Swift 如何消除泛型构造函数的歧义？

Question

考虑以下 Swift 表达式

println(Generic<Foo, Bar>(1))

Normally, one would read this as a generic call to the constructor Generic<Foo, Bar> with the arguments (1).

println( Generic<Foo,Bar>(1) )

However, when re-arranging the tokens a bit, it could also represent two separate comparisons, for example if Generic and Foo were some poorly named numeric variables:

println(Generic < Foo, Bar > (1))
// or, with proper parenthesis
println((Generic < Foo), (Bar > 1))

What we can observe here is that an expression with a generic constructor like this is highly ambiguous, and not easy to disambiguate even for humans. The problem here is that Swift doesn't have a new keyword for constructors, which makes them ambiguous with method calls and operators in some cases. Thus, I am interested in how the Swift compiler (parser) manages to disambiguate the above expressions. Is the way it is parsed dependant on the context (types, variables, functions) or can it be resolved by the parser?

Answer 1

答案很简单：编译器根本不允许您声明这些变量：

struct Generic<T, U> {

    init(_ i: Int) {}
}
struct Foo {}
struct Bar {}

print(Generic<Foo, Bar>(1))

// error
let Foo = 0      // invalid redeclaration of Foo
let Bar = 3      // invalid redeclaration of Bar
let Generic = 5  // invalid redeclaration of Generic
print(Generic<Foo, Bar>(1))

将另一个源文件中的变量或类型声明设置为当前声明将“覆盖”另一个。