CODEWITHSUNDEEP
swiftgenericstype-alias
ixx
ixx

Reputation: 32271

How to use type parameter in typealias?

I want to use a typealias for a function which is used in a generic class. One of the parameters of the function uses a generic type of the class. This is a simplified example:

class Bar {}

// no typealias - works
class TestGeneric<T: Bar> {
    let myFunc: (T) -> String

    init(myFunc: (T) -> String) {
        self.myFunc = myFunc
    }
}

let myFunc: (Bar) -> String = {(bar: Bar) in
    return "hello"
}

let test = TestGeneric<Bar>(myFunc: myFunc)
println(test.myFunc(Bar()))

// typealias - doesn't compile
class TestTypealias<T: Bar> {
    typealias MyFuncGeneric = (T) -> String

    let myFunc: MyFuncGeneric

    init(myFunc: MyFuncGeneric) {
        self.myFunc = myFunc
    }
}

let myFunc2: TestTypealias.MyFuncGeneric = {(bar: Bar) in
    return "hello"
}

let test2 = TestTypealias<Bar>(myFunc: myFunc2) // Error: Cannot invoke initializer for type 'TestTypealias<Bar>' with an argument list of type '(myFunc: T -> String)'
println(test2.myFunc(Bar()))

Is there a way to solve this? Or do I have to forgo typealias when using generics? In the actual code I have a lot of parameters and really need a typealias...

(Swift 1.2)

Upvotes: 0

Views: 1194

Answers (2)

sketchyTech
sketchyTech

Reputation: 5906

By writing

class TestGeneric<T: Bar>

all you've really done is to define T universally as being of type Bar when used inside of a method, so you can write this:

func myFunc(aValue:T) {

}

instead of this:

func myFunc<T: Bar>(aValue:T) {

    }

each time you want to define T as being of type Bar.

If you want the class itself to be generic you use only <T>, for example: class TestGeneric<T> and type is defined upon initializing. 

class Bar {}

// no typealias - works
class TestGeneric<T> {
    let myFunc: (T) -> String

    init(myFunc: (T) -> String) {
        self.myFunc = myFunc
    }
}

let myFunc: Bar -> String = {(bar: Bar) in
    return "hello"
}

let test = TestGeneric<Bar>(myFunc: myFunc)
println(test.myFunc(Bar()))

// typealias - doesn't compile
class TestTypealias<T> {
    typealias MyFuncGeneric = T -> String
    func myFunc(aValue:T) {

    }
    let myFunc: MyFuncGeneric

    init(myFunc: MyFuncGeneric) {

        self.myFunc = myFunc
    }
}

let myFunc2: TestTypealias<Bar>.MyFuncGeneric = {(bar: Bar) in
    return "hello"
}

let test2 = TestTypealias<Bar>(myFunc: myFunc2)   
println(test2.myFunc(Bar()))

Upvotes: 1

Gregzo
Gregzo

Reputation: 1194

This compiles and behaves as expected:

class Test< T >
{
    typealias GenFunc = ( thing: T ) -> Void

    let thing : T

    init( thing: T )
    {
        self.thing = thing
    }

    func ExecuteFunc( genFunc: GenFunc )
    {
        genFunc( thing: self.thing )
    }
}

Use: 

let t = Test( thing: "blah" )

t.ExecuteFunc()
{ ( thing: String ) -> Void in
    println( thing )
}

Upvotes: 1

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