Swift: Group array values into a dictionary by key mapping (Curryable version)

groupBy.swift

// Given an array collection (may be a Swift Array or an Objective-C NSArray)
// and a function that optionally maps each element to its corresponding group,
// return a Swift Dictionary of the mapped elements in their groups.
// 
// Usage:
// func keyFunc(o: V) -> K {...}
// let grouped = groupBy(keyFunc)([V])
//
// Example:
// func length(s: String) -> Int { return countElements(s) }
// let grouped = groupBy(length)(["A", "BB", "AB", [], NSDate()]) // [2: ["BB", "AB"], 1: ["A"]]

func groupBy<K,V>(keyFunc: V -> K?) -> ([AnyObject]) -> [K: [V]] {
    var grouped = [K: [V]]()
    func group(collection: [AnyObject]) -> [K: [V]] {
        for c in collection {
            if let o = c as? V {
                if let k = keyFunc(o) {
                    if var group = grouped[k] {
                        group.append(o)
                        grouped[k] = group
                    }
                    else {
                        grouped[k] = [o]
                    }
                }
            }
        }
        return grouped
    }
    return group
}

Rather than only working on arrays, it looks like it could be generalized to handle arbitrary sequence types. Perhaps a SequenceType where SequenceType.GeneratorType.Element == AnyObject

Perhaps no type constraint needed.

Any time I see for-in used in a generic finction with arrays only, I want to replace the array with the most general generic type the for-in can handle.

Any idea if a groupby method is the works to be standardized in Swift 3? It belongs with map, filter and reduce as a standard aggregate method.