julia - Julia函式 header 中的无名值是什么意思？

Question

我经常在Julia中看到类似以下内容的东西:

convert(::Type{Point{Float64}}, ::Float64)

(::如何工作？那么这是什么术语呢？

Answer 1

您的答案可以在the Julia documentation for defining conversions中找到。引用(将类型切换为使其更易于阅读):

The type of the first argument of this method is a singleton type, Type{Point{Float64}}, the only instance of which is Point{Float64}. Thus, this method is only invoked when the first argument is the type value Point{Float64}. Notice the syntax used for the first argument: the argument name is omitted prior to the :: symbol, and only the type is given. This is the syntax in Julia for a function argument whose type is specified but whose value is never used in the function body. In this example, since the type is a singleton, there would never be any reason to use its value within the body.

(强调我的)

当尝试使用错误类型的参数调用函数时，您还将在错误消息中遇到 foo(::SomeType)语法(毕竟，您无法显示不存在的变量的参数名称)。例如:

julia> foo(x::Bool) = 3
foo (generic function with 1 method)

julia> foo(5)
ERROR: `foo` has no method matching foo(::Int64)