在Julia中，关键字"where"是什么意思？

Question

struct A{T<:myType}
    arg::T
    arg1
    function A{T}(arg,arg1) where {T}
        
        return new{T}(arg,arg1)
    end
end

我的问题是，在内部构造函数旁边为什么要添加 where {T}。如果没有它，我会得到：T未定义。

- ahm5

Answer 1

您可以在REPL的帮助模式中（通过按?键）搜索where

help?> where
search: where @where with_logger

  where

  The where keyword creates a type that is an iterated union of other types, over 
  all values of some variable. For example Vector{T} where T<:Real includes all 
  Vectors where the element type is some kind of Real number.

  The variable bound defaults to Any if it is omitted:

  Vector{T} where T    # short for `where T<:Any`

  Variables can also have lower bounds:

  Vector{T} where T>:Int
  Vector{T} where Int<:T<:Real

  There is also a concise syntax for nested where expressions. For example, this:

  Pair{T, S} where S<:Array{T} where T<:Number

  can be shortened to:

  Pair{T, S} where {T<:Number, S<:Array{T}}

  This form is often found on method signatures.

  Note that in this form, the variables are listed outermost-first. This matches 
  the order in which variables are substituted when a type is "applied" to 
  parameter values using the syntax T{p1, p2, ...}.

你需要在函数签名末尾添加 where {T} 是因为A是一个带有参数T的参数化类型。如果只留下 A{T}，则函数会认为你正在创建带有参数T的 A 类型的实例，但此时 T 尚未定义。通过添加 where {T}，你让构造函数知道 T 将在调用函数时作为参数传递进来，而不是在定义时就确定。

- Jack Shannon

Answer 2

为了补充@JackShannon的回答，对于函数，where关键字放置的位置也有所不同：

foo(x::T) where {T<:Number}  = x + one(T) # ok, I can use T within the function body
foo2(x::T where {T<:Number}) = x + one(T) # run time error T not defined
foo3(x::T where {T<:Number}) = x + 1      # ok, I am not using T inside the function body

- Antonello