The Ok type is a simple error handler, basically equivalent to Either [SomeException]. This type (without the list) was used to handle conversion errors in early versions of pg.

One of the primary reasons why this type was introduced is that Either SomeException had not been provided an instance for Alternative, and it would have been a bad idea to provide an orphaned instance for a commonly-used type and typeclass included in base.

Extending the failure case to a list of SomeExceptions enables a more sensible Alternative instance definitions: <|> concatenates the list of exceptions when both cases fail, and empty is defined as 'Errors []'. Though <|> one could pick one of two exceptions, and throw away the other, and have empty provide a generic exception, this avoids cases where empty overrides a more informative exception and allows you to see all the different ways your computation has failed.