• Compiler Used: Xcode 9.2
• Operating System: macOS 10.12.6
• Architecture (ARM/x86/32bit/64bit/etc): Intel Core i5 64bit

Expected vs Actual Behavior

If you call to_json([]) (aka to_json(Vector())) or to_json(Map()) you get the string "null", whereas it makes more "semantic" sense to return an empty JSON object "{}" or array "[]", respectively, as that's what you gave it.

This kind-of also relates to the fact that a "minimal" JSON text should be an empty object or array, not a standalone value (including "null")....though some JSON parsers do support free-standing values. Even so, it makes more sense to allow a round-trip to maintain the type (e.g. from_json(to_json([])) == []).

Minimal Example to Reproduce Behavior

// The following should pass (added to new unit test):
assert_equal(from_json(to_json([])), [])
assert_equal(from_json(to_json(Map())), Map())

The above examples currently fail, because to_json([]) returns "null".