f# - 如何约束一个类型参数

Question

有没有办法约束一个类型参数从另一个类型参数派生？

type Foo<'T, 'U when 'U :> 'T> = 
    member x.Bar() : 'T = upcast Unchecked.defaultof<'U>

此代码产生以下错误:

Error 1 Invalid constraint: the type used for the constraint is sealed, which means the constraint could only be satisfied by at most one solution

Error 2 This type parameter has been used in a way that constrains it to always be ''T'

Error 3 The static coercion from type 'T to 'T0 involves an indeterminate type based on information prior to this program point. Static coercions are not allowed on some types. Further type annotations are needed.

Warning 4 This construct causes code to be less generic than indicated by the type annotations. The type variable 'U has been constrained to be type ''T'.

Answer 1

否:(。(根据我的看法，这是目前F＃最不幸的限制之一。请参阅规范的Solving Subtype Constraints部分，其中指出:

New constraints of the form type :> 'b are solved again as type = 'b.

这真的很可惜，因为否则我们可以解决F＃缺少通用方差的问题:

let cvt<'a,'b when 'a :> 'b> (s:seq<'a>) : seq<'b> = // doesn't compile
  s |> box |> unbox