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Jun 19, 2023
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Add Bidirectional Isolation section to formatting #315

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Add "Bidirectional Isolation" section to formatting
eemeli Jun 18, 2023
72f7dee
Update spec/formatting.md
eemeli Jun 18, 2023
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59 changes: 59 additions & 0 deletions spec/formatting.md
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Expand Up @@ -274,6 +274,65 @@ when * {Other match}

4. The pattern `{Exact match}` of the most preferred `1` variant is selected.

## Handling Bidirectional Text

_Messages_ contain text which can be bidirectional, that is
consisting of a mixture of left-to-right and right-to-left spans of text.

When concatenating formatted values,
the [Unicode Bidirectional Algorithm](http://www.unicode.org/reports/tr9/) [UAX9]
can produce unexpected or undesirable effects, such as "spillover".
Formatted values SHOULD be bidirectionally isolated
so that the directionality of a formatted _expression_
does not negatively affect the presentation of the overall formatted result.

An implementation MUST define methods for
determining the directionality of the message as a whole as well as each formatted _expression_.
The method of determining the directionality of a formatted _expression_
MAY rely on the introspection of its contents, or on other means.

If a formatted _expression_ itself contains spans with differing directionality,
its formatter SHOULD isolate such parts to avoid
negatively affecting the presentation of the overall formatted result.

> For example, an implementation could provide a `:number` formatting function
> which would always produce output matching the message's locale,
> allowing for its formatted string representation to never need isolation.

Implementations formatting messages as a concatenated string or a sequence of strings
MUST provide one or more strategies for bidirectional isolation.
One such strategy MUST behave as follows:

1. Let `msgdir` be the directionality of the whole message,
one of « `'LTR'`, `'RTL'`, `'unknown'` ».
These correspond to the message having left-to-right directionality,
right-to-left directionality, and to the message's directionality not being known.
1. For each _expression_ `exp` in _pattern_:
1. Let `fmt` be the formatted string representation of the resolved value of `exp`.
1. Let `dir` be the directionality of `fmt`,
one of « `'LTR'`, `'RTL'`, `'unknown'` », with the same meanings as for `msgdir`.
1. If `dir` is `'unknown'`:
1. In the formatted output,
prefix `fmt` with U+2068 FIRST STRONG ISOLATE
and postfix it with U+2069 POP DIRECTIONAL ISOLATE.
1. Else, if `dir` is `'LTR'` and `msgdir` is not `'LTR'`:
1. In the formatted output,
prefix `fmt` with U+2066 LEFT-TO-RIGHT ISOLATE
and postfix it with U+2069 POP DIRECTIONAL ISOLATE.
1. Else, if `dir` is `'RTL'` and `msgdir` is not `'RTL'`:
1. In the formatted output,
prefix `fmt` with U+2067 RIGHT-TO-LEFT ISOLATE
and postfix it with U+2069 POP DIRECTIONAL ISOLATE.

Alternatives to this "compatibility" strategy MAY be provided by implementations,
which MAY also introspect the _pattern_'s _text_ values
and identify situations where isolate characters are not needed
or where additional or different isolation would produce better results.

If an implementation provides formatting to non-string targets,
it SHOULD provide similar strategies for enabling bidirectional isolation,
where appropriate.

## Error Handling

Errors in messages and their formatting may occur and be detected
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