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Specify pattern resolution behavior for patterns with syntax errors #462

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Specify pattern resolution behavior for patterns with syntax errors #462

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23 changes: 19 additions & 4 deletions spec/formatting.md
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Expand Up @@ -237,6 +237,21 @@ _Pattern selection_ is not supported for _fallback values_.

## Pattern Selection

### Messages without Selectors

When a _message_ does not contain a _match_ construct,
and does not begin with a U+007B LEFT CURLY BRACKET `{`,
the result of pattern selection MUST be the concatenation of
a U+007B LEFT CURLY BRACKET `{`,
the contents of the message as a string,
and a U+007D RIGHT CURLY BRACKET `}`.

> For example, the following message:
> `no braces`
> would format to a string as `{no braces}`.
Comment on lines +240 to +251
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I'm not sure this is "pattern selection". This is a parse error, since the message is not, well, a message. What's more, the message "no braces {some braces}" is not a message.

I think the thing that is missing in formatting.md is the first step, which is: parsing. This should result in an syntax error. We could then go on to define what happens to the string that wants to be a message.

To be honest, I would not add the curly brackets. If you give me a string and it produces a syntax error, I should probably give you back the string as you gave it to me. If you thought it should be a valid message, you can debug it. If you are a runtime process, you can just display it (as we didn't damage it).

Note well: I argued long and hard that a plain old string with no placeholders should be a valid message. This is the old argument code-mode vs. string-mode. I am not reopening this, but we're going to tangle with the fact that you have to grope strings before feeding them to MF2 as a result.


### Messages with Selectors

When a _message_ contains a _match_ construct with one or more _expressions_,
the implementation needs to determine which _variant_ will be used
to provide the _pattern_ for the formatting operation.
Expand Down Expand Up @@ -278,7 +293,7 @@ the result of pattern selection MUST be a pattern resolving to a single _fallbac
using the message's fallback string defined in the _formatting context_
or if this is not available or empty, the U+FFFD REPLACEMENT CHARACTER `�`.

### Resolve Selectors
#### Resolve Selectors

First, resolve the values of each _selector_ _expression_:

Expand All @@ -295,7 +310,7 @@ First, resolve the values of each _selector_ _expression_:
The shape of the resolved values is determined by each implementation,
along with the manner of determining their support for selection.

### Resolve Preferences
#### Resolve Preferences

Next, using `res`, resolve the preferential order for all message keys:

Expand All @@ -320,7 +335,7 @@ The returned list MAY be empty.
The most-preferred key is first,
with each successive key appearing in order by decreasing preference.

### Filter Variants
#### Filter Variants

Then, using the preferential key orders `pref`,
filter the list of _variants_ to the ones that match with some preference:
Expand All @@ -340,7 +355,7 @@ filter the list of _variants_ to the ones that match with some preference:
1. Continue the outer loop on message _variants_.
1. Append `var` as the last element of the list `vars`.

### Sort Variants
#### Sort Variants

Finally, sort the list of variants `vars` and select the _pattern_:

Expand Down