I know I should have replied earlier to Michael's requests on serializing JSON (#26784), but I think that one issue is worth being discussed a bit more:

If sequences with zero items or more than one item are to be output on top level, I would recommend not to output them as 'null' or as array:

* To me, serialization has always been a non-blocking operation. If we don't know in advance if a query will yield more than one item, and if we treat sequences as arrays, we would from now on need to cache the first item before we know if it needs to be prefixed with a square bracket.

* In my perception, all other output methods handle sequences differently. Think of the XML output method: multiple nodes will simply be appended to each other, and the result won't be wrapped by an additional root element.

* Similarly, if a query does not yield any result, I would expect it not to return any output at all, as is the case with the other output methods.

If sequences occur on a deeper level, an error may be more appropriate. This way, parsing and serializing JSON will be more bidirectional. See the following query as example:

  map { 'A': ('B','C') }

If the value of the map object would now be serialized as array, and if it was parsed via fn:parse-json, we would get a different result.