Currently it is inefficient to detect the length of an array and it is impossible to know if an key (without subkeys) should be an array or not.

For the latter problem different workarounds exist, such as ###empty_array in yajl.

###empty_array as in yajl, problem: does not allow efficient access of first element
store length (and not last element), problem: needs prepending of #_...
store element after last element (C++-Style), problem: very unusual style
use value and not metadata array, problem: is ambiguous

Store length in metadata array of key, or keep metadata array empty if empty array. For example (ni syntax, sections used for metadata):

myarray/#0 = value0
myarray/#1 = value1
myarray/#2 = value2
myarray/#3 = value3
myarray/#4 = value4
myarray/#5 = value5
[myarray]
  array = #5