Keep Chunk.take and Chunk.drop valid for fractional counts

[fubhy]

Summary

Fractional take and drop counts produce chunks with fractional public lengths that throw RangeError when materialized.

Important

This PR starts with focused failing reproduction tests. Add the implementation fix to this same branch; CI is expected to fail until that fix is included.

Fractional counts create malformed chunks

Module: Chunk
Audit ID: core-a-f-chunk-fractional-slice-dimensions
Severity / confidence: high / high

What happens

Fractional take and drop counts produce chunks with fractional public lengths that throw RangeError when materialized.

Why it happens

Fractional counts are stored directly as ISlice.length and ISlice.offset. Materialization passes the fractional length to new Array(self.length), which throws RangeError: Invalid array length.

Expected behavior

take and drop return valid chunks containing a count-based prefix or suffix for their accepted number argument; no integer-only precondition is imposed.

Relevant implementation

These links and excerpts are pinned to audit base c9b56ab507f224426ee8388dc450da447ec4715f.

• packages/effect/src/Chunk.ts:728-767
• packages/effect/src/Chunk.ts:784-822
• packages/effect/src/Chunk.ts:408-427

View problematic code at packages/effect/src/Chunk.ts:728-767

export const take: {
  (n: number): <A>(self: Chunk<A>) => Chunk<A>
  <A>(self: Chunk<A>, n: number): Chunk<A>
} = dual(2, <A>(self: Chunk<A>, n: number): Chunk<A> => {
  if (n <= 0) {
    return _empty
  } else if (n >= self.length) {
    return self
  } else {
    switch (self.backing._tag) {
      case "ISlice": {
        return makeChunk({
          _tag: "ISlice",
          chunk: self.backing.chunk,
          length: n,
          offset: self.backing.offset
        })
      }
      case "IConcat": {
        if (n > self.left.length) {
          return makeChunk({
            _tag: "IConcat",
            left: self.left,
            right: take(self.right, n - self.left.length)
          })
        }

        return take(self.left, n)
      }
      default: {
        return makeChunk({
          _tag: "ISlice",
          chunk: self,
          offset: 0,
          length: n
        })
      }
    }
  }
})

View exact lines on GitHub

View problematic code at packages/effect/src/Chunk.ts:784-822

export const drop: {
  (n: number): <A>(self: Chunk<A>) => Chunk<A>
  <A>(self: Chunk<A>, n: number): Chunk<A>
} = dual(2, <A>(self: Chunk<A>, n: number): Chunk<A> => {
  if (n <= 0) {
    return self
  } else if (n >= self.length) {
    return _empty
  } else {
    switch (self.backing._tag) {
      case "ISlice": {
        return makeChunk({
          _tag: "ISlice",
          chunk: self.backing.chunk,
          offset: self.backing.offset + n,
          length: self.backing.length - n
        })
      }
      case "IConcat": {
        if (n > self.left.length) {
          return drop(self.right, n - self.left.length)
        }
        return makeChunk({
          _tag: "IConcat",
          left: drop(self.left, n),
          right: self.right
        })
      }
      default: {
        return makeChunk({
          _tag: "ISlice",
          chunk: self,
          offset: n,
          length: self.length - n
        })
      }
    }
  }
})

View exact lines on GitHub

View problematic code at packages/effect/src/Chunk.ts:408-427

const toReadonlyArray_ = <A>(self: Chunk<A>): ReadonlyArray<A> => {
  switch (self.backing._tag) {
    case "IEmpty": {
      return emptyArray
    }
    case "IArray": {
      return self.backing.array
    }
    default: {
      const arr = new Array<A>(self.length)
      copyToArray(self, arr, 0)
      self.backing = {
        _tag: "IArray",
        array: arr
      }
      self.left = _empty
      self.right = _empty
      self.depth = 0
      return arr
    }

View exact lines on GitHub

Reproduction

pnpm test --run packages/effect/test/Chunk.test.ts -t "produces valid chunks for fractional counts"

Observed failure: Materializing Chunk.take(Chunk.make(1, 2, 3), 1.5) threw RangeError: Invalid array length.

Implementation handoff

The initial reproduction tests on this branch are the regression specification for the implementation fix that should follow in this PR.

1. Start with the pinned implementation excerpts and the Why it happens analysis above.
2. Change the implementation so it satisfies the stated Expected behavior; do not weaken or remove the reproduction assertions.
3. Run the focused reproduction command(s) and confirm the observed failures become passing tests:

pnpm test --run packages/effect/test/Chunk.test.ts -t "produces valid chunks for fractional counts"

4. Run the affected package's existing tests, then the repository lint and type checks before requesting review.

Audit provenance

• Audit base: c9b56ab507f224426ee8388dc450da447ec4715f
• Reproduction base: c9b56ab507f224426ee8388dc450da447ec4715f
• Findings: core-a-f-chunk-fractional-slice-dimensions
• Initial patch: focused reproduction tests; implementation fix pending

Closes EFF-350

[changeset-bot]

🦋 Changeset detected

Latest commit: 91bf5ed

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 30 packages

Name	Type
effect	Patch
@effect/opentelemetry	Patch
@effect/platform-browser	Patch
@effect/platform-bun	Patch
@effect/platform-deno	Patch
@effect/platform-node-shared	Patch
@effect/platform-node	Patch
@effect/vitest	Patch
@effect/ai-anthropic	Patch
@effect/ai-openai-compat	Patch
@effect/ai-openai	Patch
@effect/ai-openrouter	Patch
@effect/atom-react	Patch
@effect/atom-solid	Patch
@effect/atom-vue	Patch
@effect/sql-clickhouse	Patch
@effect/sql-d1	Patch
@effect/sql-libsql	Patch
@effect/sql-mssql	Patch
@effect/sql-mysql2	Patch
@effect/sql-pg	Patch
@effect/sql-pglite	Patch
@effect/sql-sqlite-bun	Patch
@effect/sql-sqlite-do	Patch
@effect/sql-sqlite-node	Patch
@effect/sql-sqlite-react-native	Patch
@effect/sql-sqlite-wasm	Patch
@effect/docgen	Patch
@effect/doctest	Patch
@effect/openapi-generator	Patch

Not sure what this means? Click here to learn what changesets are.

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[pullfrog]

✅ No new issues found.

Reviewed changes — one reproduction test added to Chunk.test.ts:

• Fractional-count reproduction: adds it("produces valid chunks for fractional counts") under the existing take describe block, asserting take(1.5) → [1] and drop(1.5) → [2, 3]. The test correctly surfaces the RangeError: Invalid array length caused by the internal ISlice / IConcat backing structures using a non-integer length field.

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[tim-smart]

I think we should flip the variable names so the function parameter has the underscore.

[pullfrog]

✅ No new issues found.

Reviewed changes — Math.floor fix applied to take and drop, plus naming feedback:

• Fractional-count fix in take and drop: added const n = Math.floor(_n) at the top of both functions so slice length and offset fields are always integers. This resolves the RangeError: Invalid array length thrown during materialization of chunks produced by fractional counts.
• Variable naming flipped per review feedback: the function parameter is now _n (underscore-prefixed raw input) and the floored local is n.
• Reproduction test retained: it("produces valid chunks for fractional counts") asserts take(1.5) → [1] and drop(1.5) → [2, 3].

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[github-actions]

Bundle Size Analysis

Generated from PR build output; treat the content below as untrusted.

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