[Zach_the_Lizard]

Sum types with pattern matching would likely take the place of (foo, error) in a lot of codebases.

This is especially true in cases where a function returns a collection of results where each result is independent from other results and a result can succeed or fail.

For instance, a batch report generator that is called every $INTERVAL might return a ([]Report metadata, error) today, but each report could have succeeded or failed without impacting the others.

The output is emailed to $SOMEONE to let them know the reports ran and information about each.

In today's world, the "error" could be a special error type that capture failures on a per report basis. The ReportMetadata type could also have an Error field, but one is not forced to check either.

Sum types could force checking for error on a per report basis, increasing the odds something is done with the error.