为什么适当的类型 Any 和 Nothing 在类型不同时适合类型构造函数形状 F[_] ？
Why do proper types Any and Nothing fit type constructor shape F[_] when they are different kinds?
考虑以下采用 * -> * 种类
类型参数的方法
def g[F[_]] = ???
为什么下面的不是语法错误
g[Any] // ok
g[Nothing] // ok
因为
scala> :kind -v Any
Any's kind is A
*
This is a proper type.
scala> :kind -v Nothing
Nothing's kind is A
*
This is a proper type.
所以 Any 和 Nothing 应该是错误的形状？
引用自 Scala 规范：
For every type constructor </code> (with any number of type parameters), <code>scala.Nothing <: <: scala.Any.
https://scala-lang.org/files/archive/spec/2.13/03-types.html#conformance
Say the type parameters have lower bounds 1,…, and upper bounds 1,…,. The parameterized type is well-formed if each actual type parameter conforms to its bounds, i.e. <:<: where </code> is the substitution <code>[1:=1,…,:=].
https://scala-lang.org/files/archive/spec/2.13/03-types.html#parameterized-types
A polymorphic method type is denoted internally as [tps] where [tps] is a type parameter section [1 >: 1 <: 1,…, >: <: ] for some ≥0 and </code> is a (value or method) type. This type represents named methods that take type arguments <code>1,…, which conform to the lower bounds 1,…, and the upper bounds 1,…, and that yield results of type </code>.</p>
</blockquote>
<p><a href="https://scala-lang.org/files/archive/spec/2.13/03-types.html#polymorphic-method-types" rel="nofollow noreferrer">https://scala-lang.org/files/archive/spec/2.13/03-types.html#polymorphic-method-types</a></p>
<p>所以由于<code>Any和Nothing符合F[_]的上下界（即Any和Nothing对应），g[Any] 和 g[Nothing] 是合法的。
考虑以下采用 * -> * 种类
def g[F[_]] = ???
为什么下面的不是语法错误
g[Any] // ok
g[Nothing] // ok
因为
scala> :kind -v Any
Any's kind is A
*
This is a proper type.
scala> :kind -v Nothing
Nothing's kind is A
*
This is a proper type.
所以 Any 和 Nothing 应该是错误的形状？
引用自 Scala 规范：
For every type constructor
</code> (with any number of type parameters), <code>scala.Nothing <: <: scala.Any.
https://scala-lang.org/files/archive/spec/2.13/03-types.html#conformance
Say the type parameters have lower bounds
1,…,and upper bounds1,…,. The parameterized type is well-formed if each actual type parameter conforms to its bounds, i.e.<:<:where</code> is the substitution <code>[1:=1,…,:=].
https://scala-lang.org/files/archive/spec/2.13/03-types.html#parameterized-types
A polymorphic method type is denoted internally as
[tps]where[tps]is a type parameter section[1 >: 1 <: 1,…, >: <: ]for some≥0and</code> is a (value or method) type. This type represents named methods that take type arguments <code>1,…,which conform to the lower bounds1,…,and the upper bounds1,…,and that yield results of type</code>.</p> </blockquote> <p><a href="https://scala-lang.org/files/archive/spec/2.13/03-types.html#polymorphic-method-types" rel="nofollow noreferrer">https://scala-lang.org/files/archive/spec/2.13/03-types.html#polymorphic-method-types</a></p> <p>所以由于<code>Any和Nothing符合F[_]的上下界（即Any和Nothing对应），g[Any]和g[Nothing]是合法的。