(Design) Exact Match Selector Options

exploration/exact-match-selector-options.md

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+# Design Proposal Template
+
+Status: **Accepted**
+
+<details>
+	<summary>Metadata</summary>
+	<dl>
+		<dt>Contributors</dt>
+		<dd>@aphillips</dd>
+		<dt>First proposed</dt>
+		<dd>2023-12-08</dd>
+	</dl>
+</details>
+
+## Objective
+
+_What is this proposal trying to achieve?_
+
+We need to choose a name for the selector function used when no other selector is specified.
+
+## Background
+
+_What context is helpful to understand this proposal?_
+
+This addresses issue <a href="https://github.com/unicode-org/message-format-wg/issues/433">#433</a>
+and issues raised in various design documents, notably
+<a href="https://github.com/unicode-org/message-format-wg/pull/471">#471</a>
+(about number selection).
+
+The exact match selector function is the function used when an implementation
+cannot find another selector.
+It is also an attribute of selectors, such as `:number` or perhaps `:date`
+that need to match specific values.
+
+## Use-Cases
+
+_What use-cases do we see? Ideally, quote concrete examples._
+
+As a developer, I would like to create matches based on arbitrary enumerated string
+values in a manner similar to ICU MF1's `SelectFormat`.
+This allows me to use an ordinary enum or data values already present in my data
+to do pattern selection.
+
+As a user, I would like to create numeric matches based on a specific value
+not just the plural or ordinal category of the value.
+This is for cases where the pattern selected is tied to the actual value
+rather than just to the grammatical needs of the language.
+Consider the different between the messages in:
+>```
+>.match {$numChances :number}
+>1   {{You have one chance left}}
+>one {{You have {$numChances}} chance left}}
+>*   {{You have {$numChances}} chances left}}
+>```
+
+As an implementer, I want to create value matches that are not strictly
+tied to the string serialization for data types that I know.
+> For example, if a date value were serialized as
+> `2023-12-08T12:00:00Z[America/Los_Angeles]` and the selector were
+> `{$myDate :date dateStyle=long}` **_and_** I want to do select on the 
+> date value, I might want a match like this:
+>```
+>.match {$myDate}
+>2023-12-08 {{This only fires on this specific date}}
+>* {{This fires some other time}}
+>```
+>A different example that makes this clearer might be numbers.
+> In the `de-DE` locale, the number `42.3` is usually formatted
+> as `42,3`. Consider the message:
+> ```
+> .local $myNum = {|42.3| :number minFractionDigits=1}
+> .match {$myNum :equals}
+> 42.3 {{This always matches}
+> *    {{This never matches}}
+> ```
+
+As a user, I want to use a selector name that makes sense and feels natural to write.
+
+## Requirements
+
+_What properties does the solution have to manifest to enable the use-cases above?_
+
+## Constraints
+
+_What prior decisions and existing conditions limit the possible design?_
+
+ICU MF1 has a selector called `com.ibm.icu.text.SelectFormat` 
+associated with the keyword `select` in the MF1 syntax. 
+Our design should be compatible with this selector when used for the same use case,
+but it not constrained to use the same name or terminology.
+
+## Proposed Design
+
+_Describe the proposed solution. Consider syntax, formatting, errors, registry, tooling, interchange._
+
+The exact match selector is named `:string`.
+The exact match selector matches the string literal value of its operand against
+any sets of keys in a case-sensitive, encoding-sensitive manner.
+No Unicode normalization is performed.
+
+> [!NOTE]
+> In this context "encoding-sensitive" does not mean "character encoding"
+> but rather "sensitive to the sequence of code points used by the string".
+> See [Character Model for the World Wide Web: String Matching](https://www.w3.org/TR/charmod-norm)
+
+The `:string` selector is also a verbatim formatting function.
+That is, the message "{$var :string}" when formatted contains the resolved string
+value of the variable `$var`.
+
+## Alternatives Considered
+
+_What other solutions are available?_
+_How do they compare against the requirements?_
+_What other properties do they have?_
+
+### Option A. `:select`
+
+Pros:
+- The same as ICU MF1's keyword. Users have implementation experience.
+  Might fascilitate easier migration from MF1?
+- Not overly specific, so can conceptually fit with "non-exact" matching.
+
+Cons:
+- Somewhat generic name.
+
+### Option B. `:exact`
+
+Pros:
+- Says what the match does.
+
+Cons:
+- Unclear whether it refers to the serialization form or the imputed value.
+  There may be some cases where the match depends on options,
+  such as with `{|42.3| :integer}` and a key `42`
+
+### Option C. `:equals`
+
+Pros:
+- Says what the match does.
+
+Cons:
+- Implies object or value equality, which may not be correct in all implementations
+  or programming languages.
+  The implications might be confusing to users in those cases.
+
+### Option D. `:string`
+
+Pros:
+- Clearer about what is being compared than `select` or `exact`:
+  it's the string value?
+
+Cons:
+- Might confuse users who are comparing e.g. two numbers or two dates.
+  In typed languages, the values being compared might not actually be strings.
+- Depends on the serialization.
+
+
+### Option E. Something else
+
+It is possible that we haven't landed the best name yet. 
+If you don't like any of these, what **_do_** you like?!?