[FEEDBACK] function use outside its intent

[janishorsts]

What should happen when the function is used outside its intent?

• when func :x is defined for selection only and used for formatting
• when func :x is defined for formatting only and used for selecting

[aphillips]

This does seem to be a specification gap.

[eemeli]

when func :x is defined for selection only and used for formatting

This is addressed in Formatting:

message-format-wg/spec/formatting.md

Lines 740 to 744 in 4514e88

	Resolved values cannot always be formatted by a given implementation.
	When such an error occurs during _formatting_,
	an implementation SHOULD emit an appropriate _Message Function Error_ and produce a
	_fallback value_ for the _placeholder_ that produced the error.
	A formatting function MAY substitute a value to use instead of a _fallback value_.

when func :x is defined for formatting only and used for selecting

This is addressed in Resolve Selectors step 2.iii.:

message-format-wg/spec/formatting.md

Lines 517 to 522 in 4514e88

	1. If selection is supported for `rv`:
	1. Append `rv` as the last element of the list `res`.
	1. Else:
	1. Let `nomatch` be a resolved value for which selection always fails.
	1. Append `nomatch` as the last element of the list `res`.
	1. Emit a _Bad Selector_ error.

[janishorsts]

@eemeli Indeed, it is explained clearly. How did I overlook them?! Thanks!

[janishorsts]

When the value cannot be formatted by a given function, none of the Message Function Error errors fit the intent and purpose:

• Bad Operand,
• Bad Option or
• Bad Variant Key.

@eemeli, would you consider adding a new error to cover this scenario? Or do you rely on implementation defined Message Function Error?

message-format-wg/spec/errors.md

Line 280 in 8370709

Implementations MAY also provide implementation-defined _Message Function Error_ types.

[eemeli]

We used to have something like that (Formatting Error), but it was dropped in #774 in favour of Message Function Error, which implementations are indeed welcome to implement any necessary cases of.

The default set of functions doesn't include any that are only selectors so we've not ended up enumerating an explicit error for it. However, we do define :test:select that could be used to test for such an error.

If you're able to sign the CLA, we'd welcome a PR adding such a Message Function Error for consideration?

[aphillips]

We discussed this in the 2024-08-12 telecon. The WG believes that "Whan the value cannot be formatted by a given function" is covered by text at line 253 in formatting.md:

Implementations of a function SHOULD emit a
Bad Operand error for operands whose resolved value
or type is not supported.

The value is the operand, so this seems to cover it. What are we missing? (Closing per telecon, reopen if not satisfied)