(Design) Formatted Parts

exploration/formatted-parts.md

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+# Formatted Parts
+
+Status: **Proposed**
+
+<details>
+	<summary>Metadata</summary>
+	<dl>
+		<dt>Contributors</dt>
+		<dd>@eemeli</dd>
+		<dt>First proposed</dt>
+		<dd>2023-08-29</dd>
+		<dt>Pull Request</dt>
+		<dd><a href="https://github.com/unicode-org/message-format-wg/pull/463">#463</a></dd>
+	</dl>
+</details>
+
+## Objective
+
+Messages often include placeholders that,
+when formatted, contain internal structure ("parts").
+Preserving this structure in a formatted message
+may be helpful to the caller,
+who can then manipulate the parts.
+For example, a caller may want to style or present
+messages with the same content differently
+if those messages have different internal structure.
+
+This proposal defines a formatted-parts target for MessageFormat 2.
+
+## Background
+
+Past examples have shown us that if we don't provide a formatter to parts,
+the string output will be re-parsed and re-processed by users.
+Recent examples of web browsers needing to account for such user behaviour are available from
+[June 2022](https://github.com/WebKit/WebKit/commit/1dc01f753d89a85ee19df8e8bd75f4aece80c594) and
+[November 2022](https://bugs.chromium.org/p/v8/issues/detail?id=13494).
+
+## Use-Cases
+
+- Markup elements
+- Non-string values
+- Message post-processors
+- Decoration of placeholder interior parts.
+  For example, identifying the separate fields in these two currency values
+  (notice that the symbol, number, and fraction fields
+  are not in the same order and that the separator has been omitted):
+  ![image](https://github.com/unicode-org/message-format-wg/assets/69082/cb68c87f-9c0c-4bc6-b9a0-b1f97b2b789a)
+  ![image](https://github.com/unicode-org/message-format-wg/assets/69082/aedd4e66-7d47-4026-8b93-4ba061bb4d84)
+- Supplying bidirectional isolation of placeholders,
+  such as by using HTML's `span` element with a `dir` attribute
+  based on the direction of the placeholder.
+
+## Requirements
+
+- Define an iterable sequence of formatted part objects.
+- Include metadata for each part, such as type, source, direction, and locale.
+- Allow the representation of non-string values.
+- Allow the representation of values that consist of an iterable sequence of formatted parts.
+- Be able to represent each resolved value of a pattern with any number of formatted parts, including none.
+- Define the formatted parts in a manner that allows synonymous but appropriate implementations in different programming languages.
+
+## Constraints
+
+- The JS Intl formatters already include formatted-parts representations for each supported data type.
+  The JS implementation of the MF2 formatted-parts representation should be able to match their structure,
+  at least as far as that's possible and appropriate.
+
+## Proposed Design
+
+The formatted-parts API is included in the spec as an optional but recommended formatting target.
+
+The shape of the formatted-parts output is defined in a manner similar to the data model,
+which includes TypeScript, JSON Schema, and XML DTD definitions of the same data structure.
+
+At the top level, the formatted-parts result is an iterable sequence of parts.
+Parts corresponding to each _text_ can be simpler than those of _expressions_,
+as they do not have a `source` other than their `value`,
+or set any of the other possible metadata fields.
+
+```ts
+type MessageParts = Iterable<
+  MessageTextPart | MessageExpressionPart | MessageBiDiIsolationPart
+>;
+
+interface MessageTextPart {
+  type: "text";
+  value: string;
+}
+```
+
+For MessageExpressionPart, the `source` corresponds to the expression's fallback value.
+The `dir` and `locale` attributes of a part may be inherited from the message
+or from the operand (if present),
+or overridden by an expression attribute or formatting function,
+or otherwise set by the implementation.
+Each part should have at most one of `value` or `parts` defined;
+some may have none.
