Align List membership APIs with Array - #8530
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Aug 7, 2026
| let has: (list<'a>, 'b, ('a, 'b) => bool) => bool | ||
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| /** | ||
| `includes(list, item)` checks whether `list` includes `item` using strict equality. |
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Which is different from Array.includes which uses SameValueZero equality, but I was unsure if the NaN edge case is worth complicating the implementation.
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Summary
List.includesfor checking list membership using strict equalityList.hasAPI in favor ofList.someArray.includesStdlib_Listtests covering:NaNbehaviorMotivation
The list and array APIs currently use inconsistent names for membership checks. Arrays expose the familiar
someandincludesAPIs, while lists additionally expose the legacy comparator-basedhas.Adding
List.includesaligns the membership API names across lists and arrays. DeprecatingList.hasguides custom comparisons toward the existing predicate-basedList.someAPI without breaking existing code.The existing
Array.includesdocumentation incorrectly described its JavaScript implementation as using strict equality. It actually uses SameValueZero equality.Semantics
List.includesuses strict equality:NaNis not equal to itselfThis intentionally keeps the list implementation simple.
Array.includesretains JavaScript’s native SameValueZero semantics, under whichNaNis equal to itself. Its documentation now describes this distinction correctly.