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- Syntax: `fun f(x, y?)`, `fun f(x, y?: T)`, `fun f(x, drop y)`, and in function types `(Int, Int?) -> R` and `(T, drop Int) -> R`. - Optional parameters may be omitted by callers, in which case they are `undefined`; their annotated types therefore include `undefined`. Droppable parameters are always passed but may be ignored by implementations (e.g., the index and array passed by `Array#map`). - Runtime arity checks now follow JavaScript: only missing arguments are reported, extra arguments are ignored. Callbacks with fewer parameters than their expected function type are accepted by the new resolver. - Direct calls are checked statically against [required, positional]. - TailRecOpt resets omitted optional parameters; flow analysis, eta expansion and worker/wrapper account for optional parameters; Wasm rejects them. - Prelude: Array/Map callbacks use `drop`, and the TODOs for optional parameters (Object, Number, String, Error, Array.from, Promise#then) are resolved. Co-Authored-By: Claude Opus 5.5 <[email protected]>
- The new resolver again rejects callbacks with fewer parameters than their expected function type, unless the omitted trailing parameters are marked `drop`. Callbacks also may not require arguments that callers may omit, and extra callback parameters must be optional. - Remove the obsolete variance traverser of the elaborator, which only fed `TyParam.showDbg` and crashed on spreads in function types. Co-Authored-By: Claude Opus 5.5 <[email protected]>
Rest parameters may be marked `drop`, both in definitions (`fun f(x, drop ...xs)`) and in function types (`(Int, drop ...Array[Int]) -> Int`), in which case callbacks may omit them. Function type rest parameters (`Spd` elements) now record whether they are droppable. Co-Authored-By: Claude Opus 5.5 <[email protected]>
- `ParamList.restParam: Opt[Param]` becomes `variadic: Opt[Variadic]`, where `Variadic(param, trailing)` holds the (required) parameters following the variadic one, as in `fun f(x, ...xs, y)`. Function types support the same shape, as in `(Int, ...Array[Int], Str) -> R`. - Arguments are assigned as in Ruby: required leading and trailing parameters take the first and last arguments, optional leading parameters take the remaining ones from left to right, and the variadic parameter takes the rest. Optional parameters cannot follow a variadic parameter, and there is at most one variadic parameter. - Lowering desugars trailing parameters into a rest parameter and a prologue (`Lowering.lowerTrailingParams`), so passes running after it only see rest parameters (`ParamList.restParam` asserts this). - The new resolver matches arguments to trailing parameters from the end, and only accepts callbacks with the same structure as their expected function type when trailing parameters are involved. Co-Authored-By: Claude Opus 5.5 <[email protected]>
LPTK
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LPTK
commented
Oct 18, 2026
Join every compound-signature component at stable parameter endpoints and share conservative layout mapping with callbacks. Preserve element flows through rest slicing, keep later curried constraints connected, and document the distinction between lenient checking and flow preservation.
Observe Symbol.iterator, next, and result fields through stable per-spread sites. Preserve known element flows alongside uncertainty and share synthesized type-application interning between Array and Awaited. Support symbol-keyed members in resolution, inheritance, and JavaScript output. Compose alias argument polarity without discarding declaration variance or explicit projections, and let lenient calls retain all known result flows. Document the best-effort resolution policy and add protocol, ambiguity, variance, and bounded-growth regressions. Include regenerated golden outputs. Validation: ctest; hkmc2AllTests/test.
Declare annotations.jsSymbol in the prelude and recognize its resolved symbol. Require a stable module/val path ending in a val marked annotations.fresh, without checking Symbol types or attempting to prove freshness. Preserve key identity across receiver aliases, require matching names and keys for overrides, and carry runtime keys as explicit IR operands for reads, writes, and methods. Interpret declared-module annotation identities before their values complete, and publish completed overload sets without exposing a separate signature before its implementation. Keep Iterator limited to next, make IteratorImpl iterable, and expose native iterator results through Iterator & Iterable while preserving all component flows. Add runtime and diagnostic regressions, including an explicit FIXME for named selection after a factory-local key escapes its lexical scope. Record the requirement to confirm future language-design changes. Validation: ctest and hkmc2AllTests/test passed. Include regenerated golden output.
