add string format to 0.9 expressions/client functions

[wrenj]

This could be a good demonstration of cross platform client formatting

Gemini recommended using unicode messageformat
https://unicode-org.github.io/icu/userguide/format_parse/messages/

eg

{
  "function_name": "format_string",
  "args": {
    "params": {
      "name": "John",
      "expiresOn": "2023-03-27T19:42:51.000Z"
    },
    "template": "{Hello {$name}, your card expires on {$expiresOn :datetime skeleton=yMMMdE}!}"
  }
}

[gspencergoog]

ICU MessageFormat feels like overkill here. Yes, it's quite useful (especially for i18n), and we will want that functionality eventually, but it's a very deep API to require for implementing an A2UI renderer.

How about we start with being able to interpolate two things: a data model path or the result of a function call. Then, if a client wants to implement the entire MessageFormat API they can have a "MessageFormat" function which takes all the args and returns a string.

I'm thinking we can take any return type for the functions or data paths, and just have the client call toString on them, like most modern languages do. For objects and lists, converting them to strings could be defined as converting them to JSON (to get consistent output on clients implemented in different languages).

[gspencergoog]

Actually, I think I neglected to read your proposal closely. I took it as asking for string interpolation, because we've been talking about that. I see what you mean now: proposing to add string_format as a "standard catalog" function. I think that doesn't really change what I said, though: sure, makes sense as a function, but I'm not sure I'd require it for all clients.

[wrenj]

that's fair to not require importing a whole library. How about adding a super basic one that can be done in a small function, and should work with most languages native string formatters. I came up with this with Gemini:

string_format supports a subset of Python PEP 3101 for named placeholders, including:

• Strings: {key}
• Decimals: {key:.2f}
• Currency/Grouping: {key:,.2f}
• Dates: {key:%Y-%m-%d} (Standard Python/C strftime codes)
• Note: Alignment, fill characters, and binary/hex types are not supported.