I would like to know if there is a more elegant way to write this:
struct S { var state: [String: Any] public var amounts: [Amount] { var result: [Amount] = [] (self.state["amounts"] as? [Any]?)??.forEach({ a in result.append(Amount(a)) }) return result } } struct Amount { init(_ any: Any?) {} } I have tried using map for array, but I can't find a way to do so.
You could also use guard let and early return, which would make it look bit better.
Here is how I did it,
struct S { var state: [String: Any] public var amounts: [Amount] { guard let amounts = state["amounts"] as? [Any] else { return [] } return amounts.map(Amount.init) } } 
map(Amount.init) would be more readable than .map { Amount($0) }?
struct S { var state: [String: Any] public var amounts: [Amount] { return (self.state["amounts"] as? [Any] ?? []).map({ Amount($0) }) } } struct Amount { init(_ any: Any?) {} } You are using too many unnecessary optionals here. You should always use as? with a non-optional type. And instead of forEach, use map:
public var amounts: [Amount] { if let anyArray = self.state["amounts"] as? [Any] { return anyArray.map(Amount.init) } else { return [] } } You can get that working in a single line,
public var amounts: [Amount] { return (self.state["amounts"] as? [Any])?.map({ Amount($0) }) ?? [] } 
init(_ state: [Amount]) { self.init() guard let amounts = state["amounts"] as? [Any] else { return [] } return amounts.map(Amount.init) } 
amountsis most likely something more specific than[Any]amountsarray would be filled based on optional value fromstate, I'd suggest to declare it as an optional array ([Amount]?).