+
+```ts
+type MessageExpressionPart =
+  | MessageSingleValuePart<string, unknown>
+  | MessageMultiValuePart<string, unknown>;
+
+interface MessageSingleValuePart<T extends string, V> {
+  type: T;
+  source: string;
+  dir?: "ltr" | "rtl" | "auto";
+  locale?: string;
+  value?: V;
+}
+
+interface MessageMultiValuePart<T extends string, V> {
+  type: T;
+  source: string;
+  dir?: "ltr" | "rtl" | "auto";
+  locale?: string;
+  parts: Iterable<{ type: string; value: V; source?: string }>;
+}
+```
+
+The bidi isolation strategies included in the spec may require
+the insertion of MessageBiDiIsolationParts in the formatted-parts output.
+
+```ts
+interface MessageBiDiIsolationPart {
+  type: "bidiIsolation";
+  value: "\u2066" | "\u2067" | "\u2068" | "\u2069"; // LRI | RLI | FSI | PDI
+}
+```
+
+Some of the MessageExpressionPart instances may be further defined
+without reference to the function registry.
+
+Unannotated expressions with a _literal_ operand
+are represented by MessageStringPart.
+As with MessageTextPart,
+the `value` of MessageStringPart is always a string.
+
+```ts
+interface MessageStringPart {
+  // MessageSingleValuePart<"string", string>
+  type: "string";
+  source: string;
+  value: string;
+  dir?: "ltr" | "rtl" | "auto";
+  locale?: string;
+}
+```
+
+Unannotated expressions with a _variable_ operand
+whose type is not recognized by the implementation
+or for which no default formatter is available
+are represented by MessageUnknownPart.
+
+```ts
+interface MessageUnknownPart {
+  // MessageSingleValuePart<"unknown", unknown>
+  type: "unknown";
+  source: string;
+  value: unknown;
+}
+```
+
+When the resolution or formatting of a placeholder fails,
+it is represented in the output by MessageFallbackPart.
+No `value` is provided; when formatting to a string,
+the part's representation would be `'{' + source + '}'`.
+
+```ts
+interface MessageFallbackPart {
+  // MessageSingleValuePart<"fallback", never>
+  type: "fallback";
+  source: string;
+}
+```
+
+### Registry definition of formatted parts
+
+Each function defined in the registry MUST define its "formatted-parts" representation.
+A function can define either a unitary string `value` or a `parts` representation.
+Where possible, a function SHOULD provide a `parts` representation
+if its output might reasonably consist of multiple fields.
+In most cases, these sub-parts should not need fields beyond their `type` and a string `value`.
+Where necessary, other `value` types may be used
+and other fields such as a `source` included in the sub-parts,
+and additional fields may be included in the `MessageExpressionPart`.
+
+For example, `:datetime` and `:number` formatters could use the following formatted-parts representations.
+In many implementations, these could be further narrowed to only use `string` values.
+
+```ts
+interface MessageDateTimePart {
+  // MessageMultiValuePart<"datetime", unknown>
+  type: "datetime";
+  source: string;
+  parts: Iterable<{ type: string; value: unknown }>;
+  dir?: "ltr" | "rtl" | "auto";
+  locale?: string;
+}
+
+interface MessageNumberPart {
+  // MessageMultiValuePart<"number", unknown>
+  type: "number";
+  source: string;
+  parts: Iterable<{ type: string; value: unknown }>;
+  dir?: "ltr" | "rtl" | "auto";
+  locale?: string;
+}
+```
+
+## Alternatives Considered
+
+### Not Defining a Formatted-Parts Output
+
+Leave it to implementations.
+They will each come up with something a bit different,
+but each will mostly work.
+
+They will not be interoperable, though.
+
+### Different Parts Shapes
+
+See issue <a href="https://github.com/unicode-org/message-format-wg/issues/41">#41</a> for details.
+
+They can be considered as precursors of the current proposal,
+into which they've developed due to evolutionary pressure.
+
+### Annotated String Output
+
+Format to a string, but separately define metadata or other values.
+
+This gets really clunky for parts that are not reasonably stringifiable.