Determining the type (array, object, or scalar) of a JSON value

Hi all,

I recently wanted to declare a CHECK constraint to ensure that a JSON value
was an object with keys that were scalars (not nested objects or arrays).
This proved to be more difficult than I had expected. In the end, I had to
write a json_typeof() function in pl/pgsql. It's a simple function but
uses a rather horrifying technique: it traps the exceptions raised when
trying to apply json_array_length() to a non-array and json_each() to a
non-object! I couldn't figure out a robust technique to further determine
the type of a scalar value (int, float, text, or boolean) so this function
simply returns one of 'array', 'object', or 'scalar'.

CREATE FUNCTION json_typeof(json_value JSON)
RETURNS TEXT AS $$
BEGIN
BEGIN
PERFORM json_array_length(json_value);
RETURN 'array';
EXCEPTION WHEN invalid_parameter_value THEN NULL;
END;
BEGIN
PERFORM json_each(json_value);
RETURN 'object';
EXCEPTION WHEN invalid_parameter_value THEN NULL;
END;
RETURN 'scalar';
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;

With this function, it's fairly simple to define a domain which only
permits an array or an object as the top-level element:

CREATE DOMAIN json_document CHECK (json_typeof(VALUE) IN ('array',
'object'));

I feel that the json_typeof function is worthy of being a built-in
function. Thoughts?

Cheers,
-Andrew