Swift Codable with Different Array Types

One solution (assuming you can't change the JSON) is to implement custom decoding logic for Bet. You can use an unkeyed container (which reads from a JSON array) in order to decode each of the properties in turn (the order in which you call decode(_:) is the order they're expected to appear in the array).

import Foundation

struct OrderBook : Codable {
  let number: Int
  let bets: [Bet]
}

struct Bet : Codable {
  let price: String
  let sale: String
  let quantity: Int

  init(from decoder: Decoder) throws {
    var container = try decoder.unkeyedContainer()
    self.price = try container.decode(String.self)
    self.sale = try container.decode(String.self)
    self.quantity = try container.decode(Int.self)
  } 

  // if you need encoding (if not, make Bet Decodable
  // and remove this method)
  func encode(to encoder: Encoder) throws {
    var container = encoder.unkeyedContainer()
    try container.encode(price)
    try container.encode(sale)
    try container.encode(quantity)
  }
}

Example decoding:

let jsonString = """
{ "number": 5295, "bets": [["16","83",9], ["75","99",4], ["46","27",5]] }
"""

let jsonData = Data(jsonString.utf8)

do {
  let decoded = try JSONDecoder().decode(OrderBook.self, from: jsonData)
  print(decoded)
} catch {
  print(error)
}

// OrderBook(number: 5295, bets: [
//   Bet(price: "16", sale: "83", quantity: 9),
//   Bet(price: "75", sale: "99", quantity: 4),
//   Bet(price: "46", sale: "27", quantity: 5)
// ])