LPTK
marked this pull request as ready for review
October 19, 2026 06:35
Pin, as :fixme, the issues found while reviewing optional, droppable, and trailing parameters: - Without inlining, Runtime.Tuple.getLeading returns () instead of undefined for omitted optional parameters before a variadic one. - Stack-safe resumption drops rest parameters, which now also carry required trailing arguments. - The Wasm backend crashes on trailing parameters. - Legacy using-clause eta-expansion drops rest and trailing arguments. - A user-defined ?: operator no longer parses. Also cover the Wasm optional-parameter diagnostic and the legacy separate-signature optionality check, which had no tests. Co-Authored-By: Claude Opus 5.5 <[email protected]>
Use the call layout matcher for tuple patterns, retain enclosing generic scopes when refining constructor patterns, and validate shared type interpretations once per erasure pass. Add diff-test regressions and document the outstanding Boolean expansion problem with a commented reproducer.
Preserve keyword-based this/super parsing together with optional source identifiers, and add integration coverage for grouped optional parameters and when splits. Validation: ctest; focused diff tests; hkmc2AllTests/test.
Group callback and function-signature cases, fold small symbol, pattern, optional-value, and type-graph fixtures into their related suites, and remove redundant cases while retaining distinct resolver and backend coverage. Refresh all affected golden outputs. Validation: ctest; hkmc2AllTests/test (1,119 tests passed).
Move the 23 new AI-authored diff-test files under mlscript/slop, including the upstream optional-binding regression suite, preserving their configuration and compilation fixtures. Add hkmc2SlopTests and sdtest, exclude the directory from main discovery, and retain aggregate validation. Validation after rebasing onto 946a5b1: ctest; focused optional and variadic diff-tests; hkmc2AllTests/test (1,120 passed, including all 23 slop tests).
# Conflicts: # hkmc2/shared/src/main/scala/hkmc2/semantics/ups/SplitCompiler.scala
Share fixture compilation through a named SBT prerequisite so the Wasm and slop suites wait for the same task. This generates Wasm.mjs before slop tests run in a fresh checkout and avoids duplicate fixture compilation in aggregates. Validation: hkmc2SlopTests/test with Wasm.mjs absent (23 slop tests passed); hkmc2AllTests/test (1,120 passed, one Wasm fixture compilation). Golden output unchanged.
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Description by Codex:
This adds optional and droppable parameters and allows required parameters after a variadic parameter, with support in function types, resolution, and JavaScript lowering.
fun f(x, y?)andfun f(x, y?: T)allow callers to omit trailing arguments. Omitted arguments areundefined; an optional annotation includesundefined. Function types support both unnamed and named optional parameters.dropmarks arguments that callback implementations may ignore, including a droppable rest parameter. Prelude callback signatures now let functions such asArray.mapaccept callbacks that omit the index and array parameters.fun f(x, y?, ...xs, z)assigns required arguments from both ends, fills optional leading positions from the remaining arguments, and collects the middle inxs.Resolution checks obvious separate-signature and callback-layout mismatches (others will be reported by type the future checking pass), reports callback errors at their use sites, and retains parameter/result flows when approximating differing or compound signatures. Spread arguments are matched from both ends. Lowering extracts trailing parameters, preserves optionality through eta expansion and tail recursion, and avoids unsafe uncurrying. Runtime arity checks reject missing required arguments while allowing extra arguments to be ignored.
Non-tuple spreads now follow the JavaScript iteration protocol.
@annotations.freshidentifies stable property keys, and@annotations.jsSymbol(path)associates a source member name with such a key in nominal and structural interfaces. Computed keys remain explicit operands through lowering and transformations, with access paths recovered across lexical scopes and imports. Resolution diagnoses key collisions, inconsistent overrides, and unavailable keys.The prelude and iterable libraries adopt these features, including symbol-keyed iteration for arrays, strings, maps, and sets. Regression coverage includes callbacks, separate signatures, curried calls, spread layouts, generic element flows, keyed inheritance and imports, and bounded recursive inference growth. Optional parameters remain unsupported by the WebAssembly backend and InvalML.