Swift compiler is unable to resolve recursive use of generics

Question

I am trying to implement the chain of responsibility pattern in Swift.

public class Chain<T, U> {
    private var command: (T?, (U?) -> Void) -> Void
    private var runCommand: (() -> Void)?
    private var nextCommand: ((U?) -> Void)?

    private init(command: (T?, (U?) -> Void) -> Void) {
        self.command = command
    }

    private func next(u: U?) {
        self.nextCommand?(u)
    }

    func then<V>(command: (U?, (V?) -> Void) -> Void) -> Chain<U, V> {
        let c = Chain<U, V>(command: command)

        self.nextCommand = { command($0, c.next) }
        c.runCommand = self.runCommand

        return c
    }

    func endWith(command: (U?) -> Void) {
        self.nextCommand = command
        self.runCommand!()
    }

    static func build<V>(command: ((V?) -> Void) -> Void) -> Chain<AnyObject, V> {
        let c = Chain<AnyObject, V>(command: { _, next in command(next) })
        c.runCommand = { command(c.next) }
        return c
    }
}

My class does not raise any compilation error but a simple use case (such as the one below) does not work. It raises the following error: error: cannot invoke 'endWith' with an argument list of type '((_?) -> ()) ; expected an argument list of type '((U?) -> Void)'

Any thought?

Chain.build { next in
    print("Foo")
    next("Bar")
}
.then { o, next in
    print(o)
    next(15)
}
.endWith { o in
    print(o)
}

I know it is an edge case of generics usage in Swift. However, as it is not possible to explicitly specialize a generic type, I have not found any solution so far.

What do you mean by "does not work?" What happens? Do you receive an error? What error? On what line? Does it compile but have problems at runtime? What problems? What do you expect to happen, and what actually does happen? — Rob Napier, Dec 07 '15 at 19:53
Just edited my message with posting the compilation error I have. — Adrien Cadet, Dec 07 '15 at 19:58
Can't see how `T` is infered. I think this is one of the problems. — Jean-Baptiste Yunès, Dec 07 '15 at 20:21

score 3 · Accepted Answer · answered Dec 07 '15 at 21:26

3

The compiler isn't able to infer the types in your example. You just need to specify them where they're ambiguous:

Chain<String,Int>.build { next in
  print("Foo")
  next("Bar")
  }
  .then { (o: String?, next: Int? -> Void) in
    print(o)
    next(15)
  }
  .endWith { o in
    print(o)
}

answered Dec 07 '15 at 21:26

oisdk

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Thanks for you reply oisdk! Actually I found out typing the closures' arguments does not help at all. It was only due to the `build` method, for, in Swift, typing a generic is mandatory. Here is my full class, for people who are interested in: https://gist.github.com/acadet/dee9287e70c061a358b1 – Adrien Cadet Dec 08 '15 at 00:28

Swift compiler is unable to resolve recursive use of generics

1 Answers1