chore(pass-style): make `CopyArray` compatible with `ReadonlyArray` by michaelfig · Pull Request #3061 · endojs/endo

michaelfig · 2026-01-28T17:43:47Z

Closes: #3060

Description

This PR updates the CopyArray type definition from Array to ReadonlyArray to better reflect that CopyArrays are hardened and immutable at runtime. This allows readonly arrays and tuples to be accepted as Passable, resolving issue #3060.

Security Considerations

Types now reflect that @endo/pass-style's Passable type doesn't require CopyArrays to be mutable.

Scaling Considerations

n/a

Documentation Considerations

If you typed a mutable array as a CopyArray, Typescript will show errors if you try to mutate it directly, or call methods that would mutate it. You can achieve the original behavior by typing your array as Passable[].

Testing Considerations

Type tests have been added.

Compatibility Considerations

Makes for better compatibility.

Upgrade Considerations

No runtime change, typing only.

Copilot

Pull request overview

This PR updates the CopyArray<T> type definition from Array<T> to ReadonlyArray<T> to better reflect that CopyArrays are hardened and immutable at runtime. This allows readonly arrays and tuples to be accepted as Passable, resolving issue #3060.

Changes:

Changed CopyArray<T> type from Array<T> to ReadonlyArray<T> in type definitions
Added type tests to verify readonly arrays and tuples are accepted as Passable
Updated NEWS.md to document the type change

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/pass-style/src/types.d.ts	Changes `CopyArray<T>` from `Array<T>` to `ReadonlyArray<T>` to reflect immutability
packages/pass-style/src/types.test-d.ts	Adds type tests for readonly arrays, const tuples, and various array forms
packages/pass-style/NEWS.md	Documents the type change in the release notes

The change is semantically correct - CopyArrays are hardened at runtime and should not be mutated. The type system should reflect this. However, there is a downstream impact on the patterns package that uses incorrect type annotations for mutable accumulators.

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michaelfig · 2026-01-28T20:58:45Z

@copilot, can you specify a few examples of exactly where it is in "the patterns package that uses incorrect type annotations for mutable accumulators"?

Copilot · 2026-01-28T20:58:53Z

@michaelfig I've opened a new pull request, #3062, to work on those changes. Once the pull request is ready, I'll request review from you.

gibson042 · 2026-01-29T01:40:38Z

 export type CopyArray<T extends Passable = any> = Array<T>;

+/**
+ * A Passable and Readonly sequence of Passable values.
+ */
+export type CopyReadonlyArray<T extends Passable = any> = ReadonlyArray<T>;


Why introduce a new type? What breaks if CopyArray is instead updated to use ReadonlyArray rather than Array?

Thanks for asking!

The breaking change is that there are some uses in the wild of CopyArray as the type of a mutable array, and then the code proceeds to mutate the array. If CopyArray = ReadonlyArray, the code fails to typecheck.

At this stage in the game, I'm ~~very reluctant~~ somewhat willing to introduce breaking changes.

@copilot, for this endojs/endo repository, in *.js files containing JSDoc TypeScript types and *.ts TypeScript files, can you please identify all arrays that are declared to be of type CopyArray that are later mutated?

Use shell commands with intermediary files whenever possible to save your work and avoid consuming too much of your agent's context space.

Besides the highly enthusiastic affirmations of my AI sycophant, I also see that a mutable CopyArray type is useful to enforce that all its elements are Passable.

Having said all that, the friendly bots scoured Agoric/agoric-sdk and endojs/endo, and only @endo/patterns/src/patterns/patternMatchers.js uses a mutable CopyArray in its internals. What do you see as the best way forward?

#3065 will fix the typing that makes it look like patternMatchers.js is attempting CopyArray mutation, and any mutability associated with Passable is itself a bug whose footprint I'd rather reduce than expand. So my preference is to land #3065 and then update this PR to tighten CopyArray without introducing a new type.

Copilot · 2026-01-29T04:37:58Z

@michaelfig I've opened a new pull request, #3063, to work on those changes. Once the pull request is ready, I'll request review from you.

