array:flatten is recursive

flatten-one.xq

declare function local:flatten-one($nested as array(*)) as array(*) {
    array:fold-left($nested, [], function ($flattened as array(*), $next as item()*) as array(*) {
        typeswitch($next)
        case array(*) return array:join(($flattened, $next))
        default return array:append($flattened, $next)
    })
};

local:flatten-one([[1], 3, map{}, [1, [3,4]], ()])