fix(stdlib): ArrayIterator nullable elements + eager Array.filter_map + live for-loop tests by aaronvg · Pull Request #3749 · BoundaryML/baml

aaronvg · 2026-06-12T00:54:14Z

Summary

Follow-ups to #3747 (lazy array iterators):

Fix nullable-element early termination. ArrayIterator.next matched arr.at(idx) against null to detect the end of iteration, so an array of nullable T containing a null element stopped iterating at the first null. It now bounds-checks by index, so stored null elements are yielded. This was pre-existing, but live iterators flowing through more code paths (including every for loop) made it likelier to bite.
Add eager Array.filter_map. Eager filter/map return arrays, but filter_map only existed via .iter().filter_map(fn).collect(). This adds an eager array-returning form, implemented in BAML and mirroring for_each's snapshot-via-slice idiom; the lazy version remains on Iterator.
Pin for-loop live-view semantics. [codex] Make array iterators lazy #3747 made ArrayIterator hold the source array instead of a snapshot, and for loops lower through Iterable.iter(), so loops now see mid-loop mutations. That was only tested via explicit .iter(); this adds tests pinning that a push mid-loop is visited and a pop mid-loop terminates cleanly.

Validation

INSTA_UPDATE=always cargo insta test --test-runner nextest -p baml_tests -p baml_cli
cargo nextest run -p baml_tests --test baml_src (2 passed)
cargo nextest run -p baml_tests --lib --test bytecode_format (1599 passed)
cargo nextest run -p baml_cli describe (83 passed)
git diff --check

🤖 Generated with Claude Code

Note

Medium Risk
Changes core iterator/for-loop semantics for nullable element types and documents live-array mutation during iteration; incorrect bounds logic could affect many call sites.

Overview
Fixes array iteration when elements can be null: ArrayIterator.next now ends on an index past the length instead of treating at() returning null as exhaustion, so nullable arrays yield every slot including stored nulls.

Adds eager Array.filter_map, building a new array via a slice-bounded for loop (same snapshot pattern as for_each); lazy iter().filter_map is unchanged.

New stdlib tests lock in for-loop behavior over live array views (push mid-loop is seen; repeated pop shortens iteration), eager filter_map, and nullable-element iteration/count.

^{Reviewed by Cursor Bugbot for commit fc9a51f. Bugbot is set up for automated code reviews on this repo. Configure here.}

Summary by CodeRabbit

New Features
- Added filter_map method for arrays to combine mapping and filtering operations in a single operation.
Bug Fixes
- Fixed iterator behavior to correctly yield null elements in arrays with nullable types, rather than treating them as iteration endpoints.
Tests
- Added comprehensive iterator test coverage for array mutations and nullable element handling.

…filter_map; pin live for-loop semantics Follow-ups to #3747 (lazy array iterators): - Fix ArrayIterator.next to bounds-check by index instead of matching `at()` against null, so arrays of nullable T containing null elements iterate all elements rather than terminating early at the first null. - Add eager `Array.filter_map` returning `U[]`, restoring symmetry with eager `filter`/`map` (lazy version remains `iter().filter_map(fn)`). - Add baml_src tests pinning that `for` loops are a live view of the source array (pushes mid-loop are visited; pops mid-loop terminate cleanly), plus coverage for the nullable-element fix and eager filter_map. Co-Authored-By: Claude Fable 5 <[email protected]>

vercel · 2026-06-12T00:54:19Z

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coderabbitai · 2026-06-12T00:54:34Z

No actionable comments were generated in the recent review. 🎉

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Run ID: e4299fc7-807e-42e7-8521-6ba17916a599

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Reviewing files that changed from the base of the PR and between 725edd0 and 88de9b5.

⛔ Files ignored due to path filters (9)

baml_language/crates/baml_cli/src/snapshots/baml_cli__describe_command_tests__render_builtin_package_listing.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/baml_src/iter.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/__baml_std__/baml_tests__compiles____baml_std____03_hir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/__baml_std__/baml_tests__compiles____baml_std____04_5_mir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/__baml_std__/baml_tests__compiles____baml_std____04_tir.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/snapshots/compiles/__baml_std__/baml_tests__compiles____baml_std____06_codegen.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/src/compiler2_tir/snapshots/baml_tests__compiler2_tir__phase5__snapshot_baml_package_items.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/tests/bytecode_format/snapshots/bytecode_format__bytecode_display_expanded.snap is excluded by !**/*.snap
baml_language/crates/baml_tests/tests/bytecode_format/snapshots/bytecode_format__bytecode_display_expanded_unoptimized.snap is excluded by !**/*.snap

📒 Files selected for processing (3)

baml_language/crates/baml_builtins2/baml_std/baml/containers.baml
baml_language/crates/baml_builtins2/baml_std/baml/ns_iter/iter.baml
baml_language/crates/baml_tests/baml_src/ns_iter/iter.baml

📝 Walkthrough

Walkthrough

This PR adds Array<T>.filter_map, a new container method that maps and filters array elements using a callback producing nullable outputs. It also fixes ArrayIterator<T>.next to use explicit bounds checking instead of null matching, enabling correct iteration over arrays with nullable elements. The change includes comprehensive test coverage for mutation visibility during iteration and filter_map behavior.

