[swift-users] returning an array of Protocol-conforming objects from a function

[Ryan Walklin]

Hi Austin, 

Thanks for your reply. Sorry you’re right, my struct types don’t work if they are in in existing array of type [MyType] as below.

I’ve been trying to work round it and found that with the following code: 

extension MyType: MyProtocol {}

var array1: [MyType]

fun foo() -> [MyProtocol] {
    return array1
}

func foo2 -> [MyProtocol] {
    return [myType1, myType2]
}


The foo pattern fails, but the foo2 pattern works. My existing code with struct type was all of type foo2, so I didn’t appreciate the error. As you stay, still seems like a problem with type inference.

Ryan

> On 26 Jan 2016, at 2:47 PM, Austin Zheng <austinzheng at gmail.com> wrote:
> 
> Hi Ryan,
> 
> Are you sure it works with user-defined struct types? I tried the following code, and got the same error:
> 
> protocol MyProtocol {}
> 
> struct Blah : MyProtocol { }
> 
> func foo() -> [MyProtocol] {
>   let a = [Blah(), Blah()] as [Blah]
>   return a
> }
> 
> 'class Blah' works fine.
> 
> Even so, this is not the behavior I would have expected. Perhaps someone else can explain why [T] can't be upcasted to [P] when T is a struct and T : P.
> 
> Best,
> Austin
> 
> 
>> On Jan 25, 2016, at 7:36 PM, Ryan Walklin via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> Hi All,
>> 
>> I have a function which returns a closure, which then returns an array of objects conforming to a protocol. 
>> 
>> ie. 
>> 
>> protocol MyProtocol {}
>> 
>> extension Float: MyProtocol {}
>> extension OtherType: MyProtocol {}
>> 
>> When I try to return an array of Floats, I get the error “Cannot convert value of type '[Float]' to closure result type ‘[MyProtocol]”.
>> 
>> Other struct-based types work fine.
>> 
>> Is this not possible with standard library types?
>> 
>> Regards,
>> 
>> Ryan
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>> swift-users at swift.org <mailto:swift-users at swift.org>
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> 

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