Semantics of match failure

[littledan]

The Symbol.match protocol sometimes succeeds and sometimes fails. If it returns failure, what would the semantics be? (I'd suggest: the same kind of exception is thrown as if a match expression falls through and there's no default case.)

[rbuckton]

This is mentioned in https://github.com/rbuckton/proposal-extractors#proposed-solution: in a binding or assignment pattern, failure would throw a TypeError.

This is not only consistent with languages like Scala, but with how destructuring works in ECMAScript today:

const { x, y } = null; // throws TypeError
const [x, y] = {}; // throws TypeError

Extractors are always destructured as either an object or an array. It would be somewhat like a failing match resulting in null when the match fails, i.e.:

const Option.Some(value) = opt;

// is roughly like
const [value] = Option.Some[Symbol.matcher](opt).value;

Except we would throw a more descriptive error than TypeError: null is not iterable.

In pattern matching, a failing match would result in choosing a different alternative or trying the next match leg.

[littledan]

Ah, sorry for the redundant question from me reading too fast! I'm happy with these semantics.