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Declare the variance of iterator, generator and tagged template types - #8981
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Why:
The iterator family's type parameters were invariant, so a generator
of `[#A]` could not be coerced to one of `[#A | #B]`, and values of
library types built on `Generator.t` were not generalised under the
relaxed value restriction. Yields and return values only flow out of
an iterator and resume values only flow in. Needed for generators
(#8934).
Typing modified (coercion):
Example:
let g: Generator.t<[#A], int, [#X | #Y]> = gen()
let h = (g :> Generator.t<[#A | #B], int, [#X]>)
Before (rule):
t<y, r, n> ≤ t<y', r', n'> only if y = y', r = r', n = n'
After (rule):
y ≤ y' r ≤ r' n' ≤ n
───────────────────────── (Iterator variance)
t<y, r, n> ≤ t<y', r', n'>
t ∈ {Iterator, AsyncIterator, Generator, AsyncGenerator, IterableIterator, AsyncIterableIterator, IteratorObject}
rawResult<+y, +r> arrayLike<+a> taggedTemplate<-param, +output>
Review: Is `+'return` sound when `Generator.returnValue` takes a `'return`? Only up to a contrived case: a generator shared under two types, resumed through `returnValue` while a `finally` block yields, hands the wider value to the narrower reference. TypeScript's `Generator` has the same hole.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fc7LEuyEfynjaKCVEBPmTF
Signed-off-by: Christoph Knittel <ck@cca.io>
Semantic Diff1 commit · Full page Declare the variance of iterator, generator and tagged template types85cf497 · Christoph Knittel with Claude Opus 5.5 · Oct 23, 2026
Typing modified (coercion) let g: Generator.t<[#A], int, [#X | #Y]> = gen()
let h = (g :> Generator.t<[#A | #B], int, [#X]>)
Review
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #8981 +/- ##
==========================================
+ Coverage 81.22% 81.24% +0.01%
==========================================
Files 457 457
Lines 63862 63863 +1
==========================================
+ Hits 51875 51888 +13
+ Misses 11987 11975 -12
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2 participants
Part of the groundwork for generator functions (RFC #8934), following the variance feedback there.
The iterator and generator types were invariant, so a
Generator.t<[#A], _, _>couldn't be coerced to aGenerator.t<[#A | #B], _, _>, while the equivalentiterablecoercion worked. Library types defined as aliases ofGenerator.t, such as effect types with polymorphic-variant error rows, also weren't generalised under the relaxed value restriction. Yield and return values only flow out of an iterator and resume values only flow in, so this declares:Iterator.t,AsyncIterator.t,Generator.t,AsyncGenerator.t,IterableIterator.t,AsyncIterableIterator.t,IteratorObject.t<+'yield, +'return, -'next>Iterator.rawResult,AsyncIterator.rawResult<+'yield, +'return>Array.arrayLike<+'a>(it is only read)taggedTemplate<-'param, +'output>(it is a function)Annotations only accept more programs, so existing code keeps compiling.
Audit of the rest of the stdlib
array,dict,Map,Set,WeakMap,WeakSet,TypedArray,ref) and the phantom tags (JsxEvent.synthetic, the Dom hierarchy).iterable,asyncIterable,promise,option,list,result,null,undefined,nullableandLazy.t.Jsx.componentis already contravariant.Map,MapDict,MapInt,MapString) could be covariant in their values. But their AVL nodes havemutablefields (mutated only while building fresh trees), so the implementations are inferred invariant. Declaring+'valuewould need an abstract implementation type and casts throughout those modules.'idstay invariant everywhere, because they flow into the comparator.Caveat on
+'returnGenerator.returnValuetakes a'return. A generator shared under two types could:returnValuewith a wider value while afinallyblock yields;This case is contrived, and TypeScript's
Generatorhas the same hole. Keeping'returninvariant would lose most of the benefit.Tests
tests/tests/src/stdlib_variance_test.res:Generator.t;super_errors/GeneratorResumeTypeWidening: widening the resume type is still rejected.🤖 Generated with Claude Code
https://claude.ai/code/session_01Fc7LEuyEfynjaKCVEBPmTF