fix(type-inference)!: resolve lambda scopes and invocation types

[alexandrefimov]

Keep captured input-row references separate from lambda parameters, resolve nested parameter references through their enclosing lambda scopes, and derive lambda invocation types from their bodies, as specified in spec v0.101.0.

BREAKING CHANGE: Type inference inside lambda bodies now resolves root references against the input row and parameter references against the selected lambda scope. References to a missing enclosing lambda scope raise an error. Consumers relying on the previous parameter-row conflation must use the appropriate reference kind and steps_out.

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⚙️ Run configuration

• Configuration used: Repository: substrait-io/substrait-python/.coderabbit.yaml
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Reviewing files that changed from the base of the PR and between 33a5a7c and ac83190.

📒 Files selected for processing (2)

• src/substrait/type_inference.py
• tests/test_type_inference.py

📝 Summary

Summary by CodeRabbit

• New Features
  • List transformations now support nested callbacks that reference values from the input row or enclosing callbacks.
• Bug Fixes
  • Lambda expressions now infer result types and nullability correctly, including when used in switching fields.
  • References to parameters outside the available lambda scopes now produce an error, and callback scopes no longer affect later expression inference.

Walkthrough

Type inference now tracks nested lambda parameter scopes and resolves parameter references by steps_out. Lambda bodies and invocations use these scopes. Higher-order list expressions assign callback element references the appropriate scope offset. Tests cover nested scopes, root-row references, invocation types, and ExpandRel fields.

Changes

Lambda scope and inference

Layer / File(s)	Summary
Lambda parameter scope resolution src/substrait/type_inference.py, tests/test_type_inference.py	Type inference maintains active lambda parameter structs and resolves parameter references by steps_out. Tests cover nested and missing scopes, root-row references, and scope restoration.
Lambda body and invocation inference src/substrait/type_inference.py, tests/test_type_inference.py	Lambda bodies are inferred within their parameter scope. Invocations validate argument counts, infer arguments, and return the body type. Tests cover invocation types and ExpandRel switching fields.
Nested list-transform callback scopes src/substrait/dataframe/expr.py, tests/dataframe/test_frame.py	List-transform callbacks run within lambda scopes, and element references use an offset based on the active scope depth. Tests cover captures from the input row and enclosing callbacks.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant ListTransformBuilder
  participant TypeInference
  participant LambdaScope
  ListTransformBuilder->>TypeInference: Read active lambda depth
  ListTransformBuilder->>LambdaScope: Push callback parameter struct
  TypeInference->>LambdaScope: Resolve lambda reference by steps_out
  LambdaScope-->>TypeInference: Return matching parameter struct
  ListTransformBuilder->>LambdaScope: Restore prior scope after callback

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Suggested reviewers: nielspardon

Merge Risk: 🔵 Low · up to 33a5a

Type-incompatible lambda invocations can be accepted as valid. Add the parameter-type check before merging, or explicitly accept this bounded risk.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the lambda-scope and invocation-type fix. It uses the required Conventional Commit format and marks the change as breaking.
Description check	✅ Passed	The description explains the rationale, the lambda reference behavior, and the breaking change. It also includes a BREAKING CHANGE footer, so it provides the required information.
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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[nielspardon]

One design point inline. The builder fallback is what lets out-of-scope parameter references through, so the footer's "references to a missing enclosing lambda scope raise an error" holds today only for steps_out ≥ 1.

[nielspardon]

The fallback is gone and builder callbacks can now capture input columns, thanks. One new issue that follows from that, plus the smaller points I held back last round.

[alexandrefimov]

Fixed nested captures, added invocation argument checks, and reused _field_reference. I also checked spec v0.105.0: it still doesn’t define whether lambda scope crosses subquery boundaries, so I’ve left that behavior unchanged pending clarification.

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/substrait/type_inference.py:
- Around line 694-704: Update the lambda_invocation branch in
infer_expression_type to compare each argument’s inferred type with its
corresponding lambda parameter type, rejecting mismatches while preserving the
existing argument-count check and body_type return for matching arguments.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Repository: substrait-io/substrait-python/.coderabbit.yaml
• Review profile: CHILL
• Plan: Advanced
• Run ID: edd8b2f8-be4f-4044-9d49-eef8265fce7d

📥 Commits

Reviewing files that changed from the base of the PR and between 9470831 and 33a5a7c.

📒 Files selected for processing (4)

• src/substrait/dataframe/expr.py
• src/substrait/type_inference.py
• tests/dataframe/test_frame.py
• tests/test_type_inference.py

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