Swift 3 - Sequence ambiguous without more context

Question

I tried to create a custom iterator which returns wrapper

abcContainer

over raw data class

abc

// raw data class

class abc {

    var name : String = "";

    init( _ value : String) {

        name = value;

    }

}



// with container, only "name" is to be visible

class abcContainer {

    private var _abc : abc;

    init( _ obj : abc) {

        _abc = obj;

    }

    // + extra methods here

    func getName() -> String {

       return _abc.name

    }

}

The point would be that the dictionary would return instances of

abcContainer

instead of just the plain raw

abc

class.

I wanted to use the sequence protocol to make the conversion automatic, but I was not able to transform the

[String:abc]

into

[String:abcContainer]

automatically like this:

// the iterator is implemented just iterating the inner basic dict

// but wrapping the result value as abcContainer

class abcIterator : Sequence, IteratorProtocol {



    private var __source : [String:abc]?;

    var index = 0

    var myIterator : DictionaryIterator<String, abc>;



    init(_ ctxArray: [String:abc]) {

        self.__source = ctxArray

        index = 0;

        myIterator = (__source?.makeIterator())!

    }



    func next() -> abcContainer? {

        let nextItem = myIterator.next();

        if(nextItem != nil) {

            return abcContainer((nextItem?.value)!);

        }

        return nil;

    }

}



// this was supposed to be the wrapper over the collection

class abcCollection : Sequence {



    private var __source : [String:abc]?;



    init(_ list: [String:abc]) {

        self.__source = list

    }



    func makeIterator() -> abcIterator {

        return abcIterator(self.__source!);

    }

}

I'm probably missing something very basic here. When I try to use the collection like this:

    var dict : [String:abc] = [String:abc]();

    dict["abba"] = abc("John Smith");



    for (key,value) in abcCollection(dict) {

        print(key, value.getName());

    }

I get error: Expression type "abcCollection" is ambiguous without more context

Does anyone have idea how to make it work? What is missing? I have a feeling that this answer has the information I need...

Swift 2 to 3 Migration for Swift Sequence Protocol

Answer 1

The problem in your original code is that abcCollection(dict) returned a sequence of abcContainer objects, and those cannot be assigned to a (key, value) tuple.

You can achieve your goal with

class abcCollection : Sequence {

    private var __source : [String:abc]

    init(_ list: [String:abc]) {
        self.__source = list
    }

    public func makeIterator() -> AnyIterator<(AnyObject,abcContainer)> {
        let mapped = self.__source.lazy.map {
            ($0.key as AnyObject, abcContainer($0.value))
        }
        return AnyIterator(mapped.makeIterator())   
    }
}

Making __source non-optional makes all the (optional) unwrappings redundant, and lazy.map { ... } returns a lazily evaluated sequence of key/value pairs which is then type-erased.

Swift 3 - Sequence ambiguous without more context

Sign up