Static generic extension for Array for help function for iterated async callbacks - swift

Question

I have two help methods that I want to move to an extension to increase it's reusability. The two methods manages multiple callbacks when calling parallell http request. However when making the methods static and moving them to an extension I get this error:
Cannot convert value of type '[Action]' to expected argument type '[_]'
The code is
extension Array
{
private static func iterateObjectList<Type>(objectList:[Type], multiplier:Int=1, foreach:(object:Type, (newObject:Type?, error:NSError?) -> Void) -> (), finally: (objectList:[Type], errorList:[NSError]) -> Void)
{
var iterationsLeft = objectList.count * multiplier
var errorList:[NSError] = []
var returnObjectList:[Type] = []
if (iterationsLeft == 0) {
finally (objectList:objectList, errorList:[])
}
for object:Type in objectList {
foreach (object:object, { (requestObject, requestError) -> Void in
iterationsLeft -= 1
if (requestError != nil) {
errorList.append(requestError!);
}
if (requestObject != nil) {
returnObjectList.append(requestObject!)
}
if (iterationsLeft <= 0) {
finally (objectList:returnObjectList, errorList:errorList)
}
})
}
}
private static func simpleIterate<Type>(objectList:[Type], multiplier:Int=1, foreach:(object:Type, Void -> Void) -> (), finally: Void -> Void)
{
var iterationsLeft = objectList.count * multiplier
if (iterationsLeft == 0) {
finally ()
}
for object:Type in objectList {
foreach (object:object, { Void -> Void in
iterationsLeft -= 1
if (iterationsLeft <= 0) {
finally ()
}
})
}
}
}
The error is when using the methods:
Array.iterateObjectList(actions, foreach: { (action, iterationComplete) -> () in
self.fetchStatusAndUpdateAction(action, callback: { (error) -> Void in
iterationComplete(newObject: action, error:error)
})
}, finally: { (objectList, errorList) -> Void in
callback(error: errorList.first)
})
where actions is of type [Action] where Action is a custom object.

Answer 1

The problem is that
public struct Array<Element>
is a generic type, and in
Array.iterateObjectList(actions, foreach: { (action, iterationComplete) -> () in
// ...
}, finally: { (objectList, errorList) -> Void in
// ...
})
the compiler cannot infer what Element should be. You could
make it compile as
Array<Action>.iterateObjectList(actions, foreach: { (action, iterationComplete) -> () in
// ...
}, finally: { (objectList, errorList) -> Void in
// ...
})
or even
Array<Int>.iterateObjectList(...)
The array Element is unrelated to your generic placeholder Type,
so any type will do.
But the better solution would be to make the static method an
instance method:
func iterateObjectList(multiplier:Int=1, foreach:(object:Element, (newObject:Element?, error:NSError?) -> Void) -> (), finally: (objectList:[Element], errorList:[NSError]) -> Void)
{
// Your code with `Type` replaced by `Element`,
// and `objectList` replaced by `self`.
// ...
}
and call it on the actions array:
actions.iterateObjectList(foreach: { (action, iterationComplete) -> () in
// ...
}, finally: { (objectList, errorList) -> Void in
// ...
})