Add :test:function, :test:select, and :test:format function definitions

test/README.md

@@ -35,3 +35,78 @@ Some examples of test harnesses using these tests, from the source repository:
 - [CST parse/stringify tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/cst/cst.test.ts)
 - [Data model stringify tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/data-model/stringify.test.ts)
 - [Formatting tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/messageformat.test.ts)
+
+## Test Functions
+
+As the behaviour of `:number`, `:datetime`, and other default message functions
+is dependent on locale-specific data and may vary between implementations,
+the following functions are defined for test use only:
+
+### `test:number`
+
+This function is valid both as a selector and as a formatter.
+The only option it recognizes is `fractionDigits`,
+all other option values passed to it are ignored.
+
+When resolving a `test:number` expression,
+its internal _input_ and _fractionDigits_ values are determined as follows:
+1. If the expression argument resolves to a numerical value
+   or a string matching the `number-literal` production, then
+   1. Let _input_ be the numerical value of the argument.
+2. Else,
+   1. Emit "bad-input" error.
+   1. Use a fallback representation is used as the resolved value of the expression.
+      The _fractionDigits_ value is not determined.
+3. If the `fractionDigits` option is set, then
+   1. If its value resolves to a numerical integer value `0`, `1` or `2`,
+      or their corresponding string representations `'0'`, `'1'`, or `'2'`, then
+      1. Let _fractionDigits_ be the numerical value of the option.
+   2. Else,
+      1. Emit "bad-option" error.
+      2. Let _fractionDigits_ be `0`.
+4. Else,
+   1. Let _fractionDigits_ be `0`.
+
+When `test:number` is used as a selector,
+the behaviour of calling it as the `rv` value of MatchSelectorKeys(`rv`, `keys`)
+(see [Resolve Preferences](/spec/formatting.md#resolve-preferences) for more information)
+depends on its _input_ and _fractionDigits_ values.
+- If the _input_ is `1` and _fractionDigits_ is `0`,
+  the method will return some slice of the list « `'1'`, `'one'` »,
+  depending on whether those values are included in `keys`.
+- If the _input_ is `1` and _fractionDigits_ is not `0`,
+  the method will return the list « `'1'` » if `keys` includes `'1'`, or an empty list otherwise.
+- If the _input_ is any other value, the method will return an empty list.
+
+When an expression with a `test:number` annotation is assigned to a variable by a declaration
+and that variable is used as an argument or option value,
+its resolved value is the _input_ value.
+
+When `test:number` is used as a formatter,
+a placeholder resolving to a value with a `test:number` expression
+is formatted as a concatenation of the following parts:
+1. If _input_ is less than `0`, the character `-` U+002D Hyphen-Minus.
+2. The truncated absolute integer value of _input_, i.e. floor(abs(_input_)),
+   formatted as a sequence of decimal digit caharacters.
+3. If _fractionDigits_ is `1`, then
+   1. The character `.` U+002E Full Stop.
+   2. The single decimal digit character representing the value floor((abs(_input_) - floor(abs(_input_))) * 10)
+4. Else if _fractionDigits_ is `2`, then
+   1. The character `.` U+002E Full Stop.
+   2. The two decimal digit character string representing the value floor((abs(_input_) - floor(abs(_input_))) * 100),
+      with a `0` U+0030 Digit Zero padding at start if the result is less than `10`.
+
+If the formatting target is a sequence of parts,
+each of the above parts will be emitted separately
+rather than being concatenated into a single string.
+
+Note that for purposes of clarity, the formatting of `test:number` does not perform any rounding.
+
+### `test:plural`
+
+This function behaves exactly the same as `test:number`,
+except that it cannot be used for formatting.
+
+When `test:plural` is used as a formatter,
+a "not-formattable" error is emitted and the placeholder is formatted with
+a fallback representation.