Support "match only" mode in pattern compilation

[chengluyu]

No description provided.

[LPTK]

Ok, let's get this merged, thanks. However there is a pre-existing problem with the difference between compiled and interpreted patterns (see #422).

[LPTK]

Char should probably be a builtin pattern, right? Otherwise, because of the where clause, I guess it won't behave as expected within compiled string patterns with concatenation.

[chengluyu]

I placed it into Char.mls for now. When I implement the string compilation later, I will make it a built-in pattern.

[LPTK]

I'm leaving this "unresolved" for reference. We'll still merge the PR.

[LPTK]

Do you have any tests that shows the IR of a match-only pattern? So we can make sure it doesn't regress later.

Otherwise LGTM, thanks!

[LPTK]

Suggested change

	// :sir
	fun toSumPair(t) = if t is (@compile SumPair) as s then s else null
	// :sir
	fun toSumPair(t) = if t is (@compile SumPair) as s then s else null

[LPTK]

Why did you close this? You haven't applied the change.

[Copilot]

Pull request overview

This PR extends UPS pattern compilation to support a “match only” result mode, allowing compiled patterns to return booleans (and reuse the original scrutinee as the output) when no transformed output/bindings are required. This reduces generated code size and avoids constructing MatchSuccess/MatchFailure objects in common is @compile P cases, while adding/adjusting diagnostics and updating the test suite accordingly.

Changes:

• Add ResultMode to UPS pattern compilation and propagate an outputNeeded flag through SplitCompiler to enable boolean-only matching when safe.
• Improve constructor-pattern handling to avoid rebuilding outputs when not needed, and add warnings when extraction-argument transformation results are implicitly discarded.
• Update/add a large set of MLscript tests and compilation examples to reflect new runtime shapes (e.g. MatchSuccess(..., null)), new warnings/errors, and new match-only behavior.

Reviewed changes

Copilot reviewed 31 out of 31 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
hkmc2/shared/src/main/scala/hkmc2/semantics/ups/SplitCompiler.scala	Thread outputNeeded through matching/compilation; choose match-only vs full compilation; add warnings for discarded extraction outputs; refine inaccessible-field diagnostics.
hkmc2/shared/src/main/scala/hkmc2/semantics/ups/Compiler.scala	Introduce ResultMode and boolean match-only matcher generation; refactor field matcher plumbing; support transform-free reuse of scrutinee outputs.
hkmc2/shared/src/main/scala/hkmc2/semantics/ups/Pattern.scala	Add MatchedClassLike specialized node and preservesOriginalScrutinee analysis; adjust specialization/printing/field collection.
hkmc2/shared/src/main/scala/hkmc2/semantics/ucs/TermSynthesizer.scala	Make empty bindings represent as null (and standardize MatchSuccess bindings emission).
hkmc2/shared/src/main/scala/hkmc2/semantics/Split.scala	When expanding SimpleSplit, call makeMatchSplit with outputNeeded = false.
hkmc2/shared/src/test/mlscript/ups/TransformFree.mls	New test for compiled patterns whose output can reuse the scrutinee when transform-free.
hkmc2/shared/src/test/mlscript/ups/transformation/BindingLess.mls	New tests around transformations in subpatterns and match-only behavior.
hkmc2/shared/src/test/mlscript/ups/syntax/PatternBody.mls	New/updated diagnostics tests for inaccessible constructor params during rebuild.
hkmc2/shared/src/test/mlscript/ups/syntax/MixedParameters.mls	New warning test for implicitly discarded extraction-argument transformations.
hkmc2/shared/src/test/mlscript/ups/specialization/SimpleLiterals.mls	Add logging/side-effect tests demonstrating match-only compilation skipping transforms.
hkmc2/shared/src/test/mlscript/ups/SimpleTransform.mls	Update IR expectations and add transform side-effect tests in compiled patterns.
hkmc2/shared/src/test/mlscript/ups/SimpleConjunction.mls	Add conjunction test ensuring both sides’ transforms run when output is demanded.
hkmc2/shared/src/test/mlscript/ups/regex/Separation.mls	Update expected MatchSuccess bindings shape ({} → null).
hkmc2/shared/src/test/mlscript/ups/recursion/NatBox.mls	Update lowered IR expectation for boolean match-only compilation path.
hkmc2/shared/src/test/mlscript/ups/parametric/EtaConversion.mls	Update normalized output expectation for module/pattern references.
hkmc2/shared/src/test/mlscript/ups/MatchResult.mls	Expand coverage around @compile usage and update expected MatchSuccess bindings.
hkmc2/shared/src/test/mlscript/ups/LocalPatterns.mls	Update expected MatchSuccess bindings shape (null).
hkmc2/shared/src/test/mlscript/ups/Future.mls	Update MatchSuccess expectations and line-numbered diagnostics.
hkmc2/shared/src/test/mlscript/ups/examples/Extraction.mls	Add tests for biased disjunction + transforms and adjust outputs to preserve constructor tags when transform-free.
hkmc2/shared/src/test/mlscript/ups/examples/EvaluationContext2.mls	Adjust evaluation-context patterns/tests to align with match-only output behavior.
hkmc2/shared/src/test/mlscript/ups/examples/DoubleOrSum.mls	Update expected list output representation to preserve constructor structure.
hkmc2/shared/src/test/mlscript/ucs/patterns/where.mls	Update expected MatchSuccess bindings shape (null).
hkmc2/shared/src/test/mlscript/ucs/patterns/Parameters.mls	Add tests ensuring “keep scrutinee” works with private params when no rebuild is needed; refine rebuild error.
hkmc2/shared/src/test/mlscript/codegen/BlockPrinter.mls	Update printed block output to reflect null bindings and simplified output.
hkmc2/shared/src/test/mlscript/basics/PrefixOps.mls	Update expected MatchSuccess bindings shape (null).
hkmc2/shared/src/test/mlscript-compile/ups/TruthyFalsy.mls	New compilation-size example program exercising match-only entrypoints.
hkmc2/shared/src/test/mlscript-compile/ups/README.md	New documentation summarizing SLOC deltas for match-only vs full compiled patterns.
hkmc2/shared/src/test/mlscript-compile/ups/EvenOddTree.mls	New compilation-size example for mutually recursive tree patterns.
hkmc2/shared/src/test/mlscript-compile/ups/EvaluationContext.mls	New compilation-size example for evaluation-context patterns.
hkmc2/shared/src/test/mlscript-compile/ups/DnfCnf.mls	New compilation-size example for recursive logical-formula patterns.
hkmc2/shared/src/test/mlscript-compile/Char.mls	Add AnyChar pattern used by compilation examples/tests.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[LPTK]

I think the one below is enough.

[LPTK]

You don't need parentheses in all these examples.