What about this code causes swift to crash?

问题

I'm playing around with arrays and a flatten function in order to wrap my head around how Swift uses generics.

However, the following code gives a generic 'Error running playground' popup, and I can't work out why.

Interestingly, if I put the code into a project and try to compile it, the compiler segfaults.

import Cocoa

var arr: [[Int]] = [[1,2], [3,4], [5,6]]

func flatten<S: SequenceType, SS: SequenceType, E where S.Generator.Element == SS, SS.Generator.Element == E>(seq: S) -> [E] {
    var result = [E]()

    for subseq: SS in seq {
        for elem: E in subseq {
            result += [elem]
        }
    }

    return result
}

var flattened = flatten(arr)
println(flattened)

Has anyone seen something like this / found a workaround?

Sadly I suspect this might just be a bug with type inference in the beta :(

回答1:

This definitely seems to be some sort of bug with Swift generics (it should at least be showing a compile error instead of apparently crashing).

As for a workaround though, this seems to work:

func flatten<E>(seq: [[E]]) -> [E] {
    var result = [E]()

    for subseq: [E] in seq {
        for elem: E in subseq {
            result += [elem]
        }
    }

    return result
}

回答2:

Your flatten implementation still crashes the compiler, which I still regard as a bug. However, if you define it like this, it works:

func flatten<S: SequenceType, E where S.Generator.Element: SequenceType, S.Generator.Element.Generator.Element == E>(seq: S) -> [E] {
    var result = [E]()
    for subseq: S.Generator.Element in seq {
        for elem: E in subseq {
            result += [elem]
        }
    }
    return result
}

This achieves your goal of working with any SequenceType, not just arrays. (In fact, I wish we could return an even more general type than [E]. If I were back in C# land, I'd return IEnumerable<E>, the closest equivalent of SequenceType, to get even more sexy encapsulation. Swift is a bit leakier.)

来源：https://stackoverflow.com/questions/25194210/what-about-this-code-causes-swift-to-crash

标签

swift

swift-playground