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

["Tianyang Hu (Jira)" <ji...@apache.org>]

Tianyang Hu created BEAM-8612:
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             Summary: Convert []beam.T to the underlying type []T when passed to a DoFn with universal typed (beam.X) input
                 Key: BEAM-8612
                 URL: https://issues.apache.org/jira/browse/BEAM-8612
             Project: Beam
          Issue Type: New Feature
          Components: sdk-go
            Reporter: Tianyang Hu
            Assignee: Tianyang Hu


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 cast it to []int. This is because there's no type conversion when passing []beam.T to beam.X.

We may consider support the above case by converting []beam.T to the underlying type []T when 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, which doesn't seem to be kept at runtime?). In such case, we have to pass []beam.T to beam.X as is.



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