Copilot

Pull request overview

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

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Copilot · 2026-01-29T05:15:59Z

+arr.slice(0, 1);
+arr.shift();
+expectPassable(arr);
+
+// Not so for CopyReadonlyArray.
+const roArr: CopyReadonlyArray<number> = [];
+// @ts-expect-error not mutable
+roArr.push(1);
+roArr.slice(0, 1);
+// @ts-expect-error not mutable
+roArr.shift();
+expectPassable(roArr);


Result from call to slice ignored.

Suggested change

arr.slice(0, 1);

arr.shift();

expectPassable(arr);

// Not so for CopyReadonlyArray.

const roArr: CopyReadonlyArray<number> = [];

// @ts-expect-error not mutable

roArr.push(1);

roArr.slice(0, 1);

// @ts-expect-error not mutable

roArr.shift();

expectPassable(roArr);

const slicedArr = arr.slice(0, 1);

arr.shift();

expectPassable(arr);

expectPassable(slicedArr);

// Not so for CopyReadonlyArray.

const roArr: CopyReadonlyArray<number> = [];

// @ts-expect-error not mutable

roArr.push(1);

const slicedRoArr = roArr.slice(0, 1);

// @ts-expect-error not mutable

roArr.shift();

expectPassable(roArr);

expectPassable(slicedRoArr);

kriskowal · 2026-04-28T00:36:52Z

@gibson042 @turadg Can we confirm this is superseded by #3172 and close, or follow-up?

Related #3063

michaelfig self-assigned this Jan 28, 2026

michaelfig added the bug Something isn't working label Jan 28, 2026

michaelfig force-pushed the mfig/readonly-array-is-passable branch from 31f81e8 to e6a4c16 Compare January 28, 2026 17:52

michaelfig added a commit to Agoric/agoric-sdk that referenced this pull request Jan 28, 2026

build: do a yarn patch for endojs/endo#3061

4ad8e0c

michaelfig force-pushed the mfig/readonly-array-is-passable branch from e6a4c16 to 97a0744 Compare January 28, 2026 19:04

michaelfig requested a review from Copilot January 28, 2026 19:06

Copilot started reviewing on behalf of michaelfig January 28, 2026 19:07 View session

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Copilot AI mentioned this pull request Jan 28, 2026

chore(pass-style): make CopyArray compatible with ReadonlyArray #3062

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gibson042 reviewed Jan 29, 2026

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michaelfig added a commit to Agoric/agoric-sdk that referenced this pull request Jan 29, 2026

fixup! build: do a yarn patch for endojs/endo#3061

9968a27

michaelfig added a commit to Agoric/agoric-sdk that referenced this pull request Jan 29, 2026

build: do a yarn patch for endojs/endo#3061

29597df

michaelfig added a commit to Agoric/agoric-sdk that referenced this pull request Jan 29, 2026

build: do a yarn patch for endojs/endo#3061

3f68436

Copilot AI mentioned this pull request Jan 29, 2026

Analysis: CopyArray mutation usage in endojs/endo #3063

Draft

michaelfig force-pushed the mfig/readonly-array-is-passable branch from 03df212 to d6fc8fd Compare January 29, 2026 05:02

michaelfig requested a review from Copilot January 29, 2026 05:10

Copilot started reviewing on behalf of michaelfig January 29, 2026 05:11 View session

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gibson042 mentioned this pull request Jan 30, 2026

fix(patterns): Update containerHasSplit(copyArray, ...) to return copyArray results #3065

Merged

michaelfig force-pushed the mfig/readonly-array-is-passable branch from d6fc8fd to e5684ea Compare January 30, 2026 08:17

chore(pass-style): make CopyArray compatible with ReadonlyArray

8d8a604

michaelfig force-pushed the mfig/readonly-array-is-passable branch from e5684ea to 8d8a604 Compare January 30, 2026 10:22

kriscendobot mentioned this pull request May 11, 2026

Upstream endo: triage forgotten-approved and near-ready PRs endojs/endo-but-for-bots#220

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