Swift Functional Programming - Second Edition

<| operator

The <| operator is used to decode a value at the specified key path into the requested type. This operator uses a function named flatReduce that reduces and flattens the sequence:

public func <| <A: Decodable>(json: JSON, keys: [String]) -> Decoded<A> where A == A.DecodedType {     return flatReduce(keys, initial: json, combine: decodedJSON) >>-       A.decode }