Summary

bison/column.mojo contains ~60 visitor structs with massive code duplication across the five ColumnData type arms (Int64, Float64, Bool, String, PythonObject). Estimated 1,200–1,500 lines of near-identical code (~19% of the file).

Worst offenders

Each visitor repeats the same loop-with-null-check logic, differing only in the typed data accessor and default/sentinel value.

Proposed fix

Once the Mojo compiler fixes the typed-downcast deadlock (#642), collapse each visitor family into a single generic helper parameterized on dtype. Until then, consider:

1. Extract the loop body into a shared helper that accepts typed data by reference — this is possible today for visitors where the loop structure is truly identical.
2. Document the duplication with a comment linking back to this issue so future contributors don't add more arms without recognizing the pattern.

Impact

High — this is the largest source of duplicated code in the project. Every new dtype or operation added requires copying the same pattern into 4–5 more arms. Maintenance burden compounds over time.

Classification

• Code smell: Duplicate Code (Dispensables)
• Refactoring: Extract Method / Replace Conditional with Polymorphism

Related

• Mojo compiler deadlock: rebind typed downcasts on ArcPointer + Variant dispatch in same call graph #642 (compiler typed-downcast deadlock — root cause of the dual-storage architecture)
• tech debt: DataFrame element-wise math methods share identical loop boilerplate #612 (DataFrame element-wise math duplication — same family of issues)