Nesting not supported with custom fortmatters

[johanblumenberg]

The Format Guide states this:

All types of messageformat statements may be nested within each other, to unlimited depth

However, it doesn't work to nest anything within a custom formatter:

const chalk = require('chalk');
const MessageFormat = require('messageformat');
const mf = new MessageFormat('en-GB');
mf.addFormatters({
  error: function(v, lc, p) { return chalk.red(p); }
});
const messages = mf.compile({
  describe: 'There were {numErrors, error, {# errors}}'
});

messages.describe({ numErrors: 5 })        // 'There were 5 errors', where '5 errors' prints as red

You could also imagine nesting a plural format in there, to get 'no error', 'one error', '# errors', and so on, printed in red.

[eemeli]

You're right, custom formatters arguments don't work like you'd expect them to. They're not as such a part of the MessageFormat spec, but a relaxation/extension of the simpleArg pattern as its argType is not restricted to "number" | "date" | "time" | "spellout" | "ordinal" | "duration", but may refer to any user-determined custom formatter. However, the spec does specify the following regarding the optional argument to such a formatter:

In argStyleText, every single ASCII apostrophe begins and ends quoted literal text, and unquoted {curly braces} must occur in matched pairs. [...] Any argStyleText must be a valid pattern string for the Format subclass used.

So I'm not really sure what the "right" thing to do here would be. At a minimum, the "All types" statement should be reworded. But to address your specific use case, a slight reformulation should get you the result you're looking for:

const mf = new MessageFormat('en-GB');
mf.addFormatters({
  error: (v, lc, p) => chalk.red(p.replace(/#/g, v))
});
const messages = mf.compile({
  describe: 'There were {numErrors, error, # errors}'
});

Note that this slightly abuses the meaning of # as no check is made to make sure that it's a number. And to get proper pluralisation, you should wrap the whole thing in a plural:

There were {numErrors, plural,
  one {{numErrors, error, # error}}
  other {{numErrors, error, # errors}}
}

[johanblumenberg]

Currently, this message does not even compile:

const messages = mf.compile({
  describe: 'There were {numErrors, error, {# errors}}'
});

This message, however, does:

const messages = mf.compile({
  describe: 'There were {numErrors, error, {# errors}'
});

The second string is invalid, since an ending curly bracket is missing.

... unquoted {curly braces} must occur in matched pairs ...

It seems the parsing breaks whenever there is a new ending curly bracket, and the parsing doesn't accept any string containing an ending curly bracket.