Thinking about it, it is possible that this proposal would not work, because in the nothing-matches cases then the function would not be executed at all... But maybe this could work, by widening the matching pattern.

  all-checked = all[in: [#todo], where: [completed: true] ]
  none-checked = none[in: [#todo], where: [completed: true] ]
  todo-count = count[in: [#todo], where: [completed: false], default: 0]