional part according to this document.
(please add information about Android and WebEx here)
Definition of Done
==================
(Only applicable to design documents that describe a new feature. While the
DoD is not satisfied yet, a user-facing feature **must** be behind a feature
flag or dev-mode flag.)
* Configuration (INI) format finalized and documented in taler.conf man page
* Endpoints of libeufin-bank, fakebank, exchange and merchant return the information
* SPAs use the information to render amounts
* Wallet-core passes rendering information to wallet UIs
* Cashier, Android PoS, WebExtension, Android and iOS Wallet render amounts accordingly
Alternatives
============
None, we cannot confuse users by rendering amounts in ways that break cultural
standards, and we cannot round and have numbers in balances not add up.
Drawbacks
=========
Discussion / Q&A
================
(This should be filled in with results from discussions on mailing lists / personal communication.)
|
