可选 object 到函数式编程中的非可选数组

Question

I have an optional object1 of Type1. I would like to convert it to an array of Type2 (non optional, empty if object1 was nil).

Type2 objects are constructed with Type1 objects.

So I've tried like this:

func convert(object1: Type1?) -> [Type2] {
    object1.map {
        [
         Type2($0)
        ] 
    }
}

But I get this error:

Cannot convert return expression of type '[Type2]?' to return type '[Type2]'

Note: Type2 initialiser cannot take a an optional value as parameter.

if anyone has an idea, thanks in advance

Answer 1

Try

func convert(object1: Type1?) -> [Type2] {
    guard let res = object1 else { return [] }
    return [Type2(res)]
}

Answer 2

The error you are getting means that when you map an optional value you will either get the result of mapping (array of Type2 indoor example) or nil if the initial value (object) was nil. In such a case you could use nil coalesing operator to give a value to replace nil (in this case an empty array):

func convert(object: Type1?) -> [Type2] {
    object.map { [Type2($0)] } ?? []
}

Another possible approach would be:

func convert(object: Type1?) -> [Type2] {
    [object]
        .compactMap { $0 }
        .map { Type2($0) }
}

Answer 3

        class Type1 {}

        class Type2 {
            init(_ type1: Type1) {
            }
        }

        func convert(object1: Type1?) -> [Type2] {
            if let object1 = object1 {
                return [Type2(object1)]
            }
            return []
        }