New Issue: Should unions throw or halt?

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1

28624, "jabraham17", "Should unions throw or halt?", "2026-03-31T21:31:33Z"

In Expand and improve support for union by jabraham17 · Pull Request #28601 · chapel-lang/chapel · GitHub, I made quite a few improvements to unions. One of those is making the halts turned off via --no-checks. However, it might be really nice to have these errors throw instead

For example

union u {
  var x: int;
  var y: real;
}
var u: U;
u.x = 1;
u.y; // throws, y is not active

However, this can have other implications. For instance, now every context where a union is accessed is a throwing context, which does not sound like a good idea.

A few possible solutions

  • Introduce a tryAccess, similar to : vs tryCast
  • make the field access always throw, but add some special syntax for exceptions
    • expand the select syntax to introduce captures to acomplish the field access and check in a single way. TODO SEE ISSUE
    • u!.x uses the halting path (turned off with --no-checks) to get the previous behavior with no throwing overhead when you know the the union is active for that field.
      • this feels appealing, but implies other issues. For example, is var y = u!; legal? Whats its type? Does this imply !. becomes special syntax?