[GitHub] [beam] lostluck commented on a diff in pull request #25203: Fix interface{} in iter& emit type of DoFn in Go

["lostluck (via GitHub)" <gi...@apache.org>]

lostluck commented on code in PR #25203:
URL: https://github.com/apache/beam/pull/25203#discussion_r1102041421


##########
sdks/go/pkg/beam/core/funcx/output.go:
##########
@@ -84,6 +84,9 @@ func unfoldEmit(t reflect.Type) ([]reflect.Type, bool, error) {
 		if ok, err := isInParam(t.In(i)); !ok {
 			return nil, false, errors.Wrap(err, errIllegalParametersInEmit)
 		}
+		if t.In(i).Kind() == reflect.Interface || (t.In(i).Kind() == reflect.Pointer && t.In(i).Elem().Kind() == reflect.Interface) {

Review Comment:
   Yes. Exactly. Deny "interface{}" but allow `beam.T` et al.
   
   The "universal" types should only be referring to `beam.T` etc. Not arbitrary interfaces. Basically, it's important to distinguish between them (`beam.T` is not `beam.U`) so that for example, if a function is dropping a key or a value in a DoFn, we know which one is being used downstream./
   
   Users can use beam.T and similar to escape the type system, but then they deserve any runtime type errors they get at pipeline execution time. The strictness of knowing what types are used where is to help users avoid expensive execution time issues.



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