Feature #22108
Updated by yertto (_ yertto) 4 months ago
> [!WARNING]
> This has potentially been obsoleted by [Feature #22111](https://bugs.ruby-lang.org/issues/22111)
# Computed hash keys with `(expr):` syntax
Allow `{ (expr): value }` as a computed hash key.
Almost 20 years in the making, the missing puzzle piece for Hash's "new" colon notation:
```ruby
h = {
name: "symbol shorthand", # Ruby 1.9+
"quoted label": "symbol label", # Ruby 2.2+ (Feature #4935)
value_omission: , # Ruby 3.1+ (Feature #14579)
(expr): "computed", # THIS PROPOSAL
}
```
## Motivation
Ruby has several colon-based hash key syntaxes but the `=>` ("hash rocket") is still needed for non-symbolic keys.
Adding `(expr):` is a baby step toward allowing all-colon hashes:
```ruby
## Before -- mixed styles
n = 42; { key1: "symbol", "key-#{2}": "quoted symbol", RUBY_VERSION:, n => "bar" }.keys
# => [:key1, :"key-2", :RUBY_VERSION, 42]
## After -- uniform colon syntax
n = 42; { key1: "symbol", "key-#{2}": "quoted symbol", RUBY_VERSION:, n : "bar" }.keys
# => [:key1, :"key-2", :RUBY_VERSION, 42]
```
(And maybe a step closer to one day retiring our old friend "hash rocket" from Hashes entirely?)
### Completing the colon family
| Example | Ruby | Feature | Key type |
|---------|------|---------|----------|
| `{ name: value }` | [v1.9](https://docs.ruby-lang.org/en/3.4/NEWS/NEWS-1_9_1.html) | | Symbol |
| `{ "quoted label": value }` | [v2.2](https://docs.ruby-lang.org/en/3.4/NEWS/NEWS-2_2_0.html) | [Feature #4935](https://bugs.ruby-lang.org/issues/4935) | Symbol (quoted label) |
| `{ value_omission: }` | [v3.1](https://docs.ruby-lang.org/en/3.4/NEWS/NEWS-3_1_0_md.html) | [Feature #14579](https://bugs.ruby-lang.org/issues/14579) | Value omission |
| `{ (expr): value }` | *???* | [Feature #22108](https://bugs.ruby-lang.org/issues/22108) | Computed |
### Readability for computed-key-heavy code
Code that builds dynamic hashes currently forces a style break midway through a literal.
This is especially noticeable with interpolated keys, numeric keys, or variable-driven keys.
### Reducing `=>` overloading
The `=>` token is now being used to serve more purposes than just the Hash literal (aka "Hash rocket") it was originally used for.
- rightward assignment
```
expr => var
```
- pattern capture
```
case {name: "Alice", role: "admin"}
in {name: String => name, role:} # capture the name String value into `name`
p name # => "Alice"
end
```
- rescue variables
```
rescue SomeExceptionClass => e
```
```
rescue => e
```
Reducing its use in Hashes simplifies the language, especially for newcomers.
## Design
### `(expr):` vs `[expr]:`
Parenthesized expressions were chosen over square bracket delimited:
- Idempotent wrapping: `((x))` = `(x)`, but `[[x]]` != `[x]` (nested array)
- Array keys: `{(["a"]): "b"}` reads naturally vs `{[["a"]]: "b"}` requiring extra wrapping
- `jq` precedent: the JSON query language uses identical `{("a"+"b"): 59}` syntax (since [jq 1.6](https://jqlang.org/manual/v1.6/#object-construction), 2018)
- Parentheses signals a grouping or "evaluate this", whereas square brackets signals an Array/container
### Improving on JavaScript
JavaScript's "[computed property names](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#computed_property_names)" `{ [expr]: value }` were introduced in ECMAScript 2015 (ES6), 3 years before the jq version.
However in Ruby, `[]` already means Array literal and method call.
Rather than overloading `[]` further, `(expr):` (from jq) improves on the JavaScript design.
Parentheses naturally signal evaluation, `():` inside `{}` is unambiguous to the lexer, and it matches the well-established jq convention.
### Edge cases handled
- `(expr) : value` (space before colon) -- *syntax error* (consistent with `name : value`)
- `{ (expr): }` (value omission) -- *syntax error* (runtime expression, no compile-time local to infer)
- Mixed styles: `{ (1): "one", two: "two", "three": "three" }` -- *valid*
- Nested parens: `{ ((1 + 2) * 3): "nested" }` -- *valid*
- Nested hashes: `{ ({(1): "one"}): "two" }` -- *valid*
## Implementation
Three changes to `parse.y` (Lrama LALR(1) parser, zero new conflicts):
1. Lexer: emit `tLABEL_END` when `EXPR_ENDFN` state, no space before `:`, and inside `brace_nest > 0`
2. Grammar: new `assoc` alternative -- `tLPAREN compstmt ")" tLABEL_END arg_value`
3. Precedence: `%nonassoc ')'` to disambiguate
Reference implementation: [feature/computed-hash-keys](https://github.com/ruby/ruby/pull/17299) PR on GitHub.
## Historical context
A version of this was discussed on ruby-core in October 2007 (as part of ["General hash keys for colon notation"](https://public-inbox.org/ruby-core/E1ImQBE-00033s-IZ@x31/T/), murphy).
Unfortunately it was brought up during the v1.9 feature freeze, but it looks like [Matz's invitation to discuss for v2.0](https://public-inbox.org/ruby-core/E1ImQA8-00033T-8c@x31/) didn't end up going anywhere.
Since then, `"quoted label":` (Ruby 2.2, [Feature #4935](https://bugs.ruby-lang.org/issues/4935)) and value omission (Ruby 3.1, [Feature #14579](https://bugs.ruby-lang.org/issues/14579)) have expanded the colon family, making the computed-key gap more conspicuous.
Meanwhile, jq introduced the identical `{("a"+"b"):59}` syntax in [jq 1.6](https://jqlang.org/manual/v1.6/#object-construction) (2018), demonstrating real-world viability.
## Open questions
- Is this syntax acceptable to the community?
- Is the jq precedent compelling enough to address the "no language does this" objection?
## Future directions
All colon-based key syntaxes would now be available.
This opens up the possibility of eventually deprecating `=>` from Hash literals (while keeping it for rescue, pattern matching, and rightward assignment).
This proposal does not require that change - it is simply the enabling step, and any deprecation timeline could be a separate discussion.