Allow overriding Fluent functions

[hakastein]

Original fluent does not limit function overriding

1. Function overriding allowed
2. FluentBundle no more final

This is especially significant due to the lack of a built-in DATETIME function.

[hakastein]

So, what are you thinking about this request? In the original implementation, there were no limits (https://github.com/projectfluent/fluent.js/blob/main/fluent-bundle/src/bundle.ts#LL75C20-L75C20), and we successfully overwrote built-in functions to support square/cubic units and some messy formats like usd-per-square-meter to support $1000/m².

[hakastein]

Hello, do you plan accept this? Or do you have any questions?

[jrmajor]

Hi, I believe that as DATETIME and NUMBER are built-in functions, they're expected to work as described in Fluent Guide. Allowing to override them breaks this assumption.

Would defining a separate function, e.g. MY_NUMBER, work for your use case? Or are there any additional reasons to allow function overriding, which I haven't considered?

[hakastein]

Fluent is great for its extensibility. And the original library does not impose any restrictions on extending and redefining functions. I don't see anything wrong with overriding/extending built-in functions. Especially given that basic support for Intl.NumberFormat is only planned in the future tc39/ecma402#767

So in FronEnd we just replace built-in function and its work like a charm.

[hakastein]

So all this is necessary for uniform behavior. To make the backend implementation behave the same as the frontend implementation

[jrmajor]

Fair enough, we can allow it for the sake of compatibility with fluent.js.

Please adjust your changes to the comments I've left and I will merge it.

[hakastein]

Done

[jrmajor]

@hakastein Thank you, and sorry for the delay. I will try to release a new version soon :)