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I like that there is this left-to-right flow. I think it’s a bit nicer to read than if-let ordering where the pattern comes before the thing it will be matched

I like that there is this left-to-right flow. I think it’s a bit nicer to read than if-let ordering where the pattern comes before the thing it will be matched against. I think it’s also good for ocaml-style constructor disambiguation, which tends to go in lexical order. Another nice aspect of making guards a less special case is that it avoids complexities in deciding if a binding is unused. I believe this logic was a source of lots of compiler warning bugs in ocaml. This syntax doesn’t seem to solve the following problem with matching where there are two paths to the same binding, (say you have an option in one branch but it isn’t optional in the other, and maybe you’d like to handle both cases with the same code. Currently you can do that with a match (match …) with … pattern. I worry that the semantics around exhaustiveness and mutable values may be confusing, though I guess OCaml already has that problem: type t = { mutable x : bool } let n = function true -> 1 | false -> 0 let f t = match t with | { x = false } when ( t.x <- true; false) -> -1 | { x } -> n x * 2 + n t.x What does t { x = false } return? Similarly if you changed the second case to be two cases instead of binding x?

lptk 2y ago
As mentioned in a response to a sibling comment, we plan to support `or`, which should address the problem you mention. (If not, would you have an example of what you mean?) > I worry that the semantics around exhaustiveness and mutable values may be confusing, though I guess OCaml already has that problem Indeed, and it was until very recently a source of unsoundness: https://icfp24.sigplan.org/details/mlworkshop-2024-papers/8/Pattern-matching-on-mutable-values-danger- https://icfp24.sigplan.org/details/mlworkshop-2024-papers/8/...
- munificent 2y ago
  Oh, wow, that's interesting. We added pattern matching and exhaustiveness to Dart not that long ago, and dealing with exhaustiveness and mutability was a big concern since Dart (unlike more strictly functional languages) generally doesn't avoid mutability. Our solution was that whenever a pattern accesses a property, the language implicitly caches that value. Any future accesses to the same property in that switch statement/expression use the previously cached value. That way, an entire set of switch cases is always operating on an immutable snapshot of data so that exhaustiveness can't be violated by side effects. Spec: https://github.com/dart-lang/language/blob/main/accepted/3.0/patterns/feature-specification.md#side-effects-and-exhaustiveness https://github.com/dart-lang/language/blob/main/accepted/3.0...

lptk 2y ago

As mentioned in a response to a sibling comment, we plan to support `or`, which should address the problem you mention. (If not, would you have an example of what you mean?) > I worry that the semantics around exhaustiveness and mutable values may be confusing, though I guess OCaml already has that problem Indeed, and it was until very recently a source of unsoundness: https://icfp24.sigplan.org/details/mlworkshop-2024-papers/8/Pattern-matching-on-mutable-values-danger- https://icfp24.sigplan.org/details/mlworkshop-2024-papers/8/...

munificent 2y ago
Oh, wow, that's interesting. We added pattern matching and exhaustiveness to Dart not that long ago, and dealing with exhaustiveness and mutability was a big concern since Dart (unlike more strictly functional languages) generally doesn't avoid mutability. Our solution was that whenever a pattern accesses a property, the language implicitly caches that value. Any future accesses to the same property in that switch statement/expression use the previously cached value. That way, an entire set of switch cases is always operating on an immutable snapshot of data so that exhaustiveness can't be violated by side effects. Spec: https://github.com/dart-lang/language/blob/main/accepted/3.0/patterns/feature-specification.md#side-effects-and-exhaustiveness https://github.com/dart-lang/language/blob/main/accepted/3.0...