hypsakata opened a new issue, #48478:
URL: https://github.com/apache/arrow/issues/48478
### Describe the bug, including details regarding any error messages,
version, and platform.
When creating an `Arrow::Table` from a Ruby Hash, if a column contains
nested arrays consisting solely of non-negative Integer values, the column is
incorrectly inferred as `string` (utf8) instead of a list of integers.
However, if a negative integer is present in the data, the column is
correctly inferred as a list type.
#### Analysis (suspected root cause)
It appears the issue lies within `red-arrow/lib/arrow/array-builder.rb`.
`detect_builder_info()` returns `UIntArrayBuilder` with `detected: false`
for non-negative Integers (presumably to allow upgrading to a signed type if a
negative value appears later).
In the case of Arrays, a `ListArrayBuilder` seems to be constructed only
when `sub_builder_info[:detected]` is `true`. Consequently, nested arrays
containing only non-negative integers fail to produce a list type, causing the
column to fall back to `string` (utf8).
### Steps to reproduce the bug
```ruby
require "arrow"
# Case 1: Only non-negative integers (Bug)
p Arrow::Table.new({ id: [1, 2], values: [[0, 1, 2], [3, 4]] }).schema
# Actual: values is inferred as string (utf8)
# Output:
# #<Arrow::Schema:... id: uint8
# values: string>
```
```ruby
require "arrow"
# Case 2: Contains a negative integer (Works as expected)
p Arrow::Table.new({ id: [1, 2], values: [[0, -1, 2], [3, 4]] }).schema
# Actual: values is inferred as list<int8>
# Output:
# #<Arrow::Schema:... id: uint8
# values: list<item: int8>>
```
### Expected behavior
`values` should be inferred as a list of integers (e.g. `list<item: int*>`),
not `string`,
even when all integers are non-negative. (The exact integer bit width may
vary.)
### Actual behavior
When all integers are non-negative, `values` is inferred as `string` (utf8).
Adding a negative integer results in the correct list type inference.
### Environment
- OS: macOS 26.1
- CPU arch: Apple M4 Pro
- Ruby: 3.4.7
- Gems: red-arrow 22.0.0
- Arrow installation method: Homebrew
### Component(s)
Ruby
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