I totally get what it does, even if I hadn't seen it before. What I don't get is (a) the use of the word 'match' (that's unfamiliarity) and (b) the proposed syntax which looks like an absolute mess, something worthy of PHP, and the fact that we end up with something that isn't a function but is recursive. Or something.
'x => x = 1' in Javascript, is an expression that resolves to a function. I do understand the difference, and it's important.
'match(x) { {x} => foo }' is not a bag of functions but a new use of the '=>' operator that is confusing/surprising. I can get over it, but why should I have to?
'match(x) { {x} : foo }' would be less surprising.
You're saying 'match({x:1}) { ....}' is an expression, but is 'match(x) { {x} => foo }'?, What does it resolve to? It smells like it either isn't an expression (just as you can't write x = switch(x) {...};) or it's a new kind of function.
Apparently they're overloading the meaning of the => "fat arrow" in the proposed syntax. I agree it's confusing because it's already used with lambdas in case of JS, but many languages with pattern matching do use it (eg. Scala, Rust). Other languages like Haskell and F# use the thin arrow -> instead.
Pattern matching can be exhaustive or non-exhaustive. Basically, if a match construct is going to be an expression it has to yield a value* for any input value, ie. there has to be a case arm for every possible input. Usually match blocks have a way to declare a "default" or "else" arm for those values that don't match anything else. Some languages with static type systems can statically figure out whether a given match expression is exhaustive or not. But in JavaScript's case that's most likely not an option. The JS way is probably yielding `undefined` if a value fails to match anything.
* or diverge, ie. loop forever, or exit via nonlocal means such as exceptions.
I believe all these languages, if they use some arrow in lambda syntax (so, except Rust), use the same kind of arrow for pattern matching. It's an obvious syntax choice when lambda arguments also use the same destructuring syntax: why would anyone want to remember which of those is `[x,y]->x+y` and which is `[x,y]=>x+y`?
'x => x = 1' in Javascript, is an expression that resolves to a function. I do understand the difference, and it's important.
'match(x) { {x} => foo }' is not a bag of functions but a new use of the '=>' operator that is confusing/surprising. I can get over it, but why should I have to?
'match(x) { {x} : foo }' would be less surprising.
You're saying 'match({x:1}) { ....}' is an expression, but is 'match(x) { {x} => foo }'?, What does it resolve to? It smells like it either isn't an expression (just as you can't write x = switch(x) {...};) or it's a new kind of function.