The <|? operator is used to decode an optional value at the specified key path into the requested type:

public func <|? <A: Decodable>(json: JSON, keys: [String]) -> Decoded<A?> where A == A.DecodedType { 
  switch flatReduce(keys, initial: json, combine: decodedJSON) { 
  case .failure: return .success(.none) 
  case .success(let x): return A.decode(x) >>- {     
      .success(.some($0)) } 
  } 
}