Macro require_child 

macro_rules! require_child {
    ($V:ty, $parent:expr, $child:expr, $idx:expr => $M:ty) => { ... };
}

Require that a child array matches $M. If the child already matches, returns the same array unchanged. Otherwise, early-returns an ExecutionResult requesting execution of child $idx until it matches $M.

let codes = require_child!(Self, array, array.codes(), 0 => Primitive);
let values = require_child!(Self, array, array.values(), 1 => AnyCanonical);