Inverted Match

[AnsonYeung]

Allow Match to be inverted, which will be used to create non null checks during handler lowering.

[LPTK]

LGTM, but it's missing at least one test where this is actually used.

[AnsonYeung]

It's probably better to change the JSBuilder and not the IR, as this change make little sense with multiple branch. I will implement the change to JSBuilder directly in the handler branch as it is small enough. So we do not need to change the IR to allow inverted match then.

[LPTK]

But I don't think there's a compact/convenient way of representing that in the IR? Ideally we'd want to have such compact representation to facilitate analyses and optimizations.

Now that I think of it, though, it seems that we should allow negative match cases regardless of the pattern, not just for literal patterns.

[AnsonYeung]

Right now, I express it as Match(..., Case.Lit(...) -> End(), S(...), rst), I'm not sure how to add it so that it would be available for all patterns

[LPTK]

We would associate a Bool with each Case in the Match.

Using default branches technically works but isn't compact (can't merge several into the same match). Not sure it's that important, though.

[AnsonYeung]

For a negative match, I think it's typically not used in multiple branches as the only useful scenario I can think of is for early exit, where you expect the match to sucess and return if it's not matched.

[LPTK]

Users can write things with negated patterns, eg

fun test(x) = if x is
  0 then "0"
  ~2 & ~3 then "not 2 and not 3"
  ~3 then "not 3"
  _ then "other"

test(0)
//│ = "0"
test(1)
//│ = "not 2 and not 3"
test(2)
//│ = "not 3"
test(3)
//│ = "other"

Though currently, it seems UCS lowering already does a good job producing a negation-free result:

//│ JS (unsanitized):
//│ let test;
//│ test = function test(x2) {
//│   switch (x2) {
//│     case 0:
//│       return "0";
//│       break;
//│     case 2:
//│       return "not 3";
//│       break;
//│     case 3:
//│       return "other";
//│       break;
//│     default:
//│       return "not 2 and not 3";
//│       break;
//│   }
//│ };