Polymorphism with inherited generics cause bad access in swift

Question

I've got two classes which inherit from a generic class.

class Parser<I: CollectionType, T> {
    func parse(ts: I) -> [(T, I)] { return [] }

    func parse_all(ts: I) -> [(T, I)] {
        return parse(ts).filter { isEmpty($0.1) }
    }
}

class CharParser<I: CollectionType, T> : Parser<[Character], Character> {
    let c: Character

    init(c: Character) { self.c = c }

    override func parse(ts: [Character]) -> [(Character, [Character])] {
        println(ts)
        return ts[0] == c ? [(ts[0], Array(ts[1..<ts.count]))] : []
    }
}

class AltParser<I: CollectionType, T> : Parser<I, T> {
    let p: Parser<I, T>
    let q: Parser<I, T>

    init(p: Parser<I, T>, q: Parser<I,T>) {
        self.p = p
        self.q = q
    }

    override func parse(ts: I) -> [(T, I)] {
        println(ts)
        return p.parse(ts) + q.parse(ts)
    }
}

class SeqParser<I: CollectionType, T, S>: Parser<I, (T, S)> {
    let p: Parser<I, T>, q: Parser<I,S>

    init(p: Parser<I, T>, q: Parser<I,S>) { self.p = p; self.q = q }

    override func parse(ts: I) -> [((T, S), I)] {
        var acc = [((T, S), I)]()
        for (head1, tail1) in p.parse(ts) {
            for (head2, tail2) in q.parse(tail1) {
                acc += [((head1, head2), tail2)]
            }
        }
        return acc
    }
}

Calling

let x = CharParser<[Character], Character>(c: "a")
x.parse(Array("a"))

works fine, however trying to call it through the AltParser causes an EXC_BAD_ACCESS

let x2 = CharParser<[Character], Character>(c: "b")
let y = AltParser(p: x, q: x2)
let z = y.parse(Array("a"))

Is this a compiler bug or am I misunderstanding generics?

Answer 1

Okay, having failed completely to show that you can't do this, now I'm going to try to show that you can do it. Here's a heavily simplified version of your code that I believe works (at least it compiles and isn't crashing):

class Parser<T> {
    func parse(ts:[T]) -> [(T, [T])] { println ("super"); return []}
}
class StringParser<T> : Parser<String> {
    let c : String
    init(c: String) { self.c=c }
    override func parse(ts:[String]) -> [(String, [String])] {
        println ("sub")
        return ts[0] == c ? [(ts[0], Array(ts[1..<ts.count]))] : []
    }
}
class AltParser<T> : Parser<T> {
    let p : Parser<T>
    init(p:Parser<T>) {
        self.p = p
    }
    override func parse(ts:[T]) -> [(T, [T])] {
        return p.parse(ts)
    }
}
func test() {
    let x = StringParser<Character>(c:"a")
    x.parse(["a"])
    let z = AltParser(p:x)
    println(z.parse(["a"]))
}

I eliminated everything that seemed superfluous to the example. I turned the Character into a String because I find Character confusing. I removed parse_all and the second property q because you weren't using them for anything. Most significant, I removed the entire CollectionType from the generic and replaced it with an explicit array everywhere; that was because the CollectionType was giving me trouble compiling. I was frustrated, in particular, when I still had the CollectionType in the generic, by an inability to find a way to specify it is a collection of T; it seemed to me somehow crucial that this should be made clear. That is one reason why I removed it and replaced it with an explicit array of T. But perhaps I stumbled into something useful, because the code does seem to run now.

Go ahead and pick it apart...!