Changes

Array iteration and filter_map enhancement

Layer / File(s)	Summary
Iterator semantics fix for nullable elements `baml_language/crates/baml_builtins2/baml_std/baml/ns_iter/iter.baml`	`ArrayIterator<T>.next` now performs explicit bounds checking with `self.idx >= self.arr.length()` before retrieving the element, allowing nullable `T` elements to be yielded without being conflated with iteration termination.
Array.filter_map method implementation `baml_language/crates/baml_builtins2/baml_std/baml/containers.baml`	New `Array<T>.filter_map<U, E>` method accepts a callback that produces `U?` and returns a new array containing only non-null mapped values. Implementation iterates the source array and collects results without mutating `self`.
Iterator semantics and filter_map test coverage `baml_language/crates/baml_tests/baml_src/ns_iter/iter.baml`	Tests validate iterator mutation visibility during `for` loops (`push`/`pop` during iteration), eager `filter_map` behavior over integer arrays, and iteration over nullable array elements with element count, null count, sum of non-null values, and iterator count tracking.

Sequence Diagram

sequenceDiagram
  participant Caller
  participant Array as Array[T]
  participant Iterator as ArrayIterator[T]
  participant Callback as fn(T)->U?
  participant Result as Array[U]
  
  Caller->>Array: filter_map(fn)
  loop for each element
    Array->>Iterator: next()
    Iterator-->>Array: element (or Done)
    Array->>Callback: fn(element)
    Callback-->>Array: U? (mapped value)
    Array->>Result: append if not null
  end
  Array-->>Caller: return Array[U]

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

BoundaryML/baml#3747: Related concurrent changes to Array container helper methods and ArrayIterator semantics in iterator behavior.

Poem

🐰 A rabbit hops through arrays clear,
Now filter_map brings joy near!
Null elements won't fool the way,
Bounds check rules the iteration day! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically summarizes the three main changes: fixing nullable element handling in ArrayIterator, adding eager Array.filter_map, and adding test coverage for for-loop live-view semantics.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

Create PR with unit tests
Commit unit tests in branch aaron/iter-live-view-tests-and-nullable-fix

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github-actions · 2026-06-12T00:55:10Z

⏭️ Performance benchmarks were skipped

Perf benchmarks (CodSpeed) are opt-in on pull requests — they no longer run on every push. They always run automatically after merge to canary/main.

To run them on this PR, do any of the following, then push a commit (or re-run CI):

Add RUN_CODSPEED=1 to the PR description, or
Include run-perf or /perf in the PR title or any commit message.

Co-Authored-By: Claude Fable 5 <[email protected]>

github-actions · 2026-06-12T00:58:41Z

No description provided.

vercel Bot temporarily deployed to Preview – beps June 12, 2026 00:55 Inactive

aaronvg enabled auto-merge June 12, 2026 00:55

chore: empty commit to rerun CI

fc9a51f

Co-Authored-By: Claude Fable 5 <[email protected]>

vercel Bot temporarily deployed to Preview – promptfiddle June 12, 2026 00:57 Inactive

coderabbitai Bot approved these changes Jun 12, 2026

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vercel Bot temporarily deployed to Preview – beps June 12, 2026 00:58 Inactive

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aaronvg added this pull request to the merge queue Jun 12, 2026

Merged via the queue into canary with commit 975e4f9 Jun 12, 2026
52 checks passed

aaronvg deleted the aaron/iter-live-view-tests-and-nullable-fix branch June 12, 2026 01:18

coderabbitai Bot mentioned this pull request Jul 2, 2026

Fix generic match exhaustiveness so null can't leak into a non-nullable T (B-633) #3904

Closed

coderabbitai Bot mentioned this pull request Jul 25, 2026

Handle typeargs in pattern matching #4203

Merged

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fix(stdlib): ArrayIterator nullable elements + eager Array.filter_map + live for-loop tests - #3749

fix(stdlib): ArrayIterator nullable elements + eager Array.filter_map + live for-loop tests#3749
aaronvg merged 2 commits into
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aaron/iter-live-view-tests-and-nullable-fix

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