根据函数的位置键入与泛型相关的错误

Question

我ofType<'T>基于以下方式为序列创建了一个可管道的函数Enumerable.OfType<'T>():

let ofType<'T> (sequence : _ seq) = sequence.OfType<'T>()

在同一个.fsx文件中使用它可以正常工作; 我把它放入模块时仍然会这样做:

module Seq =
    let ofType<'T> (sequence : _ seq) = sequence.OfType<'T>()

当我将其移动到另一个脚本文件中时(它能够从其他地方访问它)将它包装在另一个顶级模块中时它停止工作:

module Prelude =
    open System.Linq

    module Seq =
        let ofType<'T> (sequence : _ seq) = sequence.OfType<'T>()

我从原始脚本文件中引用它,打开Prelude模块并调用函数如下:

let getXmlIncludes (xtype : Type) =
    xtype.GetCustomAttributes() |> Seq.ofType<XmlIncludeAttribute>

这导致Seq.ofType<XmlIncludeAttribute>被标记为错误,带有消息

error FS0001: Type mismatch. Expecting a
    Collections.Generic.IEnumerable<Attribute> -> 'a    
> but given a
    Collections.Generic.IEnumerable<Attribute> -> Collections.Generic.IEnumerable<XmlIncludeAttribute>    
The type 'obj' does not match the type 'Attribute'

当我ofType<'T>直接进入Prelude模块时,错误保持不变.

为什么会发生这种情况,我怎么能让它不发生？

(我尝试将参数的_ seq类型更改sequence为'TSeq seq,这导致了流行的版本

warning FS0064: This construct causes code to be less generic than indicated by the type annotations. The type variable 'TSeq has been constrained to be type 'obj'.

但不会改变任何有关错误的内容.)

Answer 1

Enumerable.OfType<'T>()是不是通用至于输入参数.更改_ seq为非泛型后IEnumerable,错误消失.

open System
open System.Collections
open System.Reflection
open System.Xml.Serialization

module Prelude =
    open System.Linq
    module Seq =
        let inline ofType<'T> (sequence : IEnumerable) = sequence.OfType<'T>()

open Prelude
let getXmlIncludes (xtype : Type) =
    xtype.GetCustomAttributes() |> Seq.ofType<XmlIncludeAttribute>

在你原来的代码(sequence : _ seq)被约束seq<Attribute>,但F#并 不能支持型协方差和不能正常工作seq<XmlIncludeAttribute>,如果它是seq<Attribute>,即使XmlIncludeAttribute继承Attribute.但即使F#支持协方差,您的示例也只适用于此特定情况,仅适用于继承的类型Attribute.

如果您尝试使用不同类型的Seq扩展,则可以清楚地看到错误:

let getIntsAsUints (list : List<int>) =
    list |> Seq.ofType<uint32>

Script.fsx(21,13): error FS0001: The type 'List<int>' is not compatible with the type 'seq<Attribute>'