[BEAM-8612] Convert []beam.T to the underlying type []T when passed to a DoFn with universal typed (beam.X) input

• Type: New Feature
• Status: Resolved
• Priority: P3
• Resolution: Fixed
• Fix Version/s: Not applicable
• Component/s: sdk-go
• Assignee: Tianyang Hu
• Reporter: Tianyang Hu
• Created: 11/Nov/19 23:11
• Updated: 16/May/20 13:27
• Resolved: 26/Nov/19 18:47

Description

Say there are two DoFn: f1, f2.

• f1 declares the output type as []beam.T, and each element has the underlying type int.
• f2 declares the input type as []int

Passing f1 output to f2 works well. The conversion from []beam.T to []int happens at: https://github.com/apache/beam/blob/c7be0643934a87d73483cf1fd3199a425508b03c/sdks/go/pkg/beam/core/runtime/exec/fullvalue.go#L108

But it doesn't work if f2 declares the input type as beam.X and type casts it to []int. This is because there's no type conversion when passing []beam.T to beam.X.

We may consider supporting the above case by converting []beam.T to the underlying type []T when it's passed to a universal type.

An issue is that if []beam.T is nil or empty, we don't know its underlying element type (unless we know which concrete type beam.T or beam.X is bound to, but this mapping doesn't seem to be kept at runtime?). In such case, we have to pass []beam.T to beam.X as is.

Issue Links

• links to: GitHub Pull Request #10066