朱莉娅(Julia):“&lt ;:"是什么意思符号是什么意思? [英] Julia: what does the "<:" symbol mean?

问题描述

这在函数签名中是什么意思，例如:

convert(::Type{T}, z::Complex) where {T<:Real}

解决方案

严格来说，应该区分谓词Base.:(<:)(如@Saqib的答案所述)和<:用来描述约束的句法用法. /p>

这种语法用法可以出现在方法的类型参数声明中，以将类型变量约束为其他类型的子类型:

f(x::T) where {T<:Real} = zero(x)

这种特殊情况是当您约束struct(struct Foo{T<:Real} ... end)的类型参数时-约束所生成的构造函数的方法，并允许将类型构造函数仅应用于受约束的子类型.

另一方面，在类型参数之外，<:可用于将新类型声明为某些其他(必要的是抽象)类型的子类型:

struct Foo <: Real end

尽管这两种情况都与子类型谓词的含义一致，但是您不能将它们替换为其他任意表达式(例如，您不能在f中编写... where {isreal(T)}).

What does this mean in function signatures, for example:

convert(::Type{T}, z::Complex) where {T<:Real}

解决方案

Strictly speaking, one should differentiate between the predicate Base.:(<:), as described in @Saqib's answer, and the syntactic usage of <: for describing constraints.

This syntactic usage can occur in type parameter declarations of methods, to constrain a type variable to be a subtype of some other type:

f(x::T) where {T<:Real} = zero(x)

A sort of special case of this is when you constrain the type parameter of a struct (struct Foo{T<:Real} ... end) -- that constrains the methods of the generated constructor, and allows the type constructor to be applied only to the constrained subtypes.

On the other hand, outside of type parameters, <: can be used to declare a new type as a subtype of some other (necessarily abstract) type:

struct Foo <: Real end

Although both cases are in line with the meaning of the subtyping predicate, you can't replace them with other arbitrary expressions (e.g., you can't write ... where {isreal(T)} in f).

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