[Chapel Merge] Implementation of union (in)equality operators

chapelhpcBot · May 4, 2026, 5:24pm

Branch: refs/heads/main
Revision: 9d7e5103a5fa0827318ff78f804e901f7260979f
Author: bradcray
Link: Implementation of union (in)equality operators by bradcray · Pull Request #28699 · chapel-lang/chapel · GitHub
Log Message:
Implementation of union (in)equality operators (#28699)

[reviewed by @DanilaFe]

This implements union equality and inequality as module code, building
on the support for the generic 'union' type that Jade added in #28601
and following the pattern of module-based record comparisons in #28731.
Note that pragma "last resort" was not required here as it was for the
record operators since we don't currently support generic unions
(#28758).

The implementation follows a sketch I put together in discussing

github.com/chapel-lang/chapel

Create a pattern matching syntax for unions

opened 09:41PM - 31 Mar 26 UTC

jabraham17

type: Design area: Language

While working on https://github.com/chapel-lang/chapel/pull/28601, I created som…e nicer syntax for unions ```chapel union U { var x: int; var y: real; } var u: U; select u { when U.x { writeln("x is ", u.x); } when U.y { writeln("y is ", u.y); } } ``` This gets us really nice to a pattern matching syntax for Chapel (at least for unions), but its not perfect. A few issues * The when clauses are not enforced by the compiler, I can write whatever expression I want in `when`, I just happen to use nice syntax of `U.x` * I still have to use `u.x` to access the field in the when block. It would be really nice to have the compiler introduce a temporary for me that auto-accesses the field, avoiding overhead and reducing the chance for mistakes For example, what if I could write the following ```chapel select u { when var myX = x { writeln("x is ", myX); } when y { writeln("y is ", y); } } ``` In the first when, I am pattern matching that the `x` field is active and getting the value of `x` into a temporary variable named `myX`. In the second, I am pattern matching `y` is active and getting the value of `y` into a temporary variable named `y`. This is an even nicer ergonomic improvement over the current union pattern matching. It could be further improved if we make bad union accesses throw (see https://github.com/chapel-lang/chapel/issues/28624), such that the pattern match avoids the throwing check. I see these two features as working hand in hand See https://github.com/chapel-lang/chapel/issues/26993 for similar ideas with sockets

cleaned up based on the review of #28731.

Tests added in this PR:

unionEquality.chpl: Tests that == and != work as I'd expect
userOverload.chpl: Makes sure that a user-defined overload takes
precedence over these default ones and doesn't cause an ambiguity.
unionEqDiffType.chpl: Tests that == between different unions doesn't
work as expected, whether they are structurally equivalent or not.
unionSelect.chpl: A future that shows that selects on unions that rely
on == don't work because, today, all union selects are treated as
active field selections
I also added a pair of futures that show that making a second
argument's type based on the first only inherits the first's type
constraint, not the actual type of the first argument as we think it
should (type constraint based on arg with generic formal type too loose · Issue #9310 · chapel-lang/chapel · GitHub). As I
understand it, this is already fixed in Dyno.

Note that I didn't mark these operators as being unstable or part of a
preview edition since unions are unstable in and of themselves, which I
think is sufficient.

Resolves #14139

Compare: Comparing aa8b425a2e5943e3dde90b0877a376f9af4cc011...09f33282c0b6c13817e400b20935cf086e0fc10f · chapel-lang/chapel · GitHub

Diff:
M modules/internal/ChapelUnion.chpl
A test/types/records/bugs/chainArgType.bad
A test/types/records/bugs/chainArgType.chpl
A test/types/records/bugs/chainArgType.future
A test/types/records/bugs/chainArgType.good
A test/types/records/bugs/chainArgType2.bad
A test/types/records/bugs/chainArgType2.chpl
A test/types/records/bugs/chainArgType2.future
A test/types/records/bugs/chainArgType2.good
A test/types/unions/equality/unionEqDiffType-u3.good
A test/types/unions/equality/unionEqDiffType.chpl
A test/types/unions/equality/unionEqDiffType.compopts
A test/types/unions/equality/unionEqDiffType.good
A test/types/unions/equality/unionEquality.chpl
A test/types/unions/equality/unionEquality.good
A test/types/unions/equality/unionSelect.bad
A test/types/unions/equality/unionSelect.chpl
A test/types/unions/equality/unionSelect.future
A test/types/unions/equality/unionSelect.good
A test/types/unions/equality/userOverride.chpl
A test/types/unions/equality/userOverride.good
https://github.com/chapel-lang/chapel/pull/28699.diff

[Chapel Merge] Implementation of union (in)equality operators

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