Optional object to non-optional Array in functional programming

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

So, type1 is array and it can have optional values, and you want to convert it into second array of all non-optional values am I right? — Tushar Sharma
– Tushar Sharma, Commented Jun 4, 2021 at 15:02

LuLuGaGa · Accepted Answer · 2021-06-04 15:44:12Z

1

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) }
}

answered Jun 4, 2021 at 15:44

LuLuGaGa

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1 Comment

Xys Over a year ago

Thanks, I was looking for something like the second approach !

Shehata Gamal · Accepted Answer · 2021-06-04 15:23:20Z

1

Try

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

edited Jun 4, 2021 at 15:23

answered Jun 4, 2021 at 15:02

Shehata Gamal

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2 Comments

Xys Over a year ago

Type2 initialiser cannot take a an optional value as parameter.

Xys Over a year ago

Cannot convert value of type 'Type1?' to expected element type 'Type1'

Tarun Tyagi · Accepted Answer · 2021-06-04 15:03:01Z

0

        class Type1 {}

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

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

answered Jun 4, 2021 at 15:03

Tarun Tyagi

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1 Comment

Tushar Sharma Over a year ago

I think compactMap is much simpler to read solution.

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