WIP: parse pattern matching

[fedeci]

Q	A
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
License	MIT

WIP: adding it here just to keep track of things to do and of what have been done.

• Raise for & and | at the same level (e.g. when ({ x: 200 | 201 & 300 }) {})
• Parse CallExpression + BlockStatement for newline after match
  • Disallow rest parameter for MatchExpression
• Accept simple UnaryExpressions as MatchPattern
• Support with and as
• Support expression mode

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[JLHwung]

@ljharb Can I assume match {} is a primary expression?

If so we should handle it in parseExprAtom, like #13573.

We probably need something like parseCoverMatchCallAndMatchExpression, similar to how we are disambiguating async () and async () => {}.

[ljharb]

What is a "primary" expression?

[JLHwung]

Oh I mean https://tc39.es/ecma262/#prod-PrimaryExpression

[ljharb]

hmm, i'm not super sure about this part. i would expect match () {} to be allowed anywhere any other expression is currently allowed (nested inside the pin operator in another match construct, of course, it would only be allowed with parens)

[JLHwung]

Ok, I have open tc39/proposal-pattern-matching#214 and let's continue there.

[fedeci]

@ljharb @JLHwung In cases like the ones below should we parse it as CallExpression + missing semicolon or throw unexpected?

match (x,) {
  ...
}

match (x, y) {
  ...
}

[JLHwung]

@fedeci Good question! Yes, a match call expression match() is an ambiguous match expression head iff it satisfies all the following conditions:

• Its callee match is an identifier which does not contain escape
• It has exactly one argument
• The argument is an expression
• The argument has no trailing comma

If a match call is not ambiguous, this.semicolon() will throw after the match call.

[ljharb]

Both examples in #13572 (comment) should indeed be parsed as a match expression, since x, y is a comma operator-separated expression.

[fedeci]

@ljharb Will it be matched like this?

match (x, y) {
  when ({ j }, k) { }
}

[nicolo-ribaudo]

match (x, y) { ... } is probably the same as x, match (y) { ... }: x is evaluated and then ignored, as in any sequence expression.

[ljharb]

Note that in addition to combinatory likely using and/or, it’s seeming like the pin operator will change from ^(expr)/^chainOrIdentifier to ${expr}

[fedeci]

Great, that will simplify the parsing.