Avoid methods in MessageValue

[littledan]

MessageValue has a number of methods. This is different from how the rest of Intl formatToParts works, which is entirely data-based. I don't quite understand why these methods are needed, and would prefer that we reduce the overall size of the interface. I would also prefer to remove the options from the parts, out of a similar preference for minimalism.

I'm looking forward to @ryzokuken 's effort to prototype usage of the Intl.MessageFormat API in the context of rendering to styled React components, and hope this gives us more information on what's really needed.

[eemeli]

The methods on MessageValue enable us to define how a function like :number works internally, and how a user could define another custom function like it. For example, its toString() method provides the representation for the placeholder when formatting to a string, while its toParts() method provides its formatted-parts representation.

The output of MF.p.formatToParts() does not include any methods, it's MessagePart[] rather than MessageValue[].

[littledan]

@eemeli Sorry, I missed this response and should've responded a while ago. Why do we have both formatting methods in the options bag, as well as this?

[eemeli]

Here's an example message that we need to be able to support, which uses a user-provided :moz:number function that works as both a selector and a formatter:

.input {$num :moz:number}
.match {$num}
one {{You have {$num} thing}}
* {{You have {$num} things}}

To allow for the $num to be used within the message as a selector, and to be formatted either to a string or to parts, the user needs to provide a function in the options bag that can take the actual external value of $num and return a MessageValue. That function gets called when processing the .input line, and the methods on the object it returns are called during selection and formatting.