# ICU4C MF2 formatToParts Suggestions

Author: @sffc

## Field Categories

### UFIELD_CATEGORY_MF2

Values come from the `UMessageFormatter2Field` enum:

typedef enum UMessageFormatter2Field {
    UMF2_LITERAL_FIELD,
    UMF2_PLACEHOLDER_FIELD,
    UMF2_MARKUP_FIELD,
    UMF2_ERROR_FIELD,
} UMessageFormatter2Field;

### UFIELD_CATEGORY_MF2_MARKUP

Values are indices into a data structure containing detail on the markup items, stored in the `MessageFormatter`. After parsing the message, the `MessageFormatter` should contain a vector of the markup items present in the message.

### UFIELD_CATEGORY_MF2_PLACEHOLDER

Values are indices into a data structure containing detail on the placeholders, of the `MessageFormatter`. After parsing the message, the `MessageFormatter` should contain a vector of the placeholders present in the message.

## Example

Message string: `Hello{#icon}, {#b}{$user}{/b} {$id :integer}!`

The `MessageFormatter` should contain the following structure (using JSON to illustrate):

{
    "markup": [
        {
            "name": "icon",
            "isPaired": false
        },
        {
            "name": "b",
            "isPaired": true
        }
    ],
    "placeholders": [
        {
            "name": "user",
            "type": "string"
        },
        {
            "name": "id",
            "type": "integer"
        }
    ]
}

Then, the fields in the `FormattedMessage` should be (when interpolating "Shane" and the number 1):

Hello, Shane 1!
|....|....|....|
0    5    10   15

| Start | Limit | Key | Value |
|---|---|---|---|
| 0 | 7 | UFIELD_CATEGORY_MF2 | UMF2_LITERAL_FIELD |
| 5 | 5 | UFIELD_CATEGORY_MF2 | UMF2_MARKUP_FIELD |
| 5 | 5 | UFIELD_CATEGORY_MF2_MARKUP | 0 |
| 7 | 12 | UFIELD_CATEGORY_MF2 | UMF2_PLACEHOLDER_FIELD |
| 7 | 12 | UFIELD_CATEGORY_MF2_MARKUP | 1 |
| 7 | 12 | UFIELD_CATEGORY_MF2_PLACEHOLDER | 0 |
| 12 | 13 | UFIELD_CATEGORY_MF2 | UMF2_LITERAL_FIELD |
| 13 | 14 | UFIELD_CATEGORY_MF2 | UMF2_PLACEHOLDER_FIELD |
| 13 | 14 | UFIELD_CATEGORY_MF2_PLACEHOLDER | 1 |
| 13 | 14 | UFIELD_CATEGORY_NUMBER | UNUM_INTEGER_FIELD |

Open question: Unclear whether there should be 1 or 2 literal fields at the start of the string (0-7 or 0-5, 5-7).