F# 类型推断不如类型注释指示的通用

Question

在以下代码片段的最后一行，我收到两个警告：This construct causes code to be less generic than indicated by the type annotations. The type variable 'c has been constrained to be type ''a'. 和 This construct causes code to be less generic than indicated by the type annotations. The type variable 'b has been constrained to be type ''a * 'a'.

type SomeBaseClass<'a> () =
    class end

type SomeClass<'a when 'a:equality> (getValue:unit->'a, ?arg2:SomeBaseClass<'b>) =
    inherit SomeBaseClass<'a*'a>()

    member this.Value with get () = getValue ()

    member this.Transform (transformation:'a->'c) =
        let func ():'c = transformation this.Value
        SomeClass<'c> (func, this) // warnings are attached to this line

另一方面，编译没有问题：

type SomeOtherClass<'a when 'a:equality> (getValue:unit->'a) =
    inherit SomeBaseClass<'a*'a>()

    member this.Value with get () = getValue ()

    member this.Transform (transformation:'a->'c) =
        let func ():'c = transformation this.Value
        SomeOtherClass<'c> func

我没有看到任何阻止 transformation 返回与传递的类型不同的类型。我也不明白为什么第二个警告甚至是一个警告，因为显然我已经打算将新实例的 'b 类型参数设置为 'a*'a.

我做错了什么？

Answer 1

泛型类型 SomeClass 在其构造函数中使用了泛型参数 'b，但其定义中缺少该参数。将类型定义更改为

type SomeClass<'a, 'b when 'a:equality> ...

以及带有警告的行

    SomeClass(func, this)

去除错误，返回的class类型为SomeClass<'c, ('a * 'a)>.

虽然我不知道这是要达到什么目的，所以我无法判断这是否是一个明智的修正。

F# 类型推断不如类型注释指示的通用

F# type inference less generic than indicated by the type annotations

generics

f#

type-inference