英文:
Custom use of brackets like NamedTuple
问题
以下是翻译好的部分：
- Does the creation of a NamedTuple rely on macros/metaprogramming?（创建NamedTuple是否依赖于宏/元编程？）
- What does
:splatnewdo?（:splatnew是做什么的？） - How would you approach making your own custom use of brackets?（你如何处理创建自己的自定义括号用法？）
英文:
Brackets in Julia are used for Parametric Types as far as I encountered by now.
There is however one example of curly brackets being used in a different way:
NamedTuple{(:a, :b)}((1, 2))
This constructs the NamedTuple (a=1, b=2), but you pass the field names in brackets.
The source code for the NamedTuple is a bit obscure in my opinion.
NamedTuple() = NamedTuple{(),Tuple{}}(())
NamedTuple{names}(args::Tuple) where {names} = NamedTuple{names,typeof(args)}(args)
eval(Core, :(NamedTuple{names,T}(args::T) where {names, T <: Tuple} =
$(Expr(:splatnew, :(NamedTuple{names,T}), :args))))
- Does the creation of a NamedTuple rely on macros/metaprogramming?
- What does
:splatnewdo? - How would you approach making your own custom use of brackets?
答案1
得分: 3
但是有一个例子是花括号以不同的方式使用的：
它们以相同的方式使用。字段名称是 NamedTuple 的参数。要查看这一点，请检查：
julia> typeof((a=1, b=2))
NamedTuple{(:a, :b), Tuple{Int64, Int64}}
唯一的问题是你省略了传递第二个参数，这种情况下是 Tuple{Int64, Int64}，但在这种情况下，Julia 允许省略传递所有参数。你可以省略它们的原因在你引用的这段代码中处理：
NamedTuple{names}(args::Tuple) where {names} = NamedTuple{names,typeof(args)}(args)
这将参数扩展为包括元组作为第二个参数。
创建 NamedTuple 是否依赖于宏/元编程？
是的，这段代码：
eval(Core, :(NamedTuple{names,T}(args::T) where {names, T <: Tuple} =
$(Expr(:splatnew, :(NamedTuple{names,T}), :args))))
在 Core 模块中评估表达式。这是元编程（但不是宏）。
:splatnew是什么作用？
正如你可以在 Julia 手册 中阅读的那样，它是：
类似于
new，但字段值作为单个元组传递。如果new是一个一流函数，那么它的工作方式类似于splat(new)，因此得名。
所以它本质上是 new，但接受一个位置参数，该参数会被展开。
如何制定自己的括号的自定义用法？
这不是括号的自定义用法。语义遵循标准的参数化类型规则。
英文:
> There is however one example of curly brackets being used in a different way:
They are used in the same way. Field names are parameters of a NamedTuple. To see this check:
julia> typeof((a=1, b=2))
NamedTuple{(:a, :b), Tuple{Int64, Int64}}
The only issue is that you are omitting passing the second parameter, which is Tuple{Int64, Int64} in this case but Julia allows omitting passing all parameters in this case. The reason why you can omit them is handled in this code you quoted:
NamedTuple{names}(args::Tuple) where {names} = NamedTuple{names,typeof(args)}(args)
which expands the parameters to include the tuple as a second parameter.
> Does the creation of a NamedTuple rely on macros/metaprogramming?
Yes, the code:
eval(Core, :(NamedTuple{names,T}(args::T) where {names, T <: Tuple} =
$(Expr(:splatnew, :(NamedTuple{names,T}), :args))))
evaluates the expression in Core module. This is metaprogamming (but not macro).
> What does :splatnew do?
As you can read in Julia Manual it is:
> Similar to new, except field values are passed as a single tuple. Works similarly to splat(new) if new were a first-class function, hence the name.
So essentially it is new but accepting one positional argument that is a tuple that gets splatted.
> How would you approach making your own custom use of brackets?
This is not a custom use of brackets. The semantics follows standard parametric type rules.
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