以下代码失败的一般问题已经是
answered了.但我的问题是,如果它永远不可能,为什么这不是编译器错误?
这是一个最小的例子:我可以定义这个函数,即使unsafeBitCast总是会失败,编译器也不会抱怨:
func foo() -> [Int] {
let arr: [Int?] = [1, 2, 3, 4]
guard let barr = arr as? [Int] else { return [] }
return barr
}
在我们实际尝试在运行时调用foo()之前,这不会失败.
最佳答案 这是一个编译器错误,因为[Int]不是[Int?]的子类型所以,你的代码不应该编译而没有任何错误.但是,下一个代码应该按预期工作
let i: [Int?] = [1,2,3]
let d: [Double?] = [1,2,3]
func foo<T>(arr:[T?]?) -> [T] {
guard let barr = arr where T.self == Int.self else { return [] }
return barr.flatMap{ $0 }
}
print(foo(i), foo(i).dynamicType)
print(foo(d), foo(d).dynamicType)
/*
[1, 2, 3] Array<Int>
[] Array<Double>
*/
请填写错误报告!更有趣的是,下一代码无法编译
let j = [Optional(1),Optional(2)] as? [Int]
但
let j = [Optional(1),Optional(2)]
let k = j as? [Int]
编译和崩溃……
let arr0:[Int?]? = [1,2,3]
if let r = arr0 where r.dynamicType == [Int].self {
print(r, r.dynamicType)
} else {
print("not [Int]")
}
// prints
// not [Int]
// but
func foo()->[Int] {
let arr0:[Int?]? = [1,2,3]
if let r = arr0 where r.dynamicType == [Int].self {
print(r, r.dynamicType)
// !!!!! ERROR !!!!!
// next line doesn't compile wihout flatMap !!!!
//
// error: cannot convert return expression of type '[Int?]' to return type '[Int]'
//
// even though that it is clear from the where clause that r type must be [Int]
return r.flatMap{ $0 }
} else {
print("arr0 is not [Int]")
return []
}
}
foo() // [] and prints: arr0 is not